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A Guide to MATLAB

This book is a short, focused introduction to MATLAB, a comprehen-
sive software system for mathematics and technical computing. It will
be useful to both beginning and experienced users. It contains concise
explanations of essential MATLAB commands, as well as easily under-
stood instructions for using MATLAB’s programming features, graphi-
cal capabilities, and desktop interface. It also includes an introduction
to SIMULINK, a companion to MATLAB for system simulation.

Written for MATLAB 6, this book can also be used with earlier (and
later) versions of MATLAB. This book contains worked-out examples
of applications of MATLAB to interesting problems in mathematics,
engineering, economics, and physics. In addition, it contains explicit
instructions for using MATLAB’s Microsoft Word interface to produce
polished, integrated, interactive documents for reports, presentations,
or online publishing.

This book explains everything you need to know to begin using
MATLAB to do all these things and more. Intermediate and advanced
users will find useful information here, especially if they are making
the switch to MATLAB 6 from an earlier version.

Brian R. Hunt is an Associate Professor of Mathematics at the Univer-
sity of Maryland. Professor Hunt has coauthored four books on math-
ematical software and more than 30 journal articles. He is currently
involved in research on dynamical systems and fractal geometry.

Ronald L. Lipsman is a Professor of Mathematics and Associate Dean
of the College of Computer, Mathematical, and Physical Sciences at the
University of Maryland. Professor Lipsman has coauthored five books
on mathematical software and more than 70 research articles. Professor
Lipsman was the recipient of both the NATO and Fulbright Fellowships.

Jonathan M. Rosenberg is a Professor of Mathematics at the Univer-
sity of Maryland. Professor Rosenberg is the author of two books on
mathematics (one of them coauthored by R. Lipsman and K. Coombes)
and the coeditor of Novikov Conjectures, Index Theorems, and Rigidity,
a two-volume set from the London Mathematical Society Lecture Note
Series (Cambridge University Press, 1995).

A Guide to MATLAB

for Beginners and Experienced Users

Brian R. Hunt Ronald L. Lipsman Jonathan M. Rosenberg

with Kevin R. Coombes, John E. Osborn, and Garrett J. Stuck

SH Cambridge

UNIVERSITY PRESS

CAMBRIDGE UNIVERSITY PRESS

Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, Sao Paulo
Cambridge University Press

The Edinburgh Building, Cambridge CB2 2RU, United Kingdom

Published in the United States of America by Cambridge University Press, New York

www.cambridge.org

Information on this title: www.cambridge.org/9780521 803809
© B. Hunt, R. Lipsman, J. Rosenberg, K. Coombes, J. Osborn, G. Stuck 2001

This book is in copyright. Subject to statutory exception and to the provision of
relevant collective licensing agreements, no reproduction of any part may take place
without the written permission of Cambridge University Press.

First published in print format 2001

ISBN-13 978-0-511-07792-0 eBook (NetLibrary)

ISBN-10 0-511-07792-0 eBook (NetLibrary)

ISBN-13 978-0-521-80380-9 hardback

ISBN-10 0-521-80380-2 hardback

ISBN-13 978-0-521-00859-4 paperback

ISBN-10 0-521-00859-x paperback

Cambridge University Press has no responsibility for the persistence or accuracy of
urls for external or third-party internet websites referred to in this book, and does not
guarantee that any content on such websites is, or will remain, accurate or appropriate.

MATLAB®, Simulink®, and Handle Graphics® are registered trademarks of The
MathWorks, Inc. Microsoft®, MS-DOS®, and Windows® are registered trademarks
of Microsoft Corporation. Many other proprietary names used in this book are
registered trademarks.

Portions of this book were adapted from “Differential Equations with MATLAB” by
Kevin R. Coombes, Brian R. Hunt, Ronald L. Lipsman, John E. Osborn, and Garrett J.
Stuck, copyright © 2000, John Wiley & Sons, Inc. Adapted by permission of John
Wiley & Sons, Inc.

Contents at a Glance

Preface page xiii

1 Getting Started 1

2 MATLAB Basics 8

3 Interacting with MATLAB 31

Practice Set A: Algebra and Arithmetic 48

4 Beyond the Basics 50

5 MATLAB Graphics 67

Practice Set B: Calculus, Graphics, and Linear Algebra 86

6 M-Books 91

7 MATLAB Programming 101

8 SIMULINK and GUIs 121

9 Applications 136

Practice Set C: Developing Your MATLAB Skills 204

10 MATLAB and the Internet 214

1 1 Troubleshooting 218

Solutions to the Practice Sets 235

Glossary 299

Index 317

Contents

Preface page xiii

1 Getting Started 1

Platforms and Versions 1

Installation and Location 2

Starting MATLAB 2

Typing in the Command Window 3

Online Help 4

Interrupting Calculations 5

MATLAB Windows 6

Ending a Session

2 MATLAB Basics 8

Input and Output 8

Arithmetic 8

Algebra 10

Symbolic Expressions, Variable Precision, and Exact
Arithmetic 11

Managing Variables 13

Errors in Input 14

Online Help 15

Variables and Assignments 16

Solving Equations 17

Vectors and Matrices 20

Vectors 21

Matrices 23

Suppressing Output 24

Functions 24

vii

Contents

Built-in Functions
User-Defined Functions
Graphics

Graphing with ezplot
Modifying Graphs
Graphing with plot
Plotting Multiple Curves

3 Interacting with MATLAB

The MATLAB Interface

The Desktop
Menu and Tool Bars
The Workspace
The Working Directory
Using the Command Window
M-Files

Script M-Files
Function M-Files
Loops

Presenting Your Results
Diary Files
Presenting Graphics
Pretty Printing
A General Procedure
Fine-Tuning Your M-Files
Practice Set A: Algebra and Arithmetic

4 Beyond the Basics
Suppressing Output
Data Classes

String Manipulation
Symbolic and Floating Point Numbers
Functions and Expressions
Substitution
More about M-Files

Variables in Script M-Files
Variables in Function M-Files
Structure of Function M-Files

24

25

26

26

27

28

30

31

31

31

33

33

34

35

36

37

39

41

41

42

43

45

45

46

48

50

50

51

53

53

54

56

56

56

57

57

Contents

Complex Arithmetic 58

More on Matrices 59

Solving Linear Systems 60

Calculating Eigenvalues and Eigenvectors
Doing Calculus with MATLAB 61

Differentiation 61

Integration 62

Limits 63

Sums and Products 64

Taylor Series 65

Default Variables 65

5 MATLAB Graphics 67

Two-Dimensional Plots 67

Parametric Plots 67

Contour Plots and Implicit Plots

Field Plots 71

Three-Dimensional Plots 72

Curves in Three-Dimensional Space
Surfaces in Three-Dimensional Space
Special Effects 75

Combining Figures in One Window 76

Animations 77

* Customizing and Manipulating Graphics

Change of Viewpoint 80

Change of Plot Style 80

Full-Fledged Customization 82

Quick Plot Editing in the Figure Window 84

Sound 85

Practice Set B: Calculus, Graphics, and Linear Algebra

6 M-Books 91

Enabling M-Books 92

Starting M-Books 93

Working with M-Books 95

Editing Input 95

The Notebook Menu 96

Contents

M-Book Graphics 97

More Hints for Effective Use of M-Books 98

A Warning 99

7 MATLAB Programming 101

Branching 101

Branching with if 102

Logical Expressions 104

Branching with switch 108

More about Loops 109

Open-Ended Loops 110

Breaking from a Loop 111

Other Progra mmin g Commands 112

Subfunctions 112

Commands for Parsing Input and Output 112

User Input and Screen Output 114

Evaluation 116

Debugging 117

* Interacting with the Operating System 118

Calling External Programs 118

File Input and Output 119

8 *SIMULINK and GUIs 121

SIMULINK 121

Graphical User Interfaces (GUIs) 127

GUI Layout and GUIDE 127

Saving and Running a GUI 130

GUI Callback Functions 132

9 Applications 136

Illuminating a Room 137

One 300-Watt Bulb 137

Two 150-Watt Bulbs 138

Three 100-Watt Bulbs 143

Mortgage Payments 145

Monte Carlo Simulation 149

Population Dynamics 156

Exponential Growth and Decay 157

Contents

xi

Logistic Growth 159

Rerunning the Model with SIMULINK 166

Linear Economic Models 168

Linear Programming 173

The 360° Pendulum 180

★ Numerical Solution of the Heat Equation

A Finite Difference Solution 185

The Case of Variable Conductivity 189

A SIMULINK Solution 191

Solution with pdepe 194

★ A Model of Traffic Flow 196

Practice Set C: Developing Your MATLAB Skills 204

10 MATLAB and the Internet 214

MATLAB Help on the Internet 214

Posting MATLAB Programs and Output 215

M-Files, M-Books, Reports, and HTML Files 215

Configuring Your Web Browser 216

Microsoft Internet Explorer 216

Netscape Navigator 216

1 1 Troubleshooting 218

Common Problems 218

Wrong or Unexpected Output 218

Syntax Error 220

Spelling Error 223

Error Messages When Plotting 223

A Previously Saved M-File Evaluates Differently
Computer Won’t Respond 226

The Most Common Mistakes 226

Debugging Techniques 227

Solutions to the Practice Sets 235

Practice Set A 235

Practice Set B 246

Practice Set C 266

Contents

Glossary

299

MATLAB Operators

300

Built-in Constants

301

Built-in Functions

302

MATLAB Commands

303

Graphics Commands

309

MATLAB Programming

313

Index

317

indicates an advanced chapter or section that can be skipped on a first reading.

Preface

MATLAB is an integrated technical computing environment that combines
numeric computation, advanced graphics and visualization, and a high-
level programming language.

- www . mathworks . com/products/matlab

That statement encapsulates the view of The MathWorks, Inc . , the developer of
MATLAB®. MATLAB 6 is an ambitious program. It contains hundreds of com-
mands to do mathematics. You can use it to graph functions, solve equations,
perform statistical tests, and do much more. It is a high-level programming
language that can communicate with its cousins, e.g., FORTRAN and C. You
can produce sound and animate graphics. You can do simulations and mod-
eling (especially if you have access not just to basic MATLAB but also to its
accessory SIMULINK®). You can prepare materials for export to the World
Wide Web. In addition, you can use MATLAB, in conjunction with the word
processing and desktop publishing features of Microsoft Word®, to combine
mathematical computations with text and graphics to produce a polished, in-
tegrated, and interactive document.

A program this sophisticated contains many features and options. There
are literally hundreds of useful commands at your disposal. The MATLAB
help documentation contains thousands of entries. The standard references,
whether the MathWorks User’s Guide for the product, or any of our com-
petitors, contain myriad tables describing an endless stream of commands,
options, and features that the user might be expected to learn or access.

MATLAB is more than a fancy calculator; it is an extremely useful and
versatile tool. Even if you only know a little about MATLAB, you can use it
to accomplish wonderful things. The hard part, however, is figuring out which
of the hundreds of commands, scores of help pages, and thousands of items of
documentation you need to look at to start using it quickly and effectively.

That’s where we come in.

Preface

xiv

Why We Wrote This Book

The goal of this book is to get you started using MATLAB successfully and
quickly. We point out the parts of MATLAB you need to know without over-
whelming you with details. We help you avoid the rough spots. We give you
examples of real uses of MATLAB that you can refer to when you’re doing
your own work. And we provide a handy reference to the most useful features
of MATLAB. When you’re finished reading this book, you will be able to use
MATLAB effectively. You’ll also be ready to explore more of MATLAB on your
own.

You might not be a MATLAB expert when you finish this book, but you
will be prepared to become one — if that’s what you want. We figure you’re
probably more interested in being an expert at your own specialty, whether
that’s finance, physics, psychology, or engineering. You want to use MATLAB
the way we do, as a tool. This book is designed to help you become a proficient
MATLAB user as quickly as possible, so you can get on with the business at
hand.

Who Should Read This Book

This book will be useful to complete novices, occasional users who want to
sharpen their skills, intermediate or experienced users who want to learn
about the new features of MATLAB 6 or who want to learn how to use
SIMULINK, and even experts who want to find out whether we know any-
thing they don’t.

You can read through this guide to learn MATLAB on your own. If your
employer (or your professor) has plopped you in front of a computer with
MATLAB and told you to learn how to use it, then you’ll find the book par-
ticularly useful. If you are teaching or taking a course in which you want to
use MATLAB as a tool to explore another subject — whether in mathematics,
science, engineering, business, or statistics — this book will make a perfect
supplement.

As mentioned, we wrote this guide for use with MATLAB 6. If you plan
to continue using MATLAB 5, however, you can still profit from this book.
Virtually all of the material on MATLAB commands in this book applies to
both versions. Only a small amount of material on the MATLAB interface,
found mainly in Chapters 1, 3, and 8, is exclusive to MATLAB 6.

Preface

xv

How This Book Is Organized

In writing, we drew on our experience to provide important information as
quickly as possible. The book contains a short, focused introduction to
MATLAB. It contains practice problems (with complete solutions) so you can
test your knowledge. There are several illuminating sample projects that show
you how MATLAB can be used in real-world applications, and there is an en-
tire chapter on troubleshooting.

The core of this book consists of about 75 pages: Chapters 1-4 and the begin-
ning of Chapter 5. Read that much and you’ll have a good grasp of the funda-
mentals of MATLAB. Read the rest — the remainder of the Graphics chapter
as well as the chapters on M-Books, Programming, SIMULINK and GUIs, Ap-
plications, MATLAB and the Internet, Troubleshooting, and the Glossary —
and you’ll know enough to do a great deal with MATLAB.

Here is a detailed summary of the contents of the book.

Chapter 1, Getting Started, describes how to start MATLAB on different
platforms. It tells you how to enter commands, how to access online help, how
to recognize the various MATLAB windows you will encounter, and how to
exit the application.

Chapter 2, MATLAB Basics, shows you how to do elementary mathe-
matics using MATLAB. This chapter contains the most essential MATLAB
commands.

Chapter 3, Interacting with MATLAB, contains an introduction to the
MATLAB Desktop interface. This chapter will introduce you to the basic
window features of the application, to the small program files (M-files) that you
will use to make most effective use of the software, and to a simple method
(diary files) of documenting your MATLAB sessions. After completing this
chapter, you’ll have a better appreciation of the breadth described in the quote
that opens this preface.

Practice Set A, Algebra and Arithmetic, contains some simple problems for
practicing your newly acquired MATLAB skills. Solutions are presented at
the end of the book.

Chapter 4, Beyond the Basics, contains an explanation of the finer points
that are essential for using MATLAB effectively.

Chapter 5, MATLAB Graphics, contains a more detailed look at many of
the MATLAB commands for producing graphics.

Practice Set B, Calculus, Graphics, and Linear Algebra, gives you another
chance to practice what you’ve just learned. As before, solutions are provided
at the end of the book.

Preface

xvi

Chapter 6, M-Books, contains an introduction to the word processing and
desktop publishing features available when you combine MATLAB with
Microsoft Word.

Chapter 7, MATLAB Programming, introduces you to the progr ammin g
features of MATLAB. This chapter is designed to be useful both to the novice
programmer and to the experienced FORTRAN or C programmer.

Chapter 8, SIMULINK and GUIs, consists of two parts. The first part de-
scribes the MATLAB companion software SIMULINK, a graphically oriented
package for modeling, simulating, and analyzing dynamical systems. Many
of the calculations that can be done with MATLAB can be done equally well
with SIMULINK. If you don’t have access to SIMULINK, skip this part of
Chapter 8. The second part contains an introduction to the construction and
deployment of graphical user interfaces, that is, GUIs, using MATLAB.

Chapter 9, Applications, contains examples, from many different fields, of
solutions of real-world problems using MATLAB and/or SIMULINK.

Practice Set C, Developing Your MATLAB Skills, contains practice problems
whose solutions use the methods and techniques you learned in Chapters 6-9.

Chapter 10, MATLAB and the Internet, gives tips on how to post MATLAB
output on the Web.

Chapter 11, Troubleshooting, is the place to turn when anything goes wrong.
Many common problems can be resolved by reading (and rereading) the advice
in this chapter.

Next, we have Solutions to the Practice Sets, which contains solutions to
all the problems from the three practice sets. The Glossary contains short de-
scriptions (with examples) of many MATLAB commands and objects. Though
not a complete reference, it is a handy guide to the most important features
of MATLAB. Finally, there is a complete Index.

Conventions Used in This Book

We use distinct fonts to distinguish various entities. When new terms are
first introduced, they are typeset in an italic font. Output from MATLAB
is typeset in a monospaced typewriter font; commands that you type for
interpretation by MATLAB are indicated by a boldface version of that font.
These commands and responses are often displayed on separate lines as they
would be in a MATLAB session, as in the following example:

>> x = sqrt(2*pi + 1)

x =

2.697

Preface

xvii

Selectable menu items (from the menu bars in the MATLAB Desktop, figure
windows, etc.) are typeset in a boldface font. Submenu items are separated
from menu items by a colon, as in File : Open..., Labels such as the names of
windows and buttons are quoted, in a “regular” font. File and folder names,
as well as Web addresses, are printed in a typewriter font. Finally, names
of keys on your computer keyboard are set in a small caps font.

We use four special symbols throughout the book. Here they are together
with their meanings.

Paragraphs like this one contain cross-references to other parts of the book or
suggestions of where you can skip ahead to another chapter.

^ Paragraphs like this one contain important notes. Our favorite is
“Save your work frequently.” Pay careful attention to these
paragraphs.

/ Paragraphs like this one contain useful tips or point out features of interest
in the surrounding landscape. You might not need to think carefully about
them on the first reading, but they may draw your attention to some of the
finer points of MATLAB if you go back to them later.

like this discuss features of MATLAB’s Symbolic Math
Toolbox, used for symbolic (as opposed to numerical ) calculations. If you are
not using the Symbolic Math Toolbox, you can skip these sections.

Incidentally, if you are a student and you have purchased the MATLAB
Student Version, then the Symbolic Math Toolbox and SIMULINK are auto-
matically included with your software, along with basic MATLAB. Caution:
The Student Edition of MATLAB, a different product, does not come with
SIMULINK.

About the Authors

We are mathematics professors at the University of Maryland, College Park.
We have used MATLAB in our research, in our mathematics courses, for pre-
sentations and demonstrations, for production of graphics for books and for
the Web, and even to help our kids do their homework. We hope that you’ll
find MATLAB as useful as we do and that this book will help you learn to
use it quickly and effectively. Finally, we would like to thank our editor, Alan
Harvey, for his personal attention and helpful suggestions.

Chapter 1

Getting Started

In this chapter, we will introduce you to the tools you need to begin using
MATLAB effectively. These include: some relevant information on computer
platforms and software versions; installation and location protocols; how to
launch the program, enter commands, use online help, and recover from hang-
ups; a roster of MATLAB’s various windows; and finally, how to quit the soft-
ware. We know you are anxious to get started using MATLAB, so we will keep
this chapter brief. After you complete it, you can go immediately to Chapter 2
to find concrete and simple instructions for the use of MATLAB. We describe
the MATLAB interface more elaborately in Chapter 3.

Platforms and Versions

It is likely that you will run MATLAB on a PC (running Windows or Linux)
or on some form of UNIX operating system. (The developers of MATLAB,
The MathWorks, Inc., are no longer supporting Macintosh. Earlier versions of
MATLAB were available for Macintosh; if you are running one of those, you
should find that our instructions for Windows platforms will suffice for your
needs.) Unlike previous versions of MATLAB, version 6 looks virtually identi-
cal on Windows and UNIX platforms. For definitiveness, we shall assume the
reader is using a PC in a Windows environment. In those very few instances
where our instructions must be tailored differently for Linux or UNIX users,
we shall point it out clearly.

^ We use the word Windows to refer to all flavors of the Windows
operating system, that is, Windows 95, Windows 98, Windows 2000,
Windows Millennium Edition, and Windows NT.

1

2

Chapter 1 : Getting Started

This book is written to be compatible with the current version of MATLAB,
namely version 6 (also known as Release 12). The vast majority of the MATLAB
commands we describe, as well as many features of the MATLAB interface
(M-files, diary files, M-books, etc.), are valid for version 5.3 (Release 11), and
even earlier versions in some cases. We also note that the differences between
the Professional Version and the Student Version (not the Student Edition)
of MATLAB are rather minor and virtually unnoticeable to the new, or even
mid-level, user. Again, in the few instances where we describe a MATLAB
feature that is only available in the Professional Version, we highlight that
fact clearly.

Installation and Location

If you intend to run MATLAB on a PC, especially the Student Version, it is
quite possible that you will have to install it yourself. You can easily accomplish
this using the product CDs. Follow the installation instructions as you would
with any new software you install. At some point in the installation you may
be asked which toolboxes you wish to include in your installation. Unless you
have severe space limitations, we suggest that you install any that seem of
interest to you or that you think you might use at some point in the future. We
ask only that you be sure to include the Symbolic Math Toolbox among those
you install. If possible, we also encourage you to install SIMULINK, which is
described in Chapter 8.

Depending on your version you may also be asked whether you want to
specify certain directory (i.e., folder) locations associated with Microsoft Word.
If you do, you will be able to run the M-book interface that is described in
Chapter 6. If your computer has Microsoft Word, we strongly urge you to
include these directory locations during installation.

If you allow the default settings during installation, then MATLAB will
likely be found in a directory with a name such as mat labR12 or mat lab_SR12
or MATLAB — you may have to hunt around to find it. The subdirectory
bin\win32, or perhaps the subdirectory bin, will contain the executable file
matlab . exe that runs the program, while the current working directory will
probably be set to matlabR12\work.

Starting MATLAB

You start MATLAB as you would any other software application. On a PC you
access it via the Start menu, in Programs under a folder such as MatlabR12

Typing in the Command Window

3

or Student MATLAB. Alternatively, you may have an icon set up that enables
you to start MATLAB with a simple double-click. On a UNIX machine, gen-
erally you need only type mat lab in a terminal window, though you may first
have to find the matlab/bin directory and add it to your path. Or you may
have an icon or a special button on your desktop that achieves the task.

^ On UNIX systems, you should not attempt to run MATLAB in the
background by typing matlab &. This will fail in either the current
or older versions.

However you start MATLAB, you will briefly see a window that displays
the MATLAB logo as well as some MATLAB product information, and then a
MATLAB Desktop window will launch. That window will contain a title bar, a
menu bar, a tool bar, and five embedded windows, two of which are hidden. The
largest and most important window is the Command Window on the right. We
will go into more detail in Chapter 3 on the use and manipulation of the other
four windows: the Launch Pad, the Workspace browser, the Command History
window, and the Current Directory browser. For now we concentrate on the
Command Window to get you started issuing MATLAB commands as quickly
as possible. At the top of the Command Window, you may see some general
information about MATLAB, perhaps some special instructions for getting
started or accessing help, but most important of all, a line that contains a
prompt. The prompt will likely be a double caret (>> or ;>>). If the Command
Window is “active”, its title bar will be dark, and the prompt will be followed by
a cursor (a vertical line or box, usually blinking). That is the place where you
will enter your MATLAB commands (see Chapter 2). If the Command Window
is not active, just click in it anywhere. Figure 1-1 contains an example of a
newly launched MATLAB Desktop.

^ In older versions of MATLAB, for example 5.3, there is no integrated
Desktop. Only the Command Window appears when you launch the
application. (On UNIX systems, the terminal window from which
you invoke MATLAB becomes the Command Window.) Commands
that we instruct you to enter in the Command Window inside the
Desktop for version 6 can be entered directly into the Command
Window in version 5.3 and older versions.

Typing in the Command Window

Click in the Command Window to make it active. When a window becomes
active, its titlebar darkens. It is also likely that your cursor will change from

4

Chapter 1 : Getting Started

outline form to solid, or from light to dark, or it may simply appear. Now you
can begin entering commands. Try typing 1+1; then press ENTER or RETURN.
Next try factor (1234567 89), and finally sin (10) . Your MATLAB Desktop
should look like Figure 1-2.

Online Help

MATLAB has an extensive online help mechanism. In fact, using only this
book and the online help, you should be able to become quite proficient with
MATLAB.

You can access the online help in one of several ways. Typing help at the
command prompt will reveal a long list of topics on which help is available. Just
to illustrate, try typing help general. Now you see a long list of “general
purpose” MATLAB commands. Finally, try help solve to learn about the
command solve. In every instance above, more information than your screen
can hold will scroll by. See the Online Help section in Chapter 2 for instructions
to deal with this.

There is a much more user-friendly way to access the online help, namely via
the MATLAB Help Browser. You can activate it in several ways; for example,
typing either helpwin or helpdesk at the command prompt brings it up.

Interrupting Calculations

5

Figure 1-2: Some Simple Commands.

Alternatively, it is available through the menu bar under either View or Help.
Finally, the question mark button on the tool bar will also invoke the Help
Browser. We will go into more detail on its features in Chapter 2 — suffice it
to say that as in any hypertext browser, you can, by clicking, browse through a
host of command and interface information. Figure 1-3 depicts the MATLAB
Help Browser.

^ If you are working with MATLAB version 5.3 or earlier, then typing
help, help general, or help solve at the command prompt will
work as indicated above. But the entries helpwin or helpdesk call
up more primitive, although still quite useful, forms of help
windows than the robust Help Browser available with version 6.

If you are patient, and not overly anxious to get to Chapter 2, you can type
demo to try out MATLAB’s demonstration program for beginners.

Interrupting Calculations

If MATLAB is hung up in a calculation, or is just taking too long to perform
an operation, you can usually abort it by typing CTRL+C (that is, hold down the
key labeled CTRL, or control, and press c).

6

Chapter 1 : Getting Started

Figure 1-3: The MATLAB Help Browser.

MATLAB Windows

We have already described the MATLAB Command Window and the Help
Browser, and have mentioned in passing the Command History window, Cur-
rent Directory browser, Workspace browser, and Launch Pad. These, and seve-
ral other windows you will encounter as you work with MATLAB, will allow
you to: control files and folders that you and MATLAB will need to access; write
and edit the small MATLAB programs (that is, M-files) that you will utilize to
run MATLAB most effectively; keep track of the variables and functions that
you define as you use MATLAB; and design graphical models to solve prob-
lems and simulate processes. Some of these windows launch separately, and
some are embedded in the Desktop. You can dock some of those that launch
separately inside the Desktop (through the View:Dock menu button), or you
can separate windows inside your MATLAB Desktop out to your computer
desktop by clicking on the curved arrow in the upper right.

These features are described more thoroughly in Chapter 3. For now, we
want to call your attention to the other main type of window you will en-
counter; namely graphics windows. Many of the commands you issue will
generate graphics or pictures. These will appear in a separate window. MAT-
LAB documentation refers to these as figure windows. In this book, we shall

Ending a Session

7

also call them graphics windows. In Chapter 5, we will teach you how to gen-
erate and manipulate MATLAB graphics windows most effectively.

See Figure 2-1 in Chapter 2 for a simple example of a graphics window.

^ Graphics windows cannot be embedded into the MATLAB Desktop.

Ending a Session

The simplest way to conclude a MATLAB session is to type qui t at the prompt.
You can also click on the special symbol that closes your windows (usually an x
in the upper left- or right-hand corner). Either of these may or may not close all
the other MATLAB windows (which we talked about in the last section) that
are open. You may have to close them separately. Indeed, it is our experience
that leaving MATLAB-generated windows around after closing the MATLAB
Desktop may be hazardous to your operating system. Still another way to exit
is to use the Exit MATLAB option from the File menu of the Desktop. Before
you exit MATLAB, you should be sure to save any variables, print any graphics
or other files you need, and in general clean up after yourself. Some strategies
for doing so are addressed in Chapter 3.

Chapter 2

MATLAB Basics

In this chapter, you will start learning how to use MATLAB to do mathematics.
You should read this chapter at your computer, with MATLAB running. Try
the commands in a MATLAB Command Window as you go along. Feel free to
experiment with variants of the examples we present; the best way to find out
how MATLAB responds to a command is to try it.

For further practice, you can work the problems in Practice Set A The
Glossary contains a synopsis of many MATLAB operators, constants,
functions, commands, and programming instructions.

Input and Output

You input commands to MATLAB in the MATLAB Command Window. MAT-
LAB returns output in two ways: Typically, text or numerical output is re-
turned in the same Command Window, but graphical output appears in a
separate graphics window. A sample screen, with both a MATLAB Desktop
and a graphics window, labeled Figure No. 1, is shown in Figure 2-1.

To generate this screen on your computer, first type 1/2 + 1/3. Then type
ezplot ( ' x~3 - x').

/ While MATLAB is working, it may display a “wait” symbol — for example,
an hourglass appears on many operating systems. Or it may give no visual
evidence until it is finished with its calculation.

Arithmetic

As we have just seen, you can use MATLAB to do arithmetic as you would a
calculator. You can use “+” to add, to subtract, “*” to multiply, “/” to divide,

Arithmetic

9

ans =

22.5000

MATLAB prints the answer and assigns the value to a variable called ans.
If you want to perform further calculations with the answer, you can use the
variable ans rather than retype the answer. For example, you can compute
the sum of the square and the square root of the previous answer as follows:
>> ans~2 + sqrt(ans)

ans =

510.9934

Observe that MATLAB assigns a new value to ans with each calculation.
To do more complex calculations, you can assign computed values to variables
of your choosing. For example,

>> u = cos (10)

u =

- 0.8391

10

Chapter 2: MATLAB Basics

>> v = sin(10)

- 0.5440

>> u~2 + v~2

ans =

MATLAB uses double-precision floating point arithmetic, which is accurate
to approximately 15 digits; however, MATLAB displays only 5 digits by default.
To display more digits, type format long. Then all subsequent numerical
output will have 15 digits displayed. Type format short to return to 5-digit
display.

MATLAB differs from a calculator in that it can do exact arithmetic. For
example, it can add the fractions 1/2 and 1/3 symbolically to obtain the correct
fraction 5/6. We discuss how to do this in the section Symbolic Expressions,
Variable Precision, and Exact Arithmetic on the next page.

Algebra

<-• 1/ Using MATLAB ’s Symbolic Math Toolbox, you can carry out algebraic

or symbolic calculations such as factoring polynomials or solving algebraic
equations. Type help symbolic to make sure that the Symbolic Math Tool-
box is installed on your system.

To perform symbolic computations, you must use syms to declare the vari-
ables you plan to use to be symbolic variables. Consider the following series
of commands:

>> syms x y

>> (x - y) * (x - y) ~2

(x-y) a 3

>> expand (ans)

Algebra

11

ans =

x A 3-3*x A 2*y+3*x*y A 2-y A 3

>> factor (ans)

ans =

(x-y) a 3

/ Notice that symbolic output is left-justified, while numeric output is
indented. This feature is often useful in distinguishing symbolic output
from numerical output.

Although MATLAB makes minor simplifications to the expressions you
type, it does not make major changes unless you tell it to. The command ex-
pand told MATLAB to multiply out the expression, and factor forced MAT-
LAB to restore it to factored form.

MATLAB has a command called simplify, which you can sometimes use
to express a formula as simply as possible. For example,

» simplify ( (x A 3 - y A 3)/(x - y) )

ans =

x A 2+x*y+y A 2

/ MATLAB has a more robust command, called simple, that sometimes does
a better job than simplify. Try both commands on the trigonometric
expression sin (x) *cos (y) + cos (x) *sin (y) to compare — you’ll have
to read the online help for simple to completely understand the answer.

Symbolic Expressions, Variable Precision, and Exact Arithmetic

As we have noted, MATLAB uses floating point arithmetic for its calculations.
Using the Symbolic Math Toolbox, you can also do exact arithmetic with sym-
bolic expressions. Consider the following example:

>> cos (pi/2)

ans =

6 . 1232e- 17

The answer is written in floating point format and means 6.1232 x 10 -17 .
However, we know that cos(7r/2) is really equal to 0. The inaccuracy is due
to the fact that typing pi in MATLAB gives an approximation to n accurate

Chapter 2: MATLAB Basics

to about 15 digits, not its exact value. To compute an exact answer, instead
of an approximate answer, we must create an exact symbolic representation
of tt/ 2 by typing sym('pi/2'). Now let’s take the cosine of the symbolic
representation of jt/2:

>> cos (sym(' pi/2' ) )

ans =

0

This is the expected answer.

The quotes around pi/2 in sym ( ' pi/2 ' ) create a string consisting of the
characters pi/2 and prevent MATLAB from evaluating pi/2 as a floating
point number. The command sym converts the string to a symbolic expression.

The commands sym and syms are closely related. In fact, syms x is equiv-
alent to x = sym ( ' x ' ) . The command syms has a lasting effect on its argu-
ment (it declares it to be symbolic from now on), while sym has only a tempo-
rary effect unless you assign the output to a variable, as in x = sym ( ' x' ) .
Here is how to add 1 /2 and 1 /3 symbolically:

>> sym ( ' 1/2 ' ) + sym ( ' 1/3 ' )

ans =

5/6

Finally, you can also do variable-precision arithmetic with vpa. For example,
to print 50 digits of y/2, type

>> vpa ( ' sqrt (2) ' , 50)

ans =

1.4142135623730950488016887242096980785696718753769

^ You should be wary of using sym or vpa on an expression that
MATLAB must evaluate before applying variable-precision
arithmetic. To illustrate, enter the expressions 3~45 , vpa ( 3 ~ 4 5 ) ,
and vpa ( ' 3 ~ 4 5 ' ) . The first gives a floating point approximation to
the answer, the second — because MATLAB only carries 16-digit
precision in its floating point evaluation of the exponentiation —
gives an answer that is correct only in its first 16 digits, and the
third gives the exact answer.

See the section Symbolic and Floating Point Numbers in Chapter 4 for details
about how MATLAB converts between symbolic and floating point numbers.

Managing Variables

13

Managing Variables

We have now encountered three different classes of MATLAB data: floating
point numbers, strings, and symbolic expressions. In a long MATLAB session
it may be hard to remember the names and classes of all the variables you
have defined. You can type whos to see a summary of the names and types of
your currently defined variables. Here’s the output of whos for the MATLAB
session displayed in this chapter:

>> whos

Name

ans

Size

lxl

lxl

lxl

lxl

lxl

Bytes

226

126

126

Class

sym object
double array
double array
sym object
sym object

Grand total is 58 elements using 494 bytes

We see that there are currently five assigned variables in our MATLAB
session. Three are of class “sym object”; that is, they are symbolic objects. The
variables x and y are symbolic because we declared them to be so using syms,
and ans is symbolic because it is the output of the last command we executed,
which involved a symbolic expression. The other two variables, u and v, are
of class “double array”. That means that they are arrays of double-precision
numbers; in this case the arrays are of size lxl (that is, scalars). The “Bytes”
column shows how much computer memory is allocated to each variable.

Try assigning u = pi, v = 'pi', and w = sym( 'pi' ), and then type
whos to see how the different data types are described.

The command whos shows information about all defined variables, but it
does not show the values of the variables. To see the value of a variable, simply
type the name of the variable and press enter or return.

MATLAB commands expect particular classes of data as input, and it is
important to know what class of data is expected by a given command; the help
text for a command usually indicates the class or classes of input it expects. The
wrong class of input usually produces an error message or unexpected output.
For example, type sin ( 'pi' ) to see how unexpected output can result from
supplying a string to a function that isn’t designed to accept strings.

To clear all defined variables, type clear or clear all. You can also type,
for example, clear x y to clear only x and y.

You should generally clear variables before starting a new calculation.
Otherwise values from a previous calculation can creep into the new

14

Chapter 2: MATLAB Basics

Figure 2-2: Desktop with the Workspace Browser.

calculation by accident. Finally, we observe that the Workspace browser pre-
sents a graphical alternative to whos. You can activate it by clicking on the
Workspace tab, by typing workspace at the command prompt, or through
the View item on the menu bar. Figure 2-2 depicts a Desktop in which the
Command Window and the Workspace browser contain the same information
as displayed above.

Errors in Input

If you make an error in an input line, MATLAB will beep and print an error
message. For example, here’s what happens when you try to evaluate 3u~2:

» 3u~2

??? 3u a 2

I

Error: Missing operator, comma, or semicolon.

The error is a missing multiplication operator *. The correct input would be
3*u A 2. Note that MATLAB places a marker (a vertical line segment) at the
place where it thinks the error might be; however, the actual error may have
occurred earlier or later in the expression.

Online Help

15

^ Missing multiplication operators and parentheses are among the
most common errors.

You can edit an input line by using the UP-ARROW key to redisplay the pre-
vious command, editing the command using the LEFT- and RIGHT-ARROW keys,
and then pressing return or enter. The UP- and down-arrow keys allow you
to scroll back and forth through all the commands you’ve typed in a MATLAB
session, and are very useful when you want to correct, modify, or reenter a
previous command.

Online Help

There are several ways to get online help in MATLAB. To get help on a particu-
lar command, enter help followed by the name of the command. For example,
help solve will display documentation for solve. Unless you have a large
monitor, the output of help solve will not fit in your MATLAB command
window, and the beginning of the documentation will scroll quickly past the
top of the screen. You can force MATLAB to display information one screen-
ful at a time by typing more on. You press the space bar to display the next
screenful, or ENTER to display the next line; type help more for details. Typing
more on affects all subsequent commands, until you type more off.

The command lookfor searches the first line of every MATLAB help file
for a specified string (use lookfor -all to search all lines). For example,
if you wanted to see a list of all MATLAB commands that contain the word
“factor” as part of the command name or brief description, then you would
type lookfor factor. If the command you are looking for appears in the
list, then you can use help on that command to learn more about it.

The most robust online help in MATLAB 6 is provided through the vastly
improved Help Browser. The Help Browser can be invoked in several ways: by
typing helpdesk at the command prompt, under the View item in the menu
bar, or through the question mark button on the tool bar. Upon its launch you
will see a window with two panes: the first, called the Help Navigator, used
to find documentation; and the second, called the display pane, for viewing
documentation. The display pane works much like a normal web browser. It
has an address window, buttons for moving forward and backward (among the
windows you have visited), live links for moving around in the documentation,
the capability of storing favorite sites, and other such tools.

You use the Help Navigator to locate the documentation that you will ex-
plore in the display pane. The Help Navigator has four tabs that allow you to

16

Chapter 2: MATLAB Basics

arrange your search for documentation in different ways. The first is the Con-
tents tab that displays a tree view of all the documentation topics available.
The extent of that tree will be determined by how much you (or your system
administrator) included in the original MATLAB installation (how many tool-
boxes, etc.). The second tab is an Index that displays all the documentation
available in index format. It responds to your key entry of likely items you
want to investigate in the usual alphabetic reaction mode. The third tab pro-
vides the Search mechanism. You type in what you seek, either a function
or some other descriptive term, and the search engine locates corresponding
documentation that pertains to your entry. Finally, the fourth tab is a roster
of your Favorites. Clicking on an item that appears in any of these tabs brings
up the corresponding documentation in the display pane.

The Help Browser has an excellent tutorial describing its own operation.
To view it, open the Browser; if the display pane is not displaying the “Begin
Here” page, then click on it in the Contents tab; scroll down to the “Using
the Help Browser” link and click on it. The Help Browser is a powerful and
easy-to-use aid in finding the information you need on various components of
MATLAB. Like any such tool, the more you use it, the more adept you become
at its use.

/ If you type helpwin to launch the Help Browser, the display pane will
contain the same roster that you see as the result of typing help at the
command prompt, but the entries will be links.

Variables and Assignments

In MATLAB, you use the equal sign to assign values to a variable. For instance,
>> x = 7

x =

7

will give the variable x the value 7 from now on. Henceforth, whenever MAT-
LAB sees the letter x, it will substitute the value 7. For example, if y has been
defined as a symbolic variable, then

>> x~2 - 2*x*y + y

ans =
49-13*

y

Solving Equations

17

^ To clear the value of the variable x, type clear x.

You can make very general assignments for symbolic variables and then
manipulate them. For example,

>> clear x; syms x y
>> z = x~2 - 2*x*y + y

z =

x A 2-2*x*y+y

ans =

5*y* (x A 2 -2*x*y+y)

A variable name or function name can be any string of letters, digits, and
underscores, provided it begins with a letter (punctuation marks are not al-
lowed). MATLAB distinguishes between uppercase and lowercase letters. You
should choose distinctive names that are easy for you to remember, generally
using lowercase letters. For example, you might use cubicsol as the name
of the solution of a cubic equation.

^ A common source of puzzling errors is the inadvertent reuse of
previously defined variables.

MATLAB never forgets your definitions unless instructed to do so. You can
check on the current value of a variable by simply typing its name.

Solving Equations

You can solve equations involving variables with solve or f zero. For exam-
ple, to find the solutions of the quadratic equation x 2 — 2x — 4 = 0, type

>> solve('x~2 - 2*x - 4 = O')

ans =

[ 5 * ( 1 / 2 ) + 1 ]

[ 1-5 A (1/2)1

Note that the equation to be solved is specified as a string; that is, it is sur-
rounded by single quotes. The answer consists of the exact (symbolic) solutions

18

Chapter 2: MATLAB Basics

1 ± VE. To get numerical solutions, type double (ans) , or vpa (ans) to dis-
play more digits.

The command solve can solve higher-degree polynomial equations, as well
as many other types of equations. It can also solve equations involving more
than one variable. If there are fewer equations than variables, you should spec-
ify (as strings) which variable(s) to solve for. For example, type solve ( ' 2 *x -
log (y) = 1 ' / 'y' ) to solve 2x — logy = 1 for y in terms of x. You can
specify more than one equation as well. For example,

>> [x, y] = solve ( ' x~2 - y = 2', 'y - 2*x = 5')

[ 1 + 2*2'“' ( 1/2 ) ]
[ 1 - 2 * 2 a ( 1 / 2 ) ]

y =

[ 7 +4*2 A ( 1/2 ) ]

[ 7-4*2 a (1/2) ]

This system of equations has two solutions. MATLAB reports the solution by
giving the two x values and the two y values for those solutions. Thus the first
solution consists of the first value of x together with the first value of y. You
can extract these values by typing x ( 1 ) and y ( 1 ) :

» x (1)

ans =

1+2 *2 A (1/2)

» y(l)

ans =

7+4*2 A (1/2)

The second solution can be extracted with x ( 2 ) and y ( 2 ) .

Note that in the preceding solve command, we assigned the output to the
vector [x, y] . If you use solve on a system of equations without assigning
the output to a vector, then MATLAB does not automatically display the values
of the solution:

>> sol = solve('x~2 - y = 2' , 'y - 2*x = 5')

Solving Equations

19

sol =

x: [2x1 sym]

y: [2x1 sym]

To see the vectors of x and y values of the solution, type sol . x and sol . y. To
see the individual values, type sol .x (1) , sol .y (1) , etc.

Some equations cannot be solved symbolically, and in these cases solve
tries to find a numerical answer. For example,

>> solve (' sin (x) =2 - x' )

ans =

1.1060601577062719106167372970301

Sometimes there is more than one solution, and you may not get what you
expected. For example,

>> solve (' exp ( -x) = sin(x)')

ans =

-2 . 012775662 93151116333 60706 990 971
+2 . 7030745115909622139316148044265*i

The answer is a complex number; the i at the end of the answer stands for
the number \/-l. Though it is a valid solution of the equation, there are also
real number solutions. In fact, the graphs of exp(— x) and sin(x) are shown in
Figure 2-3; each intersection of the two curves represents a solution of the
equation e~ x = sin(x).

You can numerically find the solutions shown on the graph with fzero,
which looks for a zero of a given function near a specified value of x. A solution
of the equation e~ x = sin(x) is a zero of the function e~ x — sin(x), so to find the
solution near x = 0.5 type

>> fzero (inline (' exp ( -x) - sin(x)'), 0.5)

ans =

0.5885

Replace 0 . 5 with 3 to find the next solution, and so forth.

In the example above, the command inline, which we will discuss further in
the section User-Defined Functions below, converts its string argument to a

20

Chapter 2: MATLAB Basics

exp(-x) and sin(x)

Figure 2-3

function data class. This is the type of input f zero expects as its first
argument.

y In current versions of MATLAB, f zero also accepts a string expression with
independent variable x, so that we could have run the command above
without using inline, but this feature is no longer documented in the help
text for f zero and may be removed in future versions.

Vectors and Matrices

MATLAB was written originally to allow mathematicians, scientists, and
engineers to handle the mechanics of linear algebra — that is, vectors and
matrices — as effortlessly as possible. In this section we introduce these
concepts.

Vectors and Matrices

21

Vectors

A vector is an ordered list of numbers. You can enter a vector of any length in
MATLAB by typing a list of numbers, separated by commas or spaces, inside
square brackets. For example,

z =

2 4

6 8

» Y

[4 -3 5 -2 8 1]

Y =

4 -3 5 -2 8 1

Suppose you want to create a vector of values running from 1 to 9. Here’s
how to do it without typing each number:

» X = 1:9

X =

123456789

The notation 1 : 9 is used to represent a vector of numbers running from 1 to
9 in increments of 1. The increment can be specified as the second of three
arguments:

» X = 0:2:10

X =

0 2 4 6 8 10

You can also use fractional or negative increments, for example, 0 : 0 . 1 : 1 or
100 : - 1 : 0 .

The elements of the vector x can be extracted as x ( 1 ) , x ( 2 ) , etc. For ex-
ample,

» X (3)

ans

Chapter 2: MATLAB Basics

To change the vector X from a row vector to a column vector, put a prime ( ' )
after X:

ans =

0

2

4

6

8

10

You can perform mathematical operations on vectors. For example, to square
the elements of the vector x, type

>> x .~2

ans =

0 4 16 36 64 100

The period in this expression is very important; it says that the numbers
in x should be squared individually, or element-by -element. Typing x~2 would
tell MATLAB to use matrix multiplication to multiply X by itself and would
produce an error message in this case. (We discuss matrices below and in
Chapter 4.) Similarly, you must type . * or . / if you want to multiply or di-
vide vectors element-by-element. For example, to multiply the elements of the
vector x by the corresponding elements of the vector Y, type

>> x . *Y

ans =

0 -6 20 -12 64 10

Most MATLAB operations are, by default, performed element-by-element.
For example, you do not type a period for addition and subtraction, and you
can type exp (X) to get the exponential of each number in X (the matrix ex-
ponential function is expm). One of the strengths of MATLAB is its ability to
efficiently perform operations on vectors.

Vectors and Matrices

23

Matrices

A matrix is a rectangular array of numbers. Row and column vectors, which
we discussed above, are examples of matrices. Consider the 3x4 matrix

/I 2 3 4 \

A = 5 6 7 8 .

\9 10 11 12/

It can be entered in MATLAB with the command

» A = [1, 2, 3, 4; 5, 6, 7, 8; 9, 10, 11, 12]

A =

12 3 4

5 6 7 8

9 10 11 12

Note that the matrix elements in any row are separated by commas, and the
rows are separated by semicolons. The elements in a row can also be separated
by spaces.

If two matrices A and B are the same size, their (element-by-element) sum
is obtained by typing A + B. You can also add a scalar (a single number) to a
matrix; A + c adds c to each element in A. Likewise, A - B represents the
difference of A and B, and A - c subtracts the number c from each element
of A. If A and B are multiplicatively compatible (that is, if A is n x m and B is
m x £), then their product A*B is n x i. Recall that the element of A*B in the
tth row and yth column is the sum of the products of the elements from the
tth row of A times the elements from the j th column of B, that is,

(A * B )ij = ^ Aik&kj , 1 < i < n, 1 <j<£.
k=i

The product of a number c and the matrix A is given by c*A, and A' represents
the conjugate transpose of A. (For more information, see the online help for
ctranspose and transpose.)

A simple illustration is given by the matrix product of the 3x4 matrix A
above by the 4x1 column vector z ' :

» A*z '

ans =

60

140

220

24

Chapter 2: MATLAB Basics

The result is a 3 x 1 matrix, in other words, a column vector.

MATLAB has many commands for manipulating matrices. You can read
about them in the section More about Matrices in Chapter 4 and in the online
help; some of them are illustrated in the section Linear Economic Models in
Chapter 9.

Suppressing Output

Typing a semicolon at the end of an input line suppresses printing of the
output of the MATLAB command. The semicolon should generally be used
when defining large vectors or matrices (such as X = - 1 s 0 . 1 : 2 ; ). It can
also be used in any other situation where the MATLAB output need not be
displayed.

Functions

In MATLAB you will use both built-in functions as well as functions that you
create yourself.

Built-in Functions

MATLAB has many built-in functions. These include sqrt, cos, sin, tan,
log, exp, and atan (for arctan) as well as more specialized mathematical
functions such as gamma, erf, and bessel j . MATLAB also has several built-
in constants, including pi (the number n), i (the complex number i = V— T),
and inf (oo). Here are some examples:

>> log (exp (3))

ans =

3

The function log is the natural logarithm, called “In” in many texts. Now
consider

» sin (2*pi/3)

ans =

0.8660

Functions

25

To get an exact answer, you need to use a symbolic argument:
>> sin(sym('2*pi/3' ) )

ans =

1/2*3'"' (1/2)

User-Defined Functions

In this section we will show how to use inline to define your own functions.
Here’s how to define the polynomial function f(x ) = x 2 + x + 1:

>> f = inline('x~2 + x + 1' , 'x')

Inline function:
f (x) = x A 2 + x + 1

The first argument to inline is a string containing the expression defining
the function. The second argument is a string specifying the independent
variable.

^ The second argument to inline can be omitted, in which case MATLAB will
“guess” what it should be, using the rules about “Default Variables” to be
discussed later at the end of Chapter 4.

Once the function is defined, you can evaluate it:

» f (4)

ans =

21

MATLAB functions can operate on vectors as well as scalars. To make an
inline function that can act on vectors, we use MATLAB’s vectorize function.
Here is the vectorized version of f(x) = x 2 + x + 1:

>> fl = inline (vectorize ('x~2 + x + 1'), 'x')

fl =

Inline function:
fl (x) = x.^2 + x + 1

26

Chapter 2: MATLAB Basics

Note that A has been replaced by . A . Now you can evaluate f 1 on a vector:
» fl (1:5)

ans =

3 7 13 21 31

You can plot f 1, using MATLAB graphics, in several ways that we will explore
in the next section. We conclude this section by remarking that one can also
define functions of two or more variables:

>> g = inline ( 'u~2 + v~2', 'u', 'v')

Inline function:
g(u,v) = u^2+v^2

Graphics

In this section, we introduce MATLAB’s two basic plotting commands and
show how to use them.

Graphing with ezpiot

The simplest way to graph a function of one variable is with ezpiot, which
expects a string or a symbolic expression representing the function to be plot-
ted. For example, to graph x 2 + x + 1 on the interval -2 to 2 (using the string
form of ezpiot), type

>> ezpiot ( ' x~2 + x + 1 ' , [-2 2])

The plot will appear on the screen in a new window labeled “Figure No. 1”.

We mentioned that ezpiot accepts either a string argument or a symbolic
expression. Using a symbolic expression, you can produce the plot in Figure 2-4
with the following input:

>> syms x

>> ezpiot (x~2 + x + 1, [-2 2])

/ Graphs can be misleading if you do not pay attention to the axes. For
example, the input ezpiot (x~2 + x + 3, [-2 2]) produces a graph

Graphics

27

x 2 + x + 1

Figure 2-4

that looks identical to the previous one, except that the vertical axis has
different tick marks (and MATLAB assigns the graph a different title).

Modifying Graphs

You can modify a graph in a number of ways. You can change the title above
the graph in Figure 2-4 by typing (in the Command Window, not the figure
window)

>> title 'A Parabola'

You can add a label on the horizontal axis with xlabel or change the label
on the vertical axis with ylabel. Also, you can change the horizontal and
vertical ranges of the graph with axis. For example, to confine the vertical
range to the interval from 1 to 4, type
>> axis ( [-2 2 1 4] )

The first two numbers are the range of the horizontal axis; both ranges must

28

Chapter 2: MATLAB Basics

be included, even if only one is changed. We’ll examine more options for ma-
nipulating graphs in Chapter 5.

To close the graphics window select File : Close from its menu bar, type
close in the Command Window, or kill the window the way you would close
any other window on your computer screen.

Graphing with plot

The command plot works on vectors of numerical data. The basic syntax is
plot (x, Y) where x and Y are vectors of the same length. For example,

>> X = [12 3] ;

» Y = [4 6 5] ;

>> plot (X, Y)

The command plot (x, Y) considers the vectors x and Y to be lists of the x
and y coordinates of successive points on a graph and joins the points with
line segments. So, in Figure 2-5, MATLAB connects (1, 4) to (2, 6) to (3, 5).

Figure 2-5

Graphics

29

To plot x 2 + x + 1 on the interval from —2 to 2 we first make a list X of
x values, and then type plot (x, x. ~2 + x + 1) . We need to use enough
x values to ensure that the resulting graph drawn by “connecting the dots”
looks smooth. We’ll use an increment of 0.1. Thus a recipe for graphing the
parabola is

» X = -2:0. 1:2;

» plot (X, X.~2 + X + 1)

The result appears in Figure 2-6. Note that we used a semicolon to suppress
printing of the 41-element vector x. Note also that the command

» plot (x, fl(x) )

would produce the same results (fl is defined earlier in the section User-
Defined Functions).

Figure 2-6

30

Chapter 2: MATLAB Basics

*® = ’ We describe more of MATLAB’s graphics commands in Chapter 5.

For now, we content ourselves with demonstrating how to plot a pair of
expressions on the same graph.

Plotting Multiple Curves

Each time you execute a plotting command, MATLAB erases the old plot and
draws a new one. If you want to overlay two or more plots, type hold on.
This command instructs MATLAB to retain the old graphics and draw any
new graphics on top of the old. It remains in effect until you type hold off.
Here’s an example using ezplot:

>> ezplot (' exp ( -x) ' , [0 10])

>> hold on

>> ezplot (' sin (x) ' , [0 10])

» hold off

>> title 'exp(-x) and sin(x)'

The result is shown in Figure 2-3 earlier in this chapter. The commands hold
on and hold off work with all graphics commands.

With plot, you can plot multiple curves directly. For example,

>> X = 0:0.1:10;

>> plot (X, exp(-X), X, sin (X) )

Note that the vector of x coordinates must be specified once for each function
being plotted.

Chapter 3

Interacting with
MATLAB

In this chapter we describe an effective procedure for working with MATLAB,
and for preparing and presenting the results of a MATLAB session. In parti-
cular we will discuss some features of the MATLAB interface and the use of
script M-files, function M-files, and diary files. We also give some simple hints
for debugging your M-files.

The MATLAB Interface

MATLAB 6 has a new interface called the MATLAB Desktop. Embedded inside
it is the Command Window that we described in Chapter 2. If you are using
MATLAB 5, then you will only see the Command Window. In that case you
should skip the next subsection and proceed directly to the Menu and Tool
Bars subsection below.

The Desktop

By default, the MATLAB Desktop (Figure 1-1 in Chapter 1) contains five
windows inside it, the Command Window on the right, the Launch Pad and
the Workspace browser in the upper left, and the Command History window
and Current Directory browser in the lower left. Note that there are tabs for
alternating between the Launch Pad and the Workspace browser, or between
the Command History window and Current Directory browser. Which of the
five windows are currently visible can be adjusted with the View : Desktop
Layout menu at the top of the Desktop. (For example, with the Simple option,
you see only the Command History and Command Window, side-by-side.) The
sizes of the windows can be adjusted by dragging their edges with the mouse.

31

Chapter 3: Interacting with MATLAB

The Command Window is where you type the commands and instructions
that cause MATLAB to evaluate, compute, draw, and perform all the other
wonderful magic that we describe in this book. The Command History window
contains a running history of the commands that you type into the Command
Window. It is useful in two ways. First, it lets you see at a quick glance a
record of the commands that you have entered previously. Second, it can save
you some typing time. If you click on an entry in the Command History with the
right mouse button, it becomes highlighted and a menu of options appears.
You can, for example, select Copy, then click with the right mouse button
in the Command Window and select Paste, whereupon the command you
selected will appear at the command prompt and be ready for execution or
editing. There are many other options that you can learn by experimenting;
for instance, if you double-click on an entry in the Command History then it
will be executed immediately in the Command Window.

The Launch Pad window is basically a series of shortcuts that enable you to
access various features of the MATLAB software with a double-click. You can
use it to start SIMULINK, run demos of various toolboxes, use MATLAB web
tools, open the Help Browser, and more. We recommend that you experiment
with the entries in the Launch Pad to gain familiarity with its features.

The Workspace browser and Current Directory browser will be described
in separate subsections below.

Each of the five windows in the Desktop contains two small buttons in the
upper right corner. The x allows you to close the window, while the curved
arrow will “undock” the window from the Desktop (you can return it to the
Desktop by selecting Dock from the View menu of the undocked window).
You can also customize which windows appear inside the Desktop using its
View menu.

/ While the Desktop provides some new features and a common interface for
both the Windows and UNIX versions of MATLAB 6, it may also run more
slowly than the MATLAB 5 Command Window interface, especially on older
computers. You can run MATLAB 6 with the old interface by starting the
program with the command matlab /nodesktop on a Windows system or
matlab -nodesktop on a UNIX system. If you are a Windows user, you
probably start MATLAB by double-clicking on an icon. If so, you can create
an icon to start MATLAB without the Desktop feature as follows. First, click
the right mouse button on the MATLAB icon and select Create Shortcut. A
new, nearly identical icon will appear on your screen (possibly behind a
window — you may need to hunt for it). Next, click the right mouse button
on the new icon, and select Properties. In the panel that pops up, select the

The MATLAB Interface

33

Shortcut tab, and in the “Target” box, add to the end of the executable file
name a space followed by /nodesktop. (Notice that you can also change the
default working directory in the “Start in” box.) Click OK, and your new icon
is all set; you may want to rename it by clicking on it again with the right
mouse button, selecting Rename, and typing the new name.

Menu and Tool Bars

The MATLAB Desktop includes a menu bar and a tool bar ; the tool bar contains
buttons that give quick access to some of the items you can select through the
menu bar. On a Windows system, the MATLAB 5 Command Window has a
menu bar and tool bar that are similar, but not identical, to those of MATLAB
6. For example, its menus are arranged differently and its tool bar has buttons
that open the Workspace browser and Path Browser, described below. When
referring to menu and tool bar items below, we will describe the MATLAB 6
Desktop interface.

^ Many of the menu selections and tool bar buttons cause a new
window to appear on your screen. If you are using a UNIX system,
keep in mind the following caveats as you read the rest of this
chapter. First, some of the pop-up windows that we describe are
available on some UNIX systems but unavailable on others,
depending (for instance) on the operating system. Second, we will
often describe how to use both the command line and the menu and
tool bars to perform certain tasks, though only the command line is
available on some UNIX systems.

The Workspace

In Chapter 2, we introduced the commands clear and whos, which can be
used to keep track of the variables you have defined in your MATLAB session.
The complete collection of defined variables is referred to as the Workspace,
which you can view using the Workspace browser. You can make the browser
appear by typing workspace or, in the default layout of the MATLAB Desktop,
by clicking on the Workspace tab in the Launch Pad window (in a MATLAB
5 Command Window select File: Show Workspace instead). The Workspace
browser contains a list of the current variables and their sizes (but not their
values). If you double-click on a variable, its contents will appear in a new
window called the Array Editor, which you can use to edit individual entries
in a vector or matrix. (The command openvar also will open the Array Editor.)

34 Chapter 3: Interacting with MATLAB

You can remove a variable from the Workspace by selecting it in the Workspace
browser and choosing Edit:Delete.

If you need to interrupt a session and don’t want to be forced to recompute
everything later, then you can save the current Workspace with save. For
example, typing save myf ile saves the values of all currently defined vari-
ables in a file called myf ile .mat. To save only the values of the variables X
and Y, type

>> save myfile X Y

When you start a new session and want to recover the values of those variables,
use load. For example, typing load myfile restores the values of all the
variables stored in the file myfile .mat.

The Working Directory

New files you create from within MATLAB will be stored in your current
working directory. You may want to change this directory from its default
location, or you may want to maintain different working directories for dif-
ferent projects. To create a new working directory you must use the standard
procedure for creating a directory in your operating system. Then you can
make this directory your current working directory in MATLAB by using cd,
or by selecting this directory in the “Current Directory” box on the Desktop
tool bar.

For example, on a Windows computer, you could create a directory called
C : \Pro j ectA. Then in MATLAB you would type

>> cd C:\ProjectA

to make it your current working directory. You will then be able to read and
write files in this directory in your current MATLAB session.

If you only need to be able to read files from a certain directory, an alterna-
tive to making it your working directory is to add it to the path of directories
that MATLAB searches to find files. The current working directory and the
directories in your path are the only places MATLAB searches for files, unless
you explicitly type the directory name as part of the file name. To add the
directory C : \Pro j ectA to your path, type

>> addpath C:\ProjectA

When you add a directory to the path, the files it contains remain available for
the rest of your session regardless of whether you subsequently add another

The MATLAB Interface

35

directory to the path or change the working directory. The potential disadvan-
tage of this approach is that you must be careful when naming files. When
MATLAB searches for files, it uses the first file with the correct name that it
finds in the path list, starting with the current working directory. If you use
the same name for different files in different directories in your path, you can
run into problems.

You can also control the MATLAB search path from the Path Browser.
To open the Path Browser, type editpath or pathtool, or select File:Set
Path..., The Path Browser consists of a panel, with a list of directories in the
current path, and several buttons. To add a directory to the path list, click
on Add Folder... or Add with Subfolders..., depending on whether or not
you want subdirectories to be included as well. To remove a directory, click on
Remove. The buttons Move Up and Move Down can be used to reorder the
directories in the path. Note that you can use the Current Directory browser to
examine the files in the working directory, and even to create subdirectories,
move M-files around, etc.

/ The information displayed in the main areas of the Path Browser can also be
obtained from the command line. To see the current working directory, type
pwd. To list the files in the working directory type either Is or dir. To see
the current path list that MATLAB will search for files, type path.

/ If you have many toolboxes installed, path searches can be slow, especially
with lookf or. Removing the toolboxes you are not currently using from the
MATLAB path is one way to speed up execution.

Using the Command Window

We have already described in Chapters 1 and 2 how to enter commands in the
MATLAB Command Window. We continue that description here, presenting
an example that will serve as an introduction to our discussion of M-files.
Suppose you want to calculate the values of

sin(0.1)/0.1, sin(0.01)/0.01, and sin(0.001)/0.001

to 15 digits. Such a simple problem can be worked directly in the Command
Window. Here is a typical first try at a solution, together with the response
that MATLAB displays in the Command Window:

» x = [0.1, 0.01, 0.001] ;

>> y = sin (x) . /x

36

Chapter 3: Interacting with MATLAB

y =

0.9983 1.0000 1.0000

After completing a calculation, you will often realize that the result is not
what you intended. The commands above displayed only 5 digits, not 15. To
display 15 digits, you need to type the command format long and then
repeat the line that defines y. In this case you could simply retype the latter
line, but in general retyping is time consuming and error prone, especially for
complicated problems. How can you modify a sequence of commands without
retyping them?

For simple problems, you can take advantage of the command history fea-
ture of MATLAB. Use the UP- and down- arrow keys to scroll through the list
of commands that you have used recently. When you locate the correct com-
mand line, you can use the left- and right-arrow keys to move around in the
command line, deleting and inserting changes as necessary, and then press
the enter key to tell MATLAB to evaluate the modified command. You can
also copy and paste previous command lines from the Command Window, or
in the MATLAB 6 Desktop from the Command History window as described
earlier in this chapter. For more complicated problems, however, it is better to
use M-files.

M-Files

For complicated problems, the simple editing tools provided by the Command
Window and its history mechanism are insufficient. A much better approach
is to create an M-file. There are two different kinds of M-files: script M-files
and function M-files. We shall illustrate the use of both types of M-files as we
present different solutions to the problem described above.

M-files are ordinary text files containing MATLAB commands. You can cre-
ate and modify them using any text editor or word processor that is capable of
saving files as plain ASCII text. (Such text editors include notepad in Win-
dows or emacs, textedit, and vi in UNIX.) More conveniently, you can use
the built-in Editor /Debugger, which you can start by typing edit, either by
itself (to edit a new file) or followed by the name of an existing M-file in the
current working directory. You can also use the File menu or the two leftmost
buttons on the tool bar to start the Editor/Debugger, either to create a new
file or to open an existing file. Double-clicking on an M-file in the Current
Directory browser will also open it in the Editor/Debugger.

M-Files

37

Script M-Files

We now show how to construct a script M-file to solve the mathematical prob-
lem described earlier. Create a file containing the following lines:

format long

x = [0.1, 0.01, 0.001] ;

y = sin (x) . /x

We will assume that you have saved this file with the name taskl . m in your
working directory, or in some directory on your path. You can name the file
any way you like (subject to the usual naming restrictions on your operating
system), but the “ . m” suffix is mandatory.

You can tell MATLAB to run (or execute) this script by typing taskl in
the Command Window. (You must not type the “ . m” extension here; MATLAB
automatically adds it when searching for M-files.) The output — but not the
commands that produce them — will be displayed in the Command Window.
Now the sequence of commands can easily be changed by modifying the M-file
taskl .m. For example, if you also wish to calculate sin(0.0001)/0.0001, you
can modify the M-file to read

format long

x = [0.1, 0.01, 0.001, 0.0001];

y = sin (x) . /x

and then run the modified script by typing taskl. Be sure to save your
changes to taskl .m first; otherwise, MATLAB will not recognize them. Any
variables that are set by the running of a script M-file will persist exactly
as if you had typed them into the Command Window directly. For example,
the program above will cause all future numerical output to be displayed
with 15 digits. To revert to 5-digit format, you would have to type format
short.

Echoing Commands. As mentioned above, the commands in a script M-file
will not automatically be displayed in the Command Window. If you want the
commands to be displayed along with the results, use echo:

echo on
format long

x = [0.1, 0.01, 0.001] ;

y = sin (x) . /x
echo off

38

Chapter 3: Interacting with MATLAB

Adding Comments. It is worthwhile to include comments in a lengthly script
M-file. These comments might explain what is being done in the calculation,
or they might interpret the results of the calculation. Any line in a script M-file
that begins with a percent sign is treated as a comment and is not executed by
MATLAB. Here is our new version of taskl . m with a few comments added:

echo on

% Turn on 15 digit display-
format long

x = [0.1, 0.01, 0.001] ;

y = sin (x) . /x

% These values illustrate the fact that the limit of
% sin(x)/x as x approaches 0 is 1.
echo off

When adding comments to a script M-file, remember to put a percent sign at
the beginning of each line. This is particularly important if your editor starts
a new line automatically while you are typing a comment. If you use echo
on in a script M-file, then MATLAB will also echo the comments, so they will
appear in the Command Window.

Structuring Script M-Files. For the results of a script M-file to be reproducible,
the script should be self-contained, unaffected by other variables that you
might have defined elsewhere in the MATLAB session, and uncorrupted by
leftover graphics. With this in mind, you can type the line clear all at the
beginning of the script, to ensure that previous definitions of variables do
not affect the results. You can also include the close all command at the
beginning of a script M-file that creates graphics, to close all graphics windows
and start with a clean slate.

Here is our example of a complete, careful, commented solution to the
problem described above:

% Remove old variable definitions
clear all

% Remove old graphics windows
close all

% Display the command lines in the command window
echo on

% Turn on 15 digit display
format long

M-Files

39

% Define the vector of values of the independent variable
x = [0.1, 0.01, 0.001] ;

% Compute the desired values
y = sin (x) ./x

% These values illustrate the fact that the limit of
% sin(x)/x as x approaches 0 is equal to 1.

echo off

/ Sometimes you may need to type, either in the Command Window or in an
M-file, a command that is too long to fit on one line. If so, when you get near
the end of a line you can type . . . (that is, three successive periods) followed
by ENTER, and continue the command on the next line. In the Command
Window, you will not see a command prompt on the new line.

Function M-Files

You often need to repeat a process several times for different input values of a
parameter. For example, you can provide different inputs to a built-in function
to find an output that meets a given criterion. As you have already seen, you
can use inline to define your own functions. In many situations, however,
it is more convenient to define a function using an M-file instead of an inline
function.

Let us return to the problem described above, where we computed some
values of sin(x)/x with x = 10 -6 for several values of b. Suppose, in addition,
that you want to find the smallest value of b for which sin(10 -6 )/(10 -6 ) and 1
agree to 15 digits. Here is a function M-file called sinelimit .m designed to
solve that problem:

function y = sinelimit (c)

% SINELIMIT computes sin(x)/x for x = 10~(-b),

% where b = 1 , . . . , c .

format long
b = 1 : c;
x = 10. ~ (-b) ;
y = (sin (x) ./x) ' ;

Like a script M-file, a function M-file is a plain text file that should reside in
your MATLAB working directory. The first line of the file contains a function

Chapter 3: Interacting with MATLAB

statement, which identifies the file as a function M-file. The first line specifies
the name of the function and describes both its input arguments (or parame-
ters) and its output values. In this example, the function is called sinelimit.
The file name and the function name should match.

The function sinelimit takes one input argument and returns one out-
put value, called c and y (respectively) inside the M-file. When the function
finishes executing, its output will be assigned to ans (by default) or to any other
variable you choose, just as with a built-in function. The remaining lines of
the M-file define the function. In this example, b is a row vector consisting
of the integers from 1 to c. The vector y contains the results of computing
sin(x)/x where x = 10 -6 ; the prime makes y a column vector. Notice that the
output of the lines defining b, x, and y is suppressed with a semicolon. In
general, the output of intermediate calculations in a function M-file should be
suppressed.

/ Of course, when we run the M-file above, we do want to see the results of
the last line of the file, so a natural impulse would be to avoid putting a
semicolon on this last line. But because this is a function M-file, running it
will automatically display the contents of the designated output variable y.
Thus if we did not put a semicolon at the end of the last line, we would see
the same numbers twice when we run the function!

Note that the variables used in a function M-file, such as b, x, and y in
sinelimit .m, are local variables. This means that, unlike the variables that
are defined in a script M-file, these variables are completely unrelated to any
variables with the same names that you may have used in the Command
Window, and MATLAB does not remember their values after the function
M-file is executed. For further information, see the section Variables in
Function M-files in Chapter 4.

Here is an example that shows how to use the function sinelimit:

>> sinelimit (5)

0 . 99833416646828
0 . 99998333341667
0 . 99999983333334
0 . 99999999833333
0 . 99999999998333

None of the values of b from 1 to 5 yields the desired answer, 1, to 15 digits.

Presenting Your Results

41

Judging from the output, you can expect to find the answer to the question we
posed above by typing sinelimit (10) . Try it!

Loops

A loop specifies that a command or group of commands should be repeated
several times. The easiest way to create a loop is to use a for statement. Here
is a simple example that computes and displays 10! = 10 • 9 • 8 • • • 2 • 1:

f = 1;

for n = 2:10
f = f*n;

end

f

The loop begins with the for statement and ends with the end statement. The
command between those statements is executed a total of nine times, once for
each value of n from 2 to 10. We used a semicolon to suppress intermediate
output within the loop. To see the final output, we then needed to type f after
the end of the loop. Without the semicolon, MATLAB would display each of
the intermediate values 2!, 3!,

We have presented the loop above as you might type it into an M-file; inden-
tation is not required by MATLAB, but it helps human readers distinguish the
commands within the loop. If you type the commands above directly to the
MATLAB prompt, you will not see a new prompt after entering the for state-
ment. You should continue typing, and after you enter the end statement,
MATLAB will evaluate the entire loop and display a new prompt.

/ If you use a loop in a script M-file with echo on in effect, the commands will
be echoed every time through the loop. You can avoid this by inserting the
command echo of f just before the end statement and inserting echo on
just afterward; then each command in the loop (except end) will be echoed
once.

Presenting Your Results

Sometimes you may want to show other people the results of a script M-file
that you have created. For a polished presentation, you should use an M-book,
as described in Chapter 6, or import your results into another program, such

42

Chapter 3: Interacting with MATLAB

as a word processor, or convert your results to HTML format, by the procedures
described in Chapter 10. But to share your results more informally, you can
give someone else your M-file, assuming that person has a copy of MATLAB
on which to run it, or you can provide the output you obtained. Either way,
you should remember that the reader is not nearly as familiar with the M-file
as you are; it is your responsibility to provide guidance.

/ You can greatly enhance the readability of your M-file by including frequent
comments. Your comments should explain what is being calculated, so that
the reader can understand your procedures and strategies. Once you’ve done
the calculations, you can also add comments that interpret the results.

If your audience is going to run your M-files, then you should make liberal
use of the command pause. Each time MATLAB reaches a pause statement,
it stops executing the M-file until the user presses a key. Pauses should be
placed after important comments, after each graph, and after critical points
where your script generates numerical output. These pauses allow the viewer
to read and understand your results.

Diary Files

Here is an effective way to save the output of your M-file in a way that others
(and you!) can later understand. At the beginning of a script M-file, such as
taskl .m, you can include the commands

delete taskl.txt
diary taskl.txt
echo on

The script M-file should then end with the commands

echo off
diary off

The first diary command causes all subsequent input to and output from
the Command Window to be copied into the specified file — in this case,
taskl . txt. The diary file taskl . txt is a plain text file that is suitable for
printing or importing into another program.

By using delete at the beginning of the M-file, you ensure that the file only
contains the output of the current script. If you omit the delete command,
then the diary command will add any new output to the end of an existing file,
and the file taskl . txt can end up containing the results of several runs of
the M-file. (Putting the delete command in the script will lead to a harmless

Presenting Your Results

43

warning message about a nonexistent file the first time you run the script.)
You can also get extraneous output in a diary file if you type ctrl+c to halt a
script containing a diary command. If this happens, you should type diary
of f in the Command Window before running the script again.

Presenting Graphics

As indicated in Chapters 1 and 2, graphics appear in a separate window. You
can print the current figure by selecting File : Print... in the graphics window.
Alternatively, the command print (without any arguments) causes the figure
in the current graphics window to be printed on your default printer. Since
you probably don’t want to print the graphics every time you run a script, you
should not include a bare print statement in an M-file. Instead, you should
use a form of print that sends the output to a file. It is also helpful to give
reasonable titles to your figures and to insert pause statements into your
script so that viewers have a chance to see the figure before the rest of the
script executes. For example,

xx = 2*pi* (0:0.02:1) ;
plot (xx, sin(xx))

% Put a title on the figure,
title (' Figure A: Sine Curve')
pause

% Store the graph in the file figureA.eps.
print -deps figureA

The form of print used in this script does not send anything to the printer.
Instead, it causes the current figure to be written to a file in the current
working directory called figureA. eps in Encapsulated PostScript® format.
This file can be printed later on a PostScript printer, or it can be imported into
another program that recognizes the EPS format. Type help print to see
how to save your graph in a variety of other formats that may be suitable for
your particular printer or application.

As a final example involving graphics, let’s consider the problem of plotting
the functions sin(x), sin(2x), and sin(3x) on the same set of axes. This is a
typical example; we often want to plot several similar curves whose equations
depend on a parameter. Here is a script M-file solution to the problem:

echo on

% Define the x values,
x = 2*pi* (0:0.01:1) ;

44

Chapter 3: Interacting with MATLAB

% Remove old graphics, and get ready for several new ones,
close all; axes; hold on

% Run a loop to plot three sine curves,
for c = 1:3

plot (x, sin (c*x) )
echo off

end

echo on
hold off

% Put a title on the figure,
title (' Several Sine Curves')
pause

The result is shown in Figure 3-1.

Several Sine Curves

Figure 3-1

Presenting Your Results

45

Let’s analyze this solution. We start by defining the values to use on the x
axis. The command close all removes all existing graphics windows; axes
starts a fresh, empty graphics window; and hold on lets MATLAB know that
we want to draw several curves on the same set of axes. The lines between
for and end constitute a for loop, as described above. The important part
of the loop is the plot command, which plots the desired sine curves. We
inserted an echo off command so that we only see the loop commands once
in the Command Window (or in a diary file). Finally, we turn echoing back on
after exiting the loop, use hold off to tell MATLAB that the curves we just
graphed should not be held over for the next graph that we make, title the
figure, and instruct MATLAB to pause so that the viewer can see it.

Pretty Printing

| s y Ji,/ic y Tf s is a symbolic expression, then typing pretty (s) displays s in
pretty print format, which uses multiple lines on your screen to imitate written
mathematics. The result is often more easily read than the default one-line out-
put format. An important feature of pretty is that it wraps long expressions
to fit within the margins (80 characters wide) of a standard-sized window. If
your symbolic output is long enough to extend past the right edge of your win-
dow, it probably will be truncated when you print your output, so you should
use pretty to make the entire expression visible in your printed output.

A General Procedure

In this section, we summarize the general procedure we recommend for using
the Command Window and the Editor/Debugger (or your own text editor) to
make a calculation involving many commands. We have in mind here the case
when you ultimately want to print your results or otherwise save them in
a format you can share with others, but we find that the first steps of this
procedure are useful even for exploratory calculations.

1. Create a script M-file in your current working directory to hold your com-
mands. Include echo on near the top of the file so that you can see which
commands are producing what output when you run the M-file.

2. Alternate between editing and running the M-file until you are satisfied
that it contains the MATLAB commands that do what you want. Remem-
ber to save the M-file each time between editing and running! Also, see
the debugging hints below.

46

Chapter 3: Interacting with MATLAB

3. Add comments to your M-file to explain the meaning of the intermediate
calculations you do and to interpret the results.

4. If desired, insert the delete and diary statements into the M-file as
described above.

5. If you are generating graphs, add print statements that will save the
graphs to files. Use pause statements as appropriate.

6. If needed, run the M-file one more time to produce the final output. Send
the diary file and any graphics files to the printer or incorporate them into
a document.

7. If you import your diary file into a word processing program, you can
insert the graphics right after the commands that generated them. You
can also change the fonts of text comments and input to make it easier
to distinguish comments, input, and output. This sort of polishing is done
automatically by the M-book interface; see Chapter 6.

Fine-Tuning Your M-Files

You can edit your M-file repeatedly until it produces the desired output. Gene-
rally, you will run the script each time you edit the file. If the program is long
or involves complicated calculations or graphics, it could take a while each
time. Then you need a strategy for debugging. Our experience indicates that
there is no best paradigm for debugging M-files — what you do depends on
the content of your file.

B® 5 We will discuss features of the Editor/Debugger and MATLAB debugging
commands in the section Debugging in Chapter 7 and in the section
Debugging Techniques in Chapter 11. For the moment, here are some general
tips.

• Include clear all and close all at the beginning of the M-file.

• Use echo on early in your M-file so that you can see “cause” as well as
“effect”.

• If you are producing graphics, use hold on and hold off carefully.
In general, you should put a pause statement after each hold off.
Otherwise, the next graphics command will obliterate the current one,
and you won’t see it.

• Do not include bare print statements in your M-files. Instead, print to
a file.

• Make liberal use of pause.

Fine-Tuning Your M-Files

47

The command keyboard is an interactive version of pause. If you have
the line keyboard in your M-file, then when MATLAB reaches it,
execution of your program is interrupted, and a new prompt appears
with the letter K before it. At this point you can type any normal MATLAB
command. This is useful if you want to examine or reset some variables
in the middle of a script run. To resume the execution of your script, type
return; i.e., type the six letters r-e-t-u-r-n and press the ENTER key.

In some cases, you might prefer input. For example, if you include the
line var = input ( ' Input var here : ' ) in your script, when MAT-
LAB gets to that point it will print “input var here and pause while
you type the value to be assigned to var.

Finally, remember that you can stop a running M-file by typing CTRL+C.
This is useful if, at a pause or input statement, you realize that you
want to stop execution completely.

Practice Set A

Algebra and
Arithmetic

Problems 3-8 require the Symbolic Math Toolbox. The others do not.

1. Compute:

(a) 1111-345.

(b) e 14 and 38280l7r to 15 digits each. Which is bigger?

(c) the fractions 2709/1024, 10583/4000, and 2024/765. Which of these
is the best approximation to -s/7?

2. Compute to 15 digits:

(a) cosh(O.l).

(b) ln(2). (Hint'. The natural logarithm in MATLAB is called log, not
In.)

(c) arctan(l/2). {Hint: The inverse tangent function in MATLAB is called
atan, not arctan.)

3. Solve (symbolically) the system of linear equations

3x + 4y + 5z = 2
2x — 3y + Iz = —1
x — 6y + z = 3.

Check your answer using matrix multiplication.

4. Try to solve the system of linear equations

3x — 9y + 8z = 2
2x — 3y + Iz = —1
x — 6y + z = 3.

What happens? Can you see why? Again check your answer using matrix
multiplication. Is the answer “correct”?

5. Factor the polynomial x 4 — y 4 .

48

Algebra and Arithmetic

49

6. Use simplify or simple to simplify the following expressions:

( a ) 1/(1 + 1/(1 + A ))

(b) cos 2 x — sin 2 x

7. Compute 3 301 , both as an approximate floating point number and as an
exact integer (written in usual decimal notation).

8. Use either solve or fzero, as appropriate, to solve the following equa-
tions:

(a) 8x + 3 = 0 (exact solution)

(b) 8x + 3 = 0 (numerical solution to 15 places)

(c) x 3 + px + q = 0 (Solve for x in terms of p and q )

(d) e x = 8x — 4 ( all real solutions). It helps to draw a picture first.

9. Use plot and/or ezplot, as appropriate, to graph the following functions:

(a) y = x 3 — x for — 4 < x < 4.

(b) y = sin(l/x 2 ) for — 2 < x < 2. Try this one with both plot and ezplot.
Are both results “correct”? (If you use plot, be sure to plot enough
points.)

(c) y = tan(x/2) for — n < x < tt, — 10 < y < 10 {Hint: First draw the plot;
then use axis.)

(d) y = e~ x and y = x 4 — x 2 for — 2 < x < 2 (on the same set of axes).

10. Plot the functions x 4 and 2 X on the same graph and determine how many
times their graphs intersect. {Hint: You will probably have to make several
plots, using intervals of various sizes, to find all the intersection points.)
Now find the approximate values of the points of intersection using fzero.

Chapter 4

Beyond the Basics

In this chapter, we describe some of the finer points of MATLAB and review in
more detail some of the concepts introduced in Chapter 2. We explore enough of
MATLAB’s internal structure to improve your ability to work with complicated
functions, expressions, and commands. At the end of this chapter, we introduce
some of the MATLAB commands for doing calculus.

Suppressing Output

Some MATLAB commands produce output that is superfluous. For example,
when you assign a value to a variable, MATLAB echoes the value. You can
suppress the output of a command by putting a semicolon after the command.
Here is an example:

>> syms x
>> y = x + 7

y =

x+7

= x + 7;

x+7

The semicolon does not affect the way MATLAB processes the command
internally, as you can see from its response to the command z.

50

Data Classes

51

You can also use semicolons to separate a string of commands when you are
interested only in the output of the final command (several examples appear
later in the chapter). Commas can also be used to separate commands without
suppressing output. If you use a semicolon after a graphics command, it will
not suppress the graphic.

^ The most common use of the semicolon is to suppress the printing of
a long vector, as indicated in Chapter 2.

Another object that you may want to suppress is MATLAB’s label for the
output of a command. The command disp is designed to achieve that; typing
disp (x) will print the value of the variable x without printing the label and
the equal sign. So,

» x = 7;

>> disp(x)

7

or

>> disp (solve ( 'x + tan(y)

-atan (x-5)

5', 'y'))

Data Classes

Every variable you define in MATLAB, as well as every input to, and output
from, a command, is an array of data belonging to a particular class. In this
book we use primarily four types of data: floating point numbers, symbolic
expressions, character strings, and inline functions. We introduced each of
these types in Chapter 2. In Table 4-1, we list for each type of data its class
(as given by whos ) and how you can create it.

Type of data

Class

Created by

Floating point

double

typing a number

Symbolic

sym

using sym or syms

Character string

char

typing a string inside single quotes

Inline function

inline

using inline

Table 4-1

You can think of an array as a two-dimensional grid of data. A single number
(or symbolic expression, or inline function) is regarded by MATLAB as a 1 x 1

Chapter 4: Beyond the Basics

array, sometimes called a scalar. A 1 x n array is called a row vector, and
anmxl array is called a column vector. (A string is actually a row vector of
characters.) An m x n array of numbers is called a matrix', see More on Matrices
below. You can see the class and array size of every variable you have defined
by looking in the Workspace browser or typing whos (see Managing Variables
in Chapter 2). The set of variable definitions shown by whos is called your
Workspace.

To use MATLAB commands effectively, you must pay close attention to the
class of data each command accepts as input and returns as output. The input
to a command consists of one or more arguments separated by commas; some
arguments are optional. Some commands, such as whos, do not require any
input. When you type a pair of words, such as hold on, MATLAB interprets
the second word as a string argument to the command given by the first word;
thus, hold on is equivalent to hold ( ' on ' ) . The help text (see Online Help in
Chapter 2) for each command usually tells what classes of inputs the command
expects as well as what class of output it returns.

Many commands allow more than one class of input, though sometimes
only one data class is mentioned in the online help. This flexibility can be a
convenience in some cases and a pitfall in others. For example, the integration
command, int, accepts strings as well as symbolic input, though its help
text mentions only symbolic input. However, suppose that you have already
defined a = 10, b = 5, and now you attempt to factor the expression a 2 — b 2 ,
forgetting your previous definitions and that you have to declare the variables
symbolic:

>> factor (a~2 - b~2)

3 5 5

The reason you don’t get an error message is that factor is the name of
a command that factors integers into prime numbers as well as factoring
expressions. Since a 2 — b 2 = 75 = 3 • 5 2 , the numerical version of factor is
applied. This output is clearly not what you intended, but in the course of a
complicated series of commands, you must be careful not to be fooled by such
unintended output.

/ Note that typing help factor only shows you the help text for the

numerical version of the command, but it does give a cross-reference to the
symbolic version at the bottom. If you want to see the help text for the
symbolic version instead, type help sym/ factor. Functions such as
factor with more than one version are called overloaded.

Data Classes

53

Sometimes you need to convert one data class into another to prepare the
output of one command to serve as the input for another. For example, to use
plot on a symbolic expression obtained from solve, it is convenient to use
first vectorize and then inline, because inline does not allow symbolic
input and vectorize converts symbolic expressions to strings. You can make
the same conversion without vectorizing the expression using char. Other
useful conversion commands we have encountered are double (symbolic to
numerical), sym (numerical or string to symbolic), and inline itself (string to
inline function). Also, the commands num2str and str2num convert between
numbers and strings.

String Manipulation

Often it is useful to concatenate two or more strings together. The simplest way
to do this is to use MATLAB’s vector notation, keeping in mind that a string is
a “row vector” of characters. For example, typing [stringl, string2] com-
bines stringl and string2 into one string.

Here is a useful application of string concatenation. You may need to define
a string variable containing an expression that takes more than one line to
type. (In most circumstances you can continue your MATLAB input onto the
next line by typing . . . followed by ENTER or RETURN, but this is not allowed
in the middle of a string.) The solution is to break the expression into smaller
parts and concatenate them, as in:

>> eqn = ['left hand side of equation = ', ...

'right hand side of equation']

eqn =

left hand side of equation = right hand side of equation

Symbolic and Floating Point Numbers

We mentioned above that you can convert between symbolic numbers and
floating point numbers with double and sym. Numbers that you type are,
by default, floating point. However, if you mix symbolic and floating point
numbers in an arithmetic expression, the floating point numbers are auto-
matically converted to symbolic. This explains why you can type syms x and
then x~2 without having to convert 2 to a symbolic number. Here is another
example:

» a = 1

54

Chapter 4: Beyond the Basics

>> b = a/sym(2)

b =

1/2

MATLAB was designed so that some floating point numbers are restored
to their exact values when converted to symbolic. Integers, rational numbers
with small numerators and denominators, square roots of small integers, the
number n, and certain combinations of these numbers are so restored. For
example,

>> c = sqrt(3)

1 . 7321

>> sym(c)

sqrt (3)

Since it is difficult to predict when MATLAB will preserve exact values, it is
best to suppress the floating point evaluation of a numeric argument to symby
enclosing it in single quotes to make it a string, e.g., sym ( ' 1 + sqrt (3 ) ' ) .
We will see below another way in which single quotes suppress evaluation.

Functions and Expressions

We have used the terms expression and function without carefully making a
distinction between the two. Strictly speaking, if we define fix) = x 3 — 1, then
f (written without any particular input) is a function while fix) and x 3 — 1
are expressions involving the variable x. In mathematical discourse we often
blur this distinction by calling fix) or x 3 — 1 a function, but in MATLAB the
difference between functions and expressions is important.

In MATLAB, an expression can belong to either the string or symbolic class
of data. Consider the following example:

>> f = 'x~3 - 1';

f (7)

Functions and Expressions

55

This result may be puzzling if you are expecting f to act like a function. Since
f is a string, f (7) denotes the seventh character in f, which is 1 (the spaces
count). Notice that like symbolic output, string output is not indented from
the left margin. This is a clue that the answer above is a string (consisting
of one character) and not a floating point number. Typing f ( 5 ) would yield a
minus sign and f ( - 1) would produce an error message.

You have learned two ways to define your own functions, using inline (see
Chapter 2) and using an M-file (see Chapter 3). Inline functions are most useful
for defining simple functions that can be expressed in one line and for turning
the output of a symbolic command into a function. Function M-files are useful
for defining functions that require several intermediate commands to compute
the output. Most MATLAB commands are actually M-files, and you can peruse
them for ideas to use in your own M-files — to see the M-file for, say, the
command mean you can enter type mean. See also More about M-files below.

Some commands, such as ode45 (a numerical ordinary differential equa-
tions solver), require their first argument to be a function — to be precise,
either an inline function (as in ode4 5(f, [0 2], l))ora function handle,

that is, the name of a built-in function or a function M-file preceded by the
special symbol @ (as in ode45 (@func, [0 2] , 1) ). The @ syntax is new in
MATLAB 6; in earlier versions of MATLAB, the substitute was to enclose the
name of the function in single quotes to make it a string. But with or without
quotes, typing a symbolic expression instead gives an error message. However,
most symbolic commands require their first argument to be either a string or
a symbolic expression, and not a function.

An important difference between strings and symbolic expressions is that
MATLAB automatically substitutes user-defined functions and variables into
symbolic expressions, but not into strings. (This is another sense in which the
single quotes you type around a string suppress evaluation.) For example, if
you type

>> h = inline (' t. ~3 ' , 't');

>> int ( ' h (t) ' , ' t' )

ans =

int (h (t) ,t)

then the integral cannot be evaluated because within a string h is regarded
as an unknown function. But if you type

>> syms t

» int (h (t) , t)

ans =

1/ 4*t^4

56

Chapter 4: Beyond the Basics

then the previous definition of h is substituted into the symbolic expression
h(t) before the integration is performed.

Substitution

In Chapter 2 we described how to create an inline function from an expression.
You can then plug numbers into that function, to make a graph or table of
values for instance. But you can also substitute numerical values directly into
an expression with subs. For example,

>> syms a x y;

+ y~2;
x, 2)

[x y] , [34])

>> a = x"2
>> subs (a,

4+y A 2

>> subs (a.

25

More about M-Files

Files containing MATLAB statements are called M-files. There are two kinds
of M-files: function M-files, which accept arguments and produce output, and
script M-files, which execute a series of MATLAB statements. Earlier we cre-
ated and used both types. In this section we present additional information
on M-files.

Variables in Script M-Files

When you execute a script M-file, the variables you use and define belong
to your Workspace; that is, they take on any values you assigned earlier in
your MATLAB session, and they persist after the script finishes executing.
Consider the following script M-file, called scriptexl . m:

u = [1 2 3 4];

Typing scriptexl assigns the given vector to u but displays no output. Now
consider another script, called scriptex2 .m:

n = length (u)

More about M- Files

57

If you have not previously defined u, then typing scriptex2 will produce an
error message. However, if you type scriptex2 after running scriptexl,
then the definition of u from the first script will be used in the second script
and the output n = 4 will be displayed.

If you don’t want the output of a script M-file to depend on any earlier compu-
tations in your MATLAB session, put the line clear all near the beginning
of the M-file, as we suggested in Structuring Script M-files in Chapter 3.

Variables in Function M-Files

The variables used in a function M-file are local, meaning that they are un-
affected by, and have no effect on, the variables in your Workspace. Consider
the following function M-file, called sq.m:

function z = sq(x)

% sq (x) returns the square of x.
z = x.~2;

Typing sq (3 ) produces the answer 9, whether or not x or z is already defined
in your Workspace, and neither defines them, nor changes their definitions, if
they have been previously defined.

Structure of Function M-Files

The first line in a function M-file is called the function definition line ; it defines
the function name, as well as the number and order of input and output argu-
ments. Following the function definition line, there can be several comment
lines that begin with a percent sign (%). These lines are called help text and
are displayed in response to the command help. In the M-file sq.m above,
there is only one line of help text; it is displayed when you type help sq.
The remaining lines constitute the function body, they contain the MATLAB
statements that calculate the function values. In addition, there can be com-
ment lines (lines beginning with %) anywhere in an M-file. All statements in
a function M-file that normally produce output should end with a semicolon
to suppress the output.

Function M-files can have multiple input and output arguments. Here is
an example, called polarcoordinates .m, with two input and two output
arguments:

function [r, theta] = polarcoordinates (x, y)

% polarcoordinates (x, y) returns the polar coordinates
% of the point with rectangular coordinates (x, y) .

58

Chapter 4: Beyond the Basics

r = sqrt (x~2 + y~2) ;
theta = atan2(y,x);

If you type polarcoordinates (3,4), only the first output argument is re-
turned and stored in ans; in this case, the answer is 5. To see both outputs,
you must assign them to variables enclosed in square brackets:

>> [r, theta] = polarcoordinates (3 , 4)

theta =

0.9273

By typing r = polarcoordinates (3,4) you can assign the first output ar-
gument to the variable r, but you cannot get only the second output argument;
typing theta = polarcoordinates (3,4) will still assign the first output,
5, to theta.

Complex Arithmetic

MATLAB does most of its computations using complex numbers, that is, num-
bers of the form a + bi, where i = V—l and a and b are real numbers. The
complex number i is represented as i in MATLAB. Although you may never
have occasion to enter a complex number in a MATLAB session, MATLAB
often produces an answer involving a complex number. For example, many
polynomials with real coefficients have complex roots:

>> solve('x~2 + 2 *x + 2 = O')

ans =

[ -1+i]

[ -1-i]

Both roots of this quadratic equation are complex numbers, expressed in
terms of the number i. Some common functions also return complex values
for certain values of the argument. For example,

» log ( -1)

ans =

3 . 1416i

More on Matrices

59

You can use MATLAB to do computations involving complex numbers by en-
tering numbers in the form a + b*i:

» (2 + 3*i) * (4 - i)

ans =

11.0000 + lO.OOOOi

Complex arithmetic is a powerful and valuable feature. Even if you don’t in-
tend to use complex numbers, you should be alert to the possibility of complex-
valued answers when evaluating MATLAB expressions.

More on Matrices

In addition to the usual algebraic methods of combining matrices (e.g., matrix
multiplication), we can also combine them element-wise. Specifically, if A and
B are the same size, then A. *B is the element-by-element product of A and B,
that is, the matrix whose i, j element is the product of the i, j elements of A
and B. Likewise, A . /B is the element-by-element quotient of A and B, and A. ~c
is the matrix formed by raising each of the elements of A to the power c. More
generally, if f is one of the built-in functions in MATLAB, or is a user-defined
function that accepts vector arguments, then f (A) is the matrix obtained
by applying f element-by-element to A. See what happens when you type
sqrt (A) , where A is the matrix defined at the beginning of the Matrices
section of Chapter 2.

Recall that x ( 3 ) is the third element of a vector x. Likewise, A ( 2 , 3 ) rep-
resents the 2, 3 element of A, that is, the element in the second row and third
column. You can specify submatrices in a similar way. Typing A ( 2 , [ 2 4 ] )
yields the second and fourth elements of the second row of A. To select the
second, third, and fourth elements of this row, type A (2 , 2 : 4) . The subma-
trix consisting of the elements in rows 2 and 3 and in columns 2, 3, and 4 is
generated byA(2:3,2:4).A colon by itself denotes an entire row or column.
For example, A ( : , 2 ) denotes the second column of A, and A ( 3 , : ) yields the
third row of A.

MATLAB has several commands that generate special matrices. The com-
mands zeros (n,m) and ones (n,m) producer x mmatrices of zeros and ones,
respectively. Also, eye (n) represents the n x n identity matrix.

60

Chapter 4: Beyond the Basics

Solving Linear Systems

Suppose A is a nonsingular n x n matrix and b is a column vector of length n.
Then typing x = A\b numerically computes the unique solution to A*x = b.
Type help mldivide for more information.

If either A or b is symbolic rather than numeric, then x = A\b computes
the solution to A*x = b symbolically. To calculate a symbolic solution when
both inputs are numeric, type x = sym (A) \b.

Calculating Eigenvalues and Eigenvectors

The eigenvalues of a square matrix A are calculated with eig (A) . The com-
mand [U, R] = eig (A) calculates both the eigenvalues and eigenvectors.
The eigenvalues are the diagonal elements of the diagonal matrix R, and the
columns of U are the eigenvectors. Here is an example illustrating the use of
eig:

>> A = [3 -20; 2 -20; Oil];
>> eig (A)

ans =

2

» [U, R] = eig (A)

u =

0 - 0.4082

0 - 0.8165

1.0000 0.4082

R =

10 0
0-10
0 0 2

- 0.8165

- 0.4082

- 0.4082

The eigenvector in the first column of U corresponds to the eigenvalue
in the first column of R, and so on. These are numerical values for the
eigenpairs. To get symbolically calculated eigenpairs, type [U, R] =
eig (sym (A) ) .

Doing Calculus with MATLAB

61

Doing Calculus with MATLAB

/ MATLAB has commands for most of the computations of basic calculus
in its Symbolic Math Toolbox. This toolbox includes part of a separate program
called Maple®, which processes the symbolic calculations.

Differentiation

You can use di f f to differentiate symbolic expressions, and also to approxi-
mate the derivative of a function given numerically (say by an M-file):

>> syms x; diff(x~3)

ans =

3*x^2

Here MATLAB has figured out that the variable is x. (See Default Variables
at the end of the chapter.) Alternatively,

>> f = inline ( 'x~3 ' , 'x'); diff(f(x))

ans =

3*x A 2

The syntax for second derivatives is di f f ( f (x) , 2 ) , and for nth derivatives,
dif f (f (x) , n) . The command dif f can also compute partial derivatives
of expressions involving several variables, as in dif f (x~2*y , y) , but to do
multiple partials with respect to mixed variables you must use di f f repeat-
edly, as in dif f (dif f (sin (x*y/z) , x) , y) . (Remember to declare y and
z symbolic.)

There is one instance where differentiation must be represented by the
letter D, namely when you need to specify a differential equation as input to
a command. For example, to use the symbolic ODE solver on the differential
equation xy' + 1 = y, you enter

dsolve('x*Dy + 1 = y' , 'x')

62

Chapter 4: Beyond the Basics

Integration

MATLAB can compute definite and indefinite integrals. Here is an indefinite
integral:

>> int ( 'x~2 ' , 'x' )

ans =
l/3*x'"'3

As with dif f, you can declare x to be symbolic and dispense with the char-
acter string quotes. Note that MATLAB does not include a constant of inte-
gration; the output is a single antiderivative of the integrand. Now here is a
definite integral:

>> syms x; int (asin (x) , 0, 1)

ans =

1/2 *pi - 1

You are undoubtedly aware that not every function that appears in calcu-
lus can be symbolically integrated, and so numerical integration is sometimes
necessary. MATLAB has three commands for numerical integration of a func-
tion f{x)\ quad, quad8, and quadl (the latter is new in MATLAB 6). We
recommend quadl, with quad8 as a second choice. Here’s an example:

>> syms x; int (exp ( -x~4 ) , 0, 1)

Warning: Explicit integral could not be found.

> In /data/matlabrl2/toolbox/symbolic/@sym/int .m at line 58

ans =

int (exp (-x^4) ,x =0 .. 1)

>> quadl (vectorize (exp ( -x~4) ) , 0, 1)

ans =

0.8448

^ The commands quad, quad8, and quadl will not accept Inf or -Inf as
a limit of integration (though int will). The best way to handle a
numerical improper integral over an infinite interval is to evaluate
it over a very large interval.

Doing Calculus with MATLAB

63

/ You have another option. If you type double ( int ( ) ) , then Maple’s
numerical integration routine will evaluate the integral — even over an
infinite range.

MATLAB can also do multiple integrals. The following command computes
the double integral

f f (x 2 +y 2 )dydx:

Jo Jo

» syms x y; int(int(x~2 + y~l, y, 0, sin(x)), 0, pi)

ans =
pi^2-32/9

Note that MATLAB presumes that the variable of integration in int is x
unless you prescribe otherwise. Note also that the order of integration is as in
calculus, from the “inside out”. Finally, we observe that there is a numerical
double integral command dblquad, whose properties and use we will allow
you to discover from the online help.

Limits

You can use limit to compute right- and left-handed limits and limits at
infinity. For example, here is lim sin(r)/x:

>> syms x; limit (sin (x) /x, x, 0)

ans =

1

To compute one-sided limits, use the ' right' and ' left' options. For exam-
ple,

>> limit (abs (x) /x, x, 0, 'left')

ans =

-1

Limits at infinity can be computed using the symbol Inf:

>> limit ( (x~4 + x~2 - 3)/(3*x"4 - log(x)), x. Inf)

ans :
1/3

64

Chapter 4: Beyond the Basics

Sums and Products

Finite numerical sums and products can be computed easily using the vector
capabilities of MATLAB and the commands sum and prod. For example,

>> X = 1:7;

>> sum(X)

You can do finite and infinite symbolic sums using the command symsum.
To illustrate, here is the telescoping sum

S(ra) :

syms k n; symsum(l/k - 1/ (k + 1) , 1, n)

ans =

-1/ (n+1 ) +1

And here is the well-known infinite sum

>> symsum (l/n~ 2, 1, Inf)

ans =

1/ 6*pi^2

Another familiar example is the sum of the infinite geometric series:
>> syms a k; symsum(a''k, 0, Inf)

-1/ (a-1)

Note, however, that the answer is only valid for |a| < 1.

Default Variables

65

Taylor Series

You can use taylor to generate Taylor polynomial expansions of a specified
order at a specified point. For example, to generate the Taylor polynomial up
to order 10 at 0 of the function sinx, we enter

>> syms x; taylor (sin (x) , x, 10)

ans =

x- 1/6 *x a 3+1/120*x a 5- 1/504 0*x A 7+l/362 88 0*x A 9

You can compute a Taylor polynomial at a point other than the origin. For
example,

>> taylor (exp (x) , 4, 2)

ans =

exp (2) +exp (2) * (x-2 ) +1/2 *exp (2 ) * (x-2) A 2+l/6*exp (2) * (x-2) A 3

computes a Taylor polynomial of e x centered at the point x = 2.

The command taylor can also compute Taylor expansions at infinity:

>> taylor (exp ( l/x~2 ) , 6, Inf)

ans =

l+l/x A 2+l/2/x A 4

Default Variables

You can use any letters to denote variables in functions — either MATLAB’s
or the ones you define. For example, there is nothing special about the use of
t in the following, any letter will do as well:

>> syms t; dif f (sin (t~2) )

ans =

2*cos (t A 2 ) *t

However, if there are multiple variables in an expression and you employ a
MATLAB command that does not make explicit reference to one of them,
then either you must make the reference explicit or MATLAB will use a
built-in hierarchy to decide which variable is the “one in play”. For example,

Chapter 4: Beyond the Basics

solve ( 'x + y = 3 ' ) solves for x, not y. If you want to solve for y in this
example, you need to enter solve ('x + y = 3', ' y ' ) . MATLAB’s default
variable for solve is x. If there is no x in the equation(s), MATLAB looks for
the letter nearest to x in alphabetical order (where y takes precedence over w,
but w takes precedence over z, etc). Similarly for dif f, int, and many other
symbolic commands. Thus syms w z; diff w* z yields z as an answer. On
occasion MATLAB assigns a different primary default variable — for example,
the default independent variable for MATLAB’s symbolic ODE solver dsolve
is t. This is mentioned clearly in the online help for dsolve. If you have doubt
about the default variables for any MATLAB command, you should check the
online help.

Chapter 5

MATLAB Graphics

In this chapter we describe more of MATLAB’s graphics commands and the
most common ways of manipulating and customizing them. You can get a
list of MATLAB graphics commands by typing help graphics (for general
graphics commands), help graph2d (for two-dimensional graphing), help
graph3d (for three-dimensional graphing), or help specgraph (for special-
ized graphing commands).

We have already discussed the commands plot and ezplot in Chapter 2.
We will begin this chapter by discussing more uses of these commands, as well
as the other most commonly used plotting commands in two and three dimen-
sions. Then we will discuss some techniques for customizing and manipulating
graphics.

Two-Dimensional Plots

Often one wants to draw a curve in the x-y plane, but with y not given explicitly
as a function of x. There are two main techniques for plotting such curves:
parametric plotting and contour or implicit plotting. We discuss these in turn
in the next two subsections.

Parametric Plots

Sometimes x and y are both given as functions of some parameter. For example,
the circle of radius 1 centered at (0,0) can be expressed in parametric form as
x = cos( 27 t£), y = sin(27it) where t runs from 0 to 1. Though y is not expressed
as a function ofx, you can easily graph this curve with plot, as follows:

» T = 0:0.01:1;

67

68

Chapter 5: MATLAB Graphics

>> plot (cos (2*pi*T) , sin (2*pi*T) )
>> axis square

Figure 5-1

The output is shown in Figure 5.1. If you had used an increment of only 0.1 in
the T vector, the result would have been a polygon with clearly visible corners,
an indication that you should repeat the process with a smaller increment
until you get a graph that looks smooth.

If you have version 2.1 or higher of the Symbolic Math Toolbox (cor-
responding to MATLAB version 5.3 or higher), then parametric plotting is also
possible with ezplot. Thus one can obtain almost the same picture as Figure
5-1 with the command

>> ezplot (' cos (t) ' , 'sin(t)', [0 2*pi] ) ; axis square

Two-Dimensional Plots

69

Contour Plots and Implicit Plots

A contour plot of a function of two variables is a plot of the level curves of the
function, that is, sets of points in the x-y plane where the function assumes
a constant value. For example, the level curves of x 2 + y 2 are circles centered
at the origin, and the levels are the squares of the radii of the circles. Contour
plots are produced in MATLAB with meshgrid and contour. The command
meshgrid produces a grid of points in a specified rectangular region, with a
specified spacing. This grid is used by contour to produce a contour plot in
the specified region.

We can make a contour plot of x 2 + y 2 as follows:

» [X Y] = meshgrid(-3:0.1:3, -3:0. 1:3);

» contour (X, Y, X.~2 + Y.~2)

>> axis square

The plot is shown in Figure 5-2. We have used MATLAB ’s vector notation to

- 3 - 2-1 0 12 3

Figure 5-2

70

Chapter 5: MATLAB Graphics

produce a grid with spacing 0.1 in both directions. We have also used axis
square to force the same scale on both axes.

You can specify particular level sets by including an additional vector ar-
gument to contour. For example, to plot the circles of radii 1, V2, and V3,
type

>> contour (X, Y, X.~2 + Y.~2, [123])

The vector argument must contain at least two elements, so if you want
to plot a single level set, you must specify the same level twice. This is quite
useful for implicit plotting of a curve given by an equation in x and y. For
example, to plot the circle of radius 1 about the origin, type

>> contour (X, Y, X.~2 + Y.~2, [1 1])

Or to plot the lemniscate x 2 — y 2 = (x 2 + y 2 ) 2 , rewrite the equation as

(x 2 + y 2 ) 2 - x 2 + y 2 = 0

and type

>> [X Y] = meshgrid(-l. 1:0. 01:1.1, -1.1:0.01:1.1);

>> contour (X, Y, (X.~2 + Y.~2).~2 - X.~2 + Y.~2, [0 0])

>> axis square

>> title('The lemniscate x~2-y~2= (x~2+y~2) ~2' )

The command title labels the plot with the indicated string. (In the default
string interpreter, ~ is used for inserting an exponent and . is used for sub-
scripts.) The result is shown in Figure 5-3.

have the Symbolic Math Toolbox, contour plotting can also be
done with the command ezcontour, and implicit plotting of a curve fix, y) = 0
can also be done with ezplot. One can obtain almost the same picture as
Figure 5-2 with the command

>> ezcontour ( 'x~2 + y~2' , [-3, 3], [-3, 3]); axis square

and almost the same picture as Figure 5-3 with the command

>> ezplot (' (x~2 + y~2)~2 - x~2 + y~2', ...

[-1.1, 1.1], [-1.1, 1.1]); axis square

Two-Dimensional Plots

71

The lemniscate x 2 -y 2 =(x 2 +y 2 ) 2

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

Figure 5-3

Field Plots

The MATLAB routine quiver is used to plot vector fields or arrays of arrows.
The arrows can be located at equally spaced points in the plane (if x and y
coordinates are not given explicitly), or they can be placed at specified loca-
tions. Sometimes some fiddling is required to scale the arrows so that they
don’t come out looking too big or too small. For this purpose, quiver takes an
optional scale factor argument. The following code, for example, plots a vector
field with a “saddle point,” corresponding to a combination of an attractive
force pointing toward the x axis and a repulsive force pointing away from the
y axis:

» [x, y] = meshgrid(-l.l: .2:1.1, - 1 . 1 : . 2 : 1 . 1) ;

>> quiver (x, -y) ; axis equal; axis off

The output is shown in Figure 5-4.

72

Chapter 5: MATLAB Graphics

l/ / /

i/' t/ /

iS s /

/ / l

/ll

/ / I

/ / i

\ \ \
\ \ \
\ \ \
\ \ \

\ \ \,
\ 'X 'X
\ X

^ ^ v \ \ \

V X \ \ \ t

X\ \ \ \ t

^ ^ ^ ^ I

Figure 5-4

t / / s s* ^

t t // s S'

t t ? / / S

t t t / /

Three-Dimensional Plots

MATLAB has several routines for producing three-dimensional plots.

Curves in Three-Dimensional Space

For plotting curves in 3-space, the basic command is plot3, and it works like
plot, except that it takes three vectors instead of two, one for the x coordi-
nates, one for the y coordinates, and one for the z coordinates. For example,
we can plot a helix (see Figure 5-5) with

>> T = -2:0.01:2;

>> plot3 (cos (2*pi*T) , sin (2*pi*T) , T)

Again, if you have the Symbolic Math Toolbox, there is a shortcut
using ezplot3; you can instead plot the helix with

ezplot3 ( ' cos (2*pi*t) ' , ' sin (2*pi*t) ' , ' t' , [-2, 2])

Three-Dimensional Plots

73

Surfaces in Three-Dimensional Space

There are two basic commands for plotting surfaces in 3-space: mesh and
surf. The former produces a transparent “mesh” surface; the latter produces
an opaque shaded one. There are two different ways of using each command,
one for plotting surfaces in which the 2 coordinate is given as a function of x
and y, and one for parametric surfaces in which x, y, and 2 are all given as
functions of two other parameters. Let us illustrate the former with mesh and
the latter with surf.

To plot 2 = f(x, y), one begins with a meshgrid command as in the case of
contour. For example, the “saddle surface” 2 = r 2 — y 2 can be plotted with

» [X, Y] = meshgrid(-2: .1:2, -2:. 1:2);

» Z = X.~2 - Y . "2 ;

» mesh (X, Y, Z)

The result is shown in Figure 5-6, although it looks much better on the screen
since MATLAB shades the surface with a color scheme depending on the 2
coordinate. We could have gotten an opaque surface instead by replacing mesh
with surf.

74

Chapter 5: MATLAB Graphics

Figure 5-6

With the Symbolic Math Toolbox, there is a shortcut command ezmesh,
and you can obtain a result very similar to Figure 5-6 with

>> ezmesh ( 'x~2 - y"2', [-2, 2], [-2, 2])

If one wants to plot a surface that cannot be represented by an equation
of the form z = fix, y ), for example the sphere x 2 + y 2 + z 2 = 1, then it is bet-
ter to parameterize the surface using a suitable coordinate system, in this
case cylindrical or spherical coordinates. For example, we can take as param-
eters the vertical coordinate z and the polar coordinate 0 in the x-y plane. If
r denotes the distan ce to th e z axis, then the eq uation of th e sphe re becomes
r 2 +z 2 = 1, or r = V 1 — z 2 , and so x = Vl — z 2 cos 9, y = Vl - z 2 sin6>. Thus
we can produce our plot with

[theta, Z] = meshgrid( (0 : 0 . 1 : 2) *pi, (-1:0. 1:1));
X = sqrt(l - Z . "2) . *cos (theta) ;

Special Effects

75

Figure 5-7

>> Y = sqrt(l - Z. "2) . *sin(theta) ;
>> surf(X, Y, Z) ; axis square

The result is shown in Figure 5-7.

With the Symbolic Math Toolbox, parametric plotting of surfaces has
been greatly simplified with the commands ezsurf and ezmesh, and you can
obtain a result very similar to Figure 5-7 with

>> ezsurf (' sqrt (l-s~2) *cos (t) ' , ' sqrt (l-s~2 ) *sin ( t) ' , ...
's', [-1, 1, 0, 2*pi] ) ; axis equal

Special Effects

So far we have only discussed graphics commands that produce or modify a
single static figure window. But MATLAB is also capable of combining several

76

Chapter 5: MATLAB Graphics

figures in one window, or of producing animated graphics that change with
time.

Combining Figures in One Window

The command subplot divides the figure window into an array of smaller
figures. The first two arguments give the dimensions of the array of sub-
plots, and the last argument gives the number of the subplot (counting left
to right across the first row, then left to right across the next row, and so on)
in which to put the next figure. The following example, whose output appears
as Figure 5-8, produces a 2 x 2 array of plots of the first four Bessel functions
J n , 0 < n < 3:

>> x = 0:0.05:40;

>> for j = 1:4, subplot (2 , 2 , j )

plot(x, bessel j ( j ‘ones (size (x) ) , x) )

end

Figure 5-8

Special Effects

77

Animations

The simplest way to produce an animated picture is with comet, which pro-
duces a parametric plot of a curve (the way plot does), except that you can
see the curve being traced out in time. For example,

>> t = 0 : 0 . 01*pi : 2*pi;

>> figure; axis equal; axis([-l 1 -1 1]); hold on
>> comet (cos (t) , sin(t))

displays uniform circular motion.

For more complicated animations, you can use get frame and movie. The
command get frame captures the active figure window for one frame of the
movie, and movie then plays back the result. For example, the following (in
MATLAB 5.3 or later — earlier versions of the software used a slightly differ-
ent syntax) produces a movie of a vibrating string:

» x = 0:0.01:1;

>> for j = 0:50

plot(x, sin ( j*pi/5) *sin (pi*x) ) , axis([0, 1, -2, 2])

M ( j +1) = getframe;

end

>> movie (M)

It is worth noting that the axis command here is important, to ensure that
each frame of the movie is drawn with the same coordinate axes. (Other-
wise the scale of the axes will be different in each frame and the result-
ing movie will be totally misleading.) The semicolon after the getframe
command is also important; it prevents the spewing forth of a lot of nu-
merical data with each frame of the movie. Finally, make sure that while
MATLAB executes the loop that generates the frames, you do not cover the
active figure window with another window (such as the Command Window).
If you do, the contents of the other window will be stored in the frames of the
movie.

/ MATLAB 6 has a new command movieview that you can use in place of
movie to view the animation in a separate window, with a button to replay
the movie when it is done.

78

Chapter 5: MATLAB Graphics

Customizing and Manipulating
Graphics

This is a more advanced topic; if you wish you can skip it on a first reading.

So far in this chapter, we have discussed the most commonly used MATLAB
routines for generating plots. But often, to get the results one wants, one needs
to customize or manipulate the graphics these commands produce. Knowing
how to do this requires understanding a few basic principles concerning the
way MATLAB stores and displays graphics. For most purposes, the discussion
here will be sufficient. But if you need more information, you might eventually
want to consult one of the books devoted exclusively to MATLAB graphics,
such as Using MATLAB Graphics, which comes free (in PDF format) with
the software and can be accessed in the “MATLAB Manuals” subsection of
the “Printable Documentation” section in the Help Browser (or under “Full
Documentation Set” from the helpdesk in MATLAB 5.3 and earlier versions),
or Graphics and GUIs with MATLAB, 2nd ed., by P. Marchand, CRC Press,
Boca Raton, FL, 1999.

In a typical MATLAB session, one may have many figure windows open
at once. However, only one of these can be “active” at any one time. One can
find out which figure is active with the command gcf, short for “get current
figure,” and one can change the active figure to, say, figure number 5 with the
command figure (5) , or else by clicking on figure window 5 with the mouse.
The command figure (with no arguments) creates a blank figure window.
(This is sometimes useful if you want to avoid overwriting an existing plot.)

Once a figure has been created and made active, there are two basic ways to
manipulate it. The active figure can be modified by MATLAB commands in the
command window, such as the commands title and axis square that we
have already encountered. Or one can modify the figure by using the menus
and/or tools in the figure window itself. Let’s consider a few examples. To insert
labels or text into a plot, one may use the commands text, xlabel, ylabel,
zlabel, and legend, in addition to title. As the names suggest, xlabel,
ylabel, and zlabel add text next to the coordinate axes, legend puts a
“legend” on the plot, and text adds text at a specific point. These commands
take various optional arguments that can be used to change the font family
and font size of the text. As an example, let’s illustrate how to modify our plot
of the lemniscate (Figure 5-3) by adding and modifying text:

>> figure (3)

>> title('The lemniscate x"2-y''2 = (x~2+y~2) ~2' , . . .

Customizing and Manipulating Graphics

79

The lemniscate x 2 -y 2 =(x 2 +y 2 ) 2

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

Figure 5-9

'FontSize', 16, ' FontName ' , 'Helvetica',...

' FontWeight' , 'bold')

>> text(0, 0, '\leftarrow a node, also an inflection')

>> text (0.2, -0.1, 'point for each branch')

» xlabel('x'); ylabel('y')

The result is shown in Figure 5-9. Note that many symbols (an arrow pointing
to the left in this case) can be inserted into a text string by calling them
with names starting with \. (If you’ve used the scientific typesetting program
TgX, you’ll recognize the convention here.) In most cases the names are self-
explanatory. For example, you get a Greek n by typing \pi, a summation sign
J2 by typing either \Sigma (for a capital sigma) or \sum, and arrows pointing
in various directions with \ lef tar row, \uparrow, and so on. For more details
and a complete list of available symbols, see the listing for “Text Properties”
in the Help Browser.

An alternative is to make use of the tool bar at the top of the figure window.
The button indicated by the letter “A” adds text to a figure, and the menu item

80

Chapter 5: MATLAB Graphics

Text Properties... in the Tools menu (in MATLAB 5.3), or else the menu
item Figure Properties... in the Edit menu (in MATLAB 6), can be used to
change the font style and font size.

Change of Viewpoint

Another common and important way to vary a graphic is to change the view-
point in 3-space. This can be done with the command view, and also (at least
in MATLAB 5.3 and higher) by using the Rotate 3D option in the Tools menu
at the top of the figure window. The command view (2 ) projects a figure into
the x-y plane (by looking down on it from the positive z axis), and the com-
mand view (3 ) views it from the default direction in 3-space, which is in the
direction looking toward the origin from a point far out on the ray z = 0.5t,
x = -0.5272 1, y = — 0.3044t, t > 0.

^ In MATLAB, any two-dimensional plot can be “viewed in 3D,” and
any three-dimensional plot can be projected into the plane. Thus
Figure 5-5 above (the helix), if followed by the command view (2 ) ,
produces a circle.

Change of Plot Style

Another important way to change the style of graphics is to modify the color or
line style in a plot or to change the scale on the axes. Within a plot command,
one can change the color of a graph, or plot with a dashed or dotted line, or
mark the plotted points with special symbols, simply by adding a string as a
third argument for every x-y pair. Symbols for colors are 'y' for yellow, 'm'
for magenta, ' c' for cyan, 'r' for red, ' g' for green, 'b' for blue, 'w' for
white, and ' k ' for black. Symbols for point markers include 'o' for a circle,
' x ' for an X-mark, ' + ' for a plus sign, and '*' for a star. Symbols for line
styles include ' - ' for a solid line, ' : ' for a dotted line, and ' - - ' for a dashed
line. If a point style is given but no line style, then the points are plotted but
no curve is drawn connecting them. The same methods work with plot3 in
place of plot. For example, one can produce a solid red sine curve along with a
dotted blue cosine curve, marking all the local maximum points on each curve
with a distinctive symbol of the same color as the plot, as follows:

>> X = (-2:0.02:2) *pi ; Y1 = sin(X); Y2 = cos (X) ;

>> plot (X, Yl, 'r-', X, Y2, 'b:'); hold on

>> XI = [-3*pi/2 pi/2]; Y3 = [11]; plot(Xl, Y3 , 'r+')

» X2 = [-2*pi 0 2*pi] ; Y4 = [11 1]; plot(X2, Y4 , 'b*')

Customizing and Manipulating Graphics

81

Here we would probably want the tick marks on the x axis located at mul-
tiples of n. This can be done with the set command applied to the properties
of the axes (and/or by selecting Edit : Axes Properties... in MATLAB 6,
or Tools : Axes Properties... in MATLAB 5.3). The command set is used
to change various properties of graphics. To apply it to “Axes”, it has to be
combined with the command gca, which stands for “get current axes”. The
code

>> set(gca, 'XTick', (-2:2)*pi, 'XTickLabel' , . . .

' -2pi | -pi | 0 | pi | 2pi' )

in combination with the code above gets the current axes, sets the ticks on
the x axis to go from — 2jt to 2n in multiples of n, and then labels these ticks
the way one would want (rather than in decimal notation, which is ugly here).
The result is shown in Figure 5-10. Incidentally, you might wonder how to label
the ticks as — 2n, —n, etc., instead of -2pi, -pi, and so on. This is trickier but
you can do it by typing

Figure 5-10

82 Chapter 5: MATLAB Graphics

>> set(gca, ' FontName' , 'Symbol')

>> set(gca, ' XTickLabel ' , ' -2p | -p | 0 | p | 2p' )

since in the Symbol font, n occupies the slot held by p in text fonts.

Full-Fledged Customization

What about changes to other aspects of a plot? The useful commands get and
set can be used to obtain a complete list of the properties of a graphics window,
and then to modify them. These properties are arranged in a hierarchical
structure, identified by markers (which are simply numbers) called handles.
If you type get (gcf ) , you will “get” a (rather long) list of properties of the
current figure (whose number is returned by the function gcf). Some of these
might read

Color = [0.8 0.8 0.8]

CurrentAxes = [72.0009]

PaperSize = [8.5 11]

Children = [72.0009]

Here PaperSize is self-explanatory; Color gives the background color of the
plot in RGB (red-green-blue) coordinates, where [0 0 0] is black and [1 1 1]
is white. ([0.8 0.8 0.8] is light gray.) Note that CurrentAxes and Children
in this example have the same value, the one-element vector containing the
funny-looking number 72.0009. This number would also be returned by the
command gca (“get current axes”); it is the handle to the axis properties of
the plot. The fact that this also shows up under Children indicates that the
axis properties are “children” of the figure, this is, they lie one level down in the
hierarchical structure. Typing get (gca) or get (72 . 0009) would then give
you a list of axis properties, including further Children such as Line objects,
within which you would find the XData and YData encoding the actual plot.

Once you have located the properties you’re interested in, they can be
changed with set. For example,

>> set (gcf, 'Color', [1 0 0])

changes the background color of the border of the figure window to red, and
>> set (gca, 'Color', [1 1 0])

changes the background color of the plot itself (a child of the figure window)
to yellow (which in the RGB scheme is half red, half green).

Customizing and Manipulating Graphics

83

This “one at a time” method for locating and modifying figure properties
can be speeded up using the command f indob j to locate the handles of all
the descendents (the main figure window, its children, children of children,
etc.) of the current figure. One can also limit the search to handles containing
elements of a specific type. For example, f indob j ( ' Type' , ' Line' ) hunts
for all handles of objects containing a Line element. Once one has located
these, set can be used to change the LineStyle from solid to dashed, etc.
In addition, the low-level graphics commands line, rectangle, fill,
surface, and image can be used to create new graphics elements within a
figure window.

As an example of these techniques, the following code creates a chessboard
on a white background, as shown in Figure 5-11:

>> white =[11 1]; gray = 0.7*white;

» a = [0 1 1 0] ; b = [0 0 1 1] ; c = [1111];

Figure 5-1 1

84

Chapter 5: MATLAB Graphics

>> figure; hold on

>> for k = 0:1, for j = 0:2:6

fill(a'*c + c' * (0 : 2 : 6) + k, b'*c + j + k, gray)
end, end

>> plot (8*a' , 8*b', 'k')

>> set(gca, ' XTickLabel ' , [] , ' YTickLabel ' , [] )

>> set(gcf, 'Color', white); axis square

Here white and gray are the RGB codings for white and gray. The double
for loop draws the 32 dark squares on the chessboard, using fill, with j
indexing the dark squares in a single vertical column, with k = 0 giving the
odd-numbered rows, and with k = 1 giving the even-numbered rows. Note
that fill here takes three arguments: a matrix, each of whose columns gives
the x coordinates of the vertices of a polygon to be filled (in this case a square),
a second matrix whose corresponding columns give the y coordinates of the
vertices, and a color. We’ve constructed the matrices with four columns, one
for each of the solid squares in a single horizontal row. The plot command
draws the solid black line around the outside of the board. Finally, the first
set command removes the printed labels on the axes, and the second set
command resets the background color to white.

Quick Plot Editing in the Figure Window

Almost all of the command-line changes one can make in a figure have coun-
terparts that can be executed using the menus in the figure window. So why
bother learning both techniques? The reason is that editing in the figure win-
dow is often more convenient, especially when one wishes to “experiment” with
various changes, while editing a figure with MATLAB code is often required
when writing M-files. So the true MATLAB expert uses both techniques. The
figure window menus are a bit different in MATLAB 6 than in MATLAB 5.3.
In MATLAB 6, you can zoom in and out and rotate the figure using the Tools
menu, you can insert labels and text with the Insert menu, and you can view
and edit the figure properties (just as you would with set) with the Edit
menu. For example you can change the ticks and labels on the axes by se-
lecting Edit : Edit Axes.... In MATLAB 5.3, editing of the figure properties is
done with the Property Editor, located under the File menu of the figure
window. By default this opens to the figure properties, and double-clicking on
“Children” then enables you to access the axes properties, etc.

Sound

85

Sound

You can use sound to generate sound on your computer (provided that your
computer is suitably equipped). Although, strictly speaking, sound is not a
graphics command, we have placed it in this chapter since we think of “sight”
and “sound” as being allied features. The command sound takes a vector, views
it as the waveform of a sound, and “plays” it. The length of the vector, divided by
8192, is the length of the sound in seconds. A “sinusoidal” vector corresponds
to a pure tone, and the frequency of the sinusoidal signal determines the pitch.
Thus the following example plays the motto from Beethoven’s 5th Symphony:

» x=0:0.1*pi:250*pi; y=zeros (1, 200) ; z=0 : 0 . l*pi : 1000*pi ;

>> sound( [sin(x) ,y,sin(x) ,y,sin(x) , y, sin (z*4/5) ,y, . . .
sin (8/9*x) ,y, sin (8/9*x) ,y, sin (8/9*x) ,y, sin (z*3/4) ] ) ;

Note that the zero vector y in this example creates a very short pause between
successive notes.

Practice Set B

Calculus, Graphics,
and Linear Algebra

x Problems 2, 3, 5-7, and parts of 10-12 require the Symbolic Math Tool-
box. The others do not.

1. Use contour to do the following:

(a) Plot the level curves of the function f(x,y ) = 3y + y 3 — x 3 in the
region where x and y are between —1 and 1 (to get an idea of what
the curves look like near the origin), and in some larger regions (to
get the big picture).

(b) Plot the curve 3y + y 3 — x 3 = 5.

(c) Plot the level curve of the function fix, y) = y lnx + x lny that con-
tains the point (1, 1).

2. Find the derivatives of the following functions. If possible, simplify each
answer.

(a) f{x) = 6x 3 - 5x 2 + 2x - 3.

(b) fix) = §Sg.

(c) fix) = sin(3x 2 + 2).

(d) fix) = arcsin(2x + 3).

(e) fix) = VTTrL

(f) fix) = x r .

(g) fix) = arctan(x 2 + 1).

3. See if MATLAB can do the following integrals symbolically. For the indef-
inite integrals, check the results by differentiating.

(a) Jq / 2 cosxdx.

(b) /xsin(x 2 )c(x.

(c) / sin(3x)Vl — cos(3x)dx.

86

Practice Set B: Calculus, Graphics, and Linear Algebra

87

(d) /x 2 V^+4dx.

(e) dx.

4. Compute the following integrals numerically, using quad8 or quadl:

(a) J” e sinx dx.

(b) [q Vx 3 + 1 dx.

(c) e~ x2 dx. In this case, also approximate the error in the numerical
answer, by comparing with the exact answer found in Problem 3.

5. Evaluate the following limits:

(a) lim^o

(b) linw*

(c) lim^ 00 x 2 e _ *.

(d) linw- jij.

(e) lim x _ > o+ sin

6. Compute the following sums:

(a) £Li* 2 .

(b)

(c) J2T=o It • You may need the gamma function r(x) = / 0 °° e^t*^ 1 dt,
called gamma in MATLAB, which satisfies T{k + 1) = k\.

(d) IX-oo {zZkf-

7. Find the Taylor polynomial of the indicated order n at the indicated point
c for the following functions:

(a) f{x) = e x ,n = l,c = 0.

(b) fix) = sinx, n = 5 and 6, c = 0.

(c) fix) = sinx, n = 6, c = 2.

(d) fix) = tanx, n = 7, c = 0.

(e) fix) = lnx, n = 5, c = 1.

(f) /Tx) = erf(x), n = 9, c = 0.

8. Plot the following surfaces:

(a) z = sinx siny for — 37r <x<3jt and —3n < y < 3n.

(b) z = (x 2 + y 2 ) cos(x 2 + y 2 ) for — 1 < x < 1 and — 1 < y < 1.

9. Create a 17-frame movie, whose frames show filled red circles of radius
1/2 centered at the points (4 cos(j7r/8), 4 sin(j7r/8)), j = 0, 1, . . . , 16. Make
sure all the circles are drawn on the same set of axes, and that they look
like circles, not ellipses.

10. In this problem we use the backslash operator, or “left-matrix-divide” op-
erator introduced in the Solving Linear Systems section of Chapter 4.

(a) Use the backslash operator to solve the system of linear equations
in Problem 3 of Practice Set A.

88

Practice Set B: Calculus, Graphics, and Linear Algebra

(b) Now try the same method on Problem 4 of Practice Set A. MATLAB
finds one, but not all, answer(s). Can you explain why? If not, see
Problem 11 below, as well as part (d) of this problem.

(c) Next try the method on this problem:

w + 3x - 2y + Az = 1
— 2w + 3x + Ay — z = 1
—Aw — 3x + y + 2z = 1
2w + 3x — Ay + z = 1.

Check your answer by matrix multiplication.

(d) Finally, try the matrix division method on:

ax + by = u
cx + dy = v.

Don’t forget to declare the variables to be symbolic. Your answer
should involve a fraction, and so will be valid only when its de-
nominator is nonzero. Evaluate det on the coefficient matrix of the
system. Compare with the denominator.

11. We deal in this problem with 3x3 matrices, although the concepts are
valid in any dimension.

(a) Consider the rows of a square matrix A. They are vectors in 3-space
and so span a subspace of dimension 3, 2, 1, or possibly 0 (if all
the entries of A are zero). That number is called the rank of A. The
MATLAB command rank computes the rank of a matrix. Try it
on the four coefficient matrices in each of the parts of Problem 10.
Comment on MATLAB’s answer for the fourth one.

(b) An n x n matrix is nonsingular if its rank is n. Which of the four
matrices you computed in part (a) are nonsingular?

(c) Another measure of nonsingularity is given by the determinant — a
fundamental result in linear algebra is that a matrix is nonsingular
precisely when its determinant is nonzero. In that case a unique
matrix B exists that satisfies AB = BA = the identity matrix. We
denote this inverse matrix by A -1 . MATLAB can compute inverses
with inv. Compute det (A) for the four coefficient matrices, and for
the nonsingular ones, find their inverses. Note: The matrix equation
Ax = b has a unique solution, namely x = A l b = A\b, when A is
nonsingular.

12. As explained in Chapter 4, when you compute [U, R] = eig (A) , each
column of U is an eigenvector of A associated to the eigenvalue that

Practice Set B: Calculus, Graphics, and Linear Algebra

appears in the corresponding column of the diagonal matrix R. This says
exactly that AU = UR.

(a) Verify the equality AU = UR for each of the coefficient matrices in
Problem 10.

(b) In fact, rank(A) = rank(CT), so when A is nonsingular, then

U~ l AU = R.

Thus if two diagonalizable matrices A and B have the same set of
eigenvectors, then the fact that diagonal matrices commute implies
the same for A and B. Verify these facts for the two matrices

( 1

0

2\

( 5

2

-8\

-1

0

4 .

B= 3

6

-10

V -1

-1

5/

\3

3

"7/

that is, show that the matrices of eigenvectors are the “same” —
that is, the columns are the same up to scalar multiples — and
verify that AB = BA.

13. This problem, having to do with genetic inheritance, is based on Chapter
12 in Applications of Linear Algebra, 3rd ed., by C. Rorres and H. Anton,
John Wiley & Sons, 1984. In a typical inheritance model, a trait in the off-
spring is determined by the passing of a genotype from the parents, where
there are two independent possibilities from each parent, say A and a,
and each is equally likely. (A is the dominant gene, and a is recessive.)
Then we have the following table of probabilities of the possible geno-
types for the offspring for all possible combinations of the genotypes of the
parents:

Genotype of Parents

AA-AA AA-Aa AA-aa Aa-Aa Aa-aa aa-aa
Genotype AA 1 1/2 0 1/4 0 0

of Aa 0 1/2 1 1/2 1/2 0

Offspring aa 0 0 0 1/4 1/2 1

Now suppose one has a population in which mating only occurs with
one’s identical genotype. (That’s not far-fetched if we are considering con-
trolled plant or vegetable populations.) Next suppose that xo, yo> and zo
denote the percentage of the population with genotype AA, Aa, and aa
respectively at the outset of observation. We then denote by x n , y n , and
z n the percentages in the nth generation. We are interested in knowing

90

Practice Set B: Calculus, Graphics, and Linear Algebra

these numbers for large n and how they depend on the initial population.
Clearly

Xn + y n + Zn = ^ n> 0.

Now we can use the table to express a relationship between the nth and
(; n + l)st generations. Because of our presumption on mating, only the first,
fourth, and sixth columns are relevant. Indeed a moment’s reflection re-
veals that we have

1

•*'n+l =X n +

y n + 1 = y n (*)

l

Zn+ 1 =z n + -y n .

4

(a) Write the equations (*) as a single matrix equation X n+1 = MX n ,
n> 0. Explain carefully what the entries of the column matrix X„
are and what the coefficients of the square matrix M are.

(b) Apply the matrix equation recursively to express X n in terms of X 0
and powers of M.

(c) Next use MATLAB to compute the eigenvalues and eigenvectors of
M.

(d) From Problem 12 you know that MU = UR, where R is the diag-
onal matrix of eigenvalues of M. Solve that equation for M. Now
it should be evident to you what R^ = lim,,^.^ R n is. Use that and
your expression of M in terms of R to compute = lim^oo M n .

(e) Describe the eventual population distribution by computing M x X 0 .

(f) Check your answer by directly computing M n for large specific val-
ues of M. ( Hint : MATLAB can compute the powers of a matrix M by
entering M~10, for example.)

(g) You can alter the fundamental presumption in this problem by as-
suming, alternatively, that all members of the nth generation must
mate only with a parent whose genotype is purely dominant. Com-
pute the eventual population distribution of that model. Chapters
12-14 in Rorres and Anton have other interesting models.

Chapter 6

M-Books

MATLAB is exceptionally strong in linear algebra, numerical methods, and
graphical interpretation of data. It is easily programmed and relatively easy
to learn to use. As such it has proven invaluable to engineers and scientists
who are working on problems that rely on scientific techniques and methods at
which MATLAB excels. Very often the individuals and groups that so employ
MATLAB are primarily interested in the numbers and graphs that emerge
from MATLAB commands, processes, and programs. Therefore, it is enough
for them to work in a MATLAB Command Window, from which they can eas-
ily print or export their desired output. At most, the production technique
described in Chapter 3 involving diary files is sufficient for their presentation
needs.

However, other practitioners of mathematical software find themselves with
two additional requirements. They need a mathematical software package em-
bedded in an interactive environment — one in which the output is not nec-
essarily “linear”, that is, one that they can manipulate and massage without
regard to chronology or geographical location. Second, they need a higher-level
presentation mode, which affords graphics integrated with text, with different
formats for input and output, and one that can communicate effortlessly with
other software applications. Some of MATLAB’s competitors have focused on
such needs in designing the interfaces (or front ends) behind which their math-
ematical software runs. MATLAB has decided to concentrate on the software
rather than the interface — and for the reasons and purposes outlined above,
that is clearly a wise decision. But for academic users (both faculty and espe-
cially students), for authors, and even for applied scientists who want to use
MATLAB to generate slick presentations, the interface demands can become
very important. For them, MATLAB has provided the M-book interface, which
we describe in this chapter.

91

92

Chapter 6: M-Books

The M-book interface allows the user to operate MATLAB from a special
Microsoft Word document instead of from a MATLAB Command Window. In
this mode, the user should think of Word as running in the foreground and
MATLAB as running in the background. Lines that you enter into your Word
document are passed to the MATLAB engine in the background and executed
there, whereupon the output is returned to Word (through the intermediary of
Visual Basic®), and then both input and output are automatically formatted.
One obtains a living document in the sense that one can edit the document as
one normally edits a word processing document. So one can revisit input lines
that need adjustment, change them, and reexecute on the spot — after which
the old outdated output is automatically overwritten with new output. The
graphical output that results from MATLAB graphics commands appear in
the Word document, immediately after the commands that generated them.
Erroneous input and output are easily expunged, enhanced formatting can
be done in a way that is no more complicated than what one does in a word
processor, and in the end the result of your MATLAB session can be an at-
tractive, easily readable, and highly informative document. Of course, one can
“cheat” by editing one’s output — we shall discuss that and other pitfalls and
strengths in what follows.

Enabling M-Books

To run the M-book interface you must have Microsoft Word on your com-
puter. It is possible to run the interface with earlier versions of Word, but
we find that it works best if you have Word 97. (In fact, we find that it
runs better in Word 97 than it does in Word 2000, though the difference is
not usually significant.) The interface is enabled when you install MATLAB.
This is done in one of three ways depending on which version of MATLAB
you have. In some instances, during installation, you will be prompted to
enter the location of the Word executable file and the Word template direc-
tory. These are usually easily located; for example, on many PCs the former
is in MSOffice\Office\Winword.exe, and the latter is in MSOffice\
Templates. You may also be asked to specify a template file — in that case,
select normal . dot in the Templates directory. The installation program will
create a new template called m-book . dot, which is the Word template file as-
sociated with M-book documents.

/ If you don’t know where the Word files are located on your PC, go to Find
from the Start menu on the Task Bar, and search your hard drive for the
files Winword.exe and normal . dot.

Starting M-Books

93

In other instances, you may not notice any prompt for Word information
during installation. This can mean that your computer found the Word
executable and template information and set up the associations automat-
ically; or it can mean that it ignored the M-book configuration completely.
In either eventuality, it is best, after installation, to type notebook - setup
from the Command Window. Follow the ensuing instructions, which will be
essentially the same as in the first possibility described in the last paragraph.

Starting M-Books

The most common way to start up the M-book interface is to type notebook
at the Command Window prompt. This is the only way to start the M-book
interface if it is your first foray into the venue. After you type notebook,
you will see Microsoft Word launch and a blank Word document will fill your
screen. We will refer to this document as an M-book. The difference between a
blank M-book and a normal Word document is only apparent if you peruse the
menu bar. There you will see an entry that is not present in a normal Word
document — namely, the Notebook menu. Click on it and examine the menu
items that appear. We will describe each of them and their functions in our
discussion below. If this is not your first experience with M-books, and you
have already saved an M-book, say under the name Probleml . doc, then you
can open it by typing notebook Probleml.doc at the Command Window
prompt. Even though you may not see it, the MATLAB Command Window is
alive, but it is hidden behind the M-book.

^ On some systems, you may see a DOS command window appear after
typing notebook, but before the M-book appears. We recommend
that you close that window before working in the M-book.

/ For M-books to work properly, you need to have “Macros Enabled” in your
Word installation. If an M-book opens as a regular Word document, without
M-book functionality, it probably means that macros have been disabled. To
enable them, first close the document (without saving changes), then go to
Tools : Macro : Security... from the Word menu bar, and reset your security
level to Medium or Low. Then reopen the M-book.

/ An alternate, and on some systems (especially networked systems) a
preferable, launch method is first to open a previously saved M-book —
either directly through File : Open... in Word or by double-clicking on the
file name in Windows Explorer. Word recognizes that the document is an
M-book, so automatically launches MATLAB if it is not already running. A

94

Chapter 6: M-Books

word of caution: If you have more than one version of MATLAB installed,
Word will launch the version you installed last. To override this, you can
open the MATLAB version you want before you open the M-book.

You can now type into the M-book in the usual way. In fact you could pre-
pare a document in this screen in precisely the same manner that you would
in a normal Word screen. The background features of MATLAB are only ac-
tivated if you do one of two things: either access the items in the Notebook
menu or press the key combination CTRL+ENTER. Type into your M-book the
line 23/45 and press ctrl+enter. After a short delay you will see what you
entered change font to bold New Courier, encased in brackets, and then the
output

0.5111

will appear below, also in New Courier font (but not bold). It is also likely that
the input and output will be colored (the input in green, the output in blue).
Your cursor should be on the line following the output, but if it is at the end
of the output line, move it down a line and type solve ( 'x~ 2 - 5*x + 5 =
0 ' ) followed by ctrl+enter. After some thought MATLAB feeds the answer
to the M-book:

ans=

[5/2 + l/2*5 A (1/2) ]

[5/2-l/2*5 /k (1/2) ]

Finally, try typing ezplot ( ' x~3 - x' ) , then CTRL+ENTER, and watch the
graph appear. At this point your M-book should look like Figure 6-1.

You may note that your commands take a little longer to evaluate than
they would inside a normal MATLAB Command Window. This is not sur-
prising considering the amount of information that is passing back and forth
between MATLAB and Word. Continue entering MATLAB commands that are
familiar to you (always followed by ctrl+enter), and observe that you obtain
the output you expect, except that it is formatted and integrated into your
M-book.

/ If you want to start a fresh M-book, click on File : New M-book in the Menu
Bar, or File : New, and then click on m-book . dot.

Working with M- Books

95

Working with M-Books

You interact with data in your M-book in two ways — via the keyboard or
through the menu bar.

Editing Input

Place your cursor in the line containing the second command of the previous
section — where we solved the quadratic equation

x 2 - 5x + 5 = 0.

Click to the left of the equal sign, hit backspace, type 6 (that is, replace the
second 5 by a 6), and press CTRL+ENTER. You will see your output replaced by

ans =

[ 2 ]

[ 3]

By changing the quadratic equation we have altered its roots. You can edit
any of the input lines in your M-book in this way, including the one that
generated the graph. See what happens if you click in the ezplot command
line, change the cubic expression, and press ctrl+enter.

96

Chapter 6: M-Books

It is important to understand that your M-book can be handled in exactly
the same way that you would any Word document. In particular, you can
save the file, print the document, change fonts or margins, move or export a
graphic, etc. This has the advantage of allowing you to present the results
of your MATLAB session in an attractively formatted style. It also has the
disadvantage of affording the user the opportunity to muck with MATLAB’s
input or output and so to create input and output that may not truly correspond
to each other. One must be very careful!

/ Note that the help item on the menu bar is Word help, not MATLAB help. If
you want to invoke MATLAB help, then either type help (with CTRL+ENTER
of course) or bring the MATLAB Command Window to the foreground (see
below) and use MATLAB help in the usual fashion.

The Notebook Menu

Next let’s examine the items in the Notebook menu. First comes Define
Input Cell. If you put your cursor on any line and select Define Input Cell,
then that line will become an input line. But to evaluate it, you still need to
press ctrl+enter. The advantage to this item is apparent when you want to
create an input cell containing more than one line. For example, type

syms x y

factor (x~2 - y~2)

and then select both lines (by clicking and dragging over them) and choose
Define Input Cell, ctrl+enter will then cause both lines to be evaluated. You
can recognize that both lines are incorporated into one input cell by looking at
the brackets, or Cell Markers. The menu item Hide Cell Markers will cause
the Cell Markers to disappear; in fact that menu item is a toggle switch that
turns the Markers on and off. If you have several input cells, you can convert
them into one input cell by selecting them and choosing Group Cells. You can
break them apart by choosing Ungroup Cells. If you click in an input cell
and choose Undefine Cells, that cell ceases to be an input cell; its formatting
reverts to the default Word format, as does the corresponding output cell. If
you “undefine” an output cell, it loses its format, but the corresponding input
cell remains unchanged.

If you select some portion of your M-book (for example, the entire M-book
by using Edit : Select All) and then choose Purge Output Cells, all output
cells in the selection will be deleted. This is particularly useful if you wish
to change some data on which the output in your selection depends, and then

M-Book Graphics

97

reevaluate the entire selection by choosing Evaluate Cell. You can reevaluate
the entire M-book at any time by choosing Evaluate M-book. If your M-book
contains a loop, you can evaluate it by selecting it and choosing Evaluate
Loop, or for that matter Evaluate Cell, provided the entire loop is inside a
single input cell.

It is often handy to purge all output from an M-book before saving, to
economize on storage space or on time upon reopening, especially if there
are complicated graphs in the document. If there are any input cells that you
want to automatically evaluate upon opening of the M-book, select them and
click on Define Auto Init Cell. The color of the text in those cells will change.
If you want to separate out a series of commands, say for repeated evaluation,
then select the cells and click on Define Calc Zone. The commands selected
will be encased in a Word section (with section breaks before and after it). If
you click in the section and select Evaluate Calc Zone from the Notebook
menu, the commands in only that zone will be (re)evaluated.

The last two buttons are also useful. The button Bring MATLAB to Front
does exactly that; it reveals the MATLAB Command Window that has been
hiding behind the M-book. You may want to enter a command directly into the
Command Window (for example, a help entry) and not have it in your M-book.
Finally, the last button, Notebook Options brings up a panel in which you
can do some customization of your M-book: set the numerical format, establish
the size of graphics figures, etc. We find it most useful to decrease the default
graphics size — the “factory setting” is generally too large. Decreasing the
figure size with Notebook Options may not work with Word 2000, though it
is still possible to change the size of figures one at a time, by right clicking on
the figure and then choosing the “Size” tab from Format Object....

M-Book Graphics

All MATLAB commands that generate graphics work in M-books. The figure
produced by a graphics command appears immediately below that command.
However, one must be a little careful in planning and executing graphics
statements. For example, if in an attempt to reproduce Figure 5-3, you type
ezcontour ( ' x~2 + y~2', [-3 3], [-3 3]) and CTRL+ENTER, this will

yield the level curves of x 2 +y 2 , but they will appear elliptical because you
forgot the command axis square. If you enter that command on the next
line, you will get a second picture that will be correct. But a much better
strategy — and one that we strongly recommend — is to return to the original
input cell and edit it by adding a semicolon (or a carriage return) and the axis

98

Chapter 6: M-Books

square statement. In general, as you refine your graphics in an M-book, you
will find it is more desirable to modify the input cells that generated them,
rather than to produce more pictures by repeating the command with new
options. So when adding things such as xlabel, ylabel, legend, title,
etc., it is usually best to just add them to the graphics input cell and reevalu-
ate. As a result, input cells generating graphics in M-books often end up being
several lines long.

In instances where you really do want to generate a new picture, then you
need to think about whether you want to have hold set to on or off. This
feature works exactly as in a Command Window — if hold is set to on, what-
ever graphic results from your next command will be combined with whatever
last graphic you produced; and if hold is off, then previous graphics will not
influence any graphic you generate.

Since there are no separate graphics windows, the command figure is of
limited use in M-books; you probably should not use it. If you do, it will produce
a blank graph. Similarly, there are other graphics commands that are not so
suitable for use in M-books, for example close.

/ There is one exception to this rule: Sometimes you might want to use a
figure window along with an M-book, for example to rotate a plot with the
mouse. If you type figure from the Command Window to open a figure
window, then subsequent graphics from the M-book will appear
simultaneously in the figure window and in the M-book itself.

Finally, we note the button Toggle Graph Output for Cell, the only button
on the Notebook menu not previously described. If you select a cell contain-
ing a graphics command and click on this button, no graphical output will
result from the evaluation of this command. This can be useful when used
in conjunction with hold on if you want to produce a single graphic using
multiple command lines.

More Hints for Effective Use of M-Books

If an interactive mode and/or attractive output beyond what you can achieve
with M-files and diary files is your goal, then you should get used to working in
the M-book interface rather than in a Command Window. Even experienced
MATLAB users will find that in time they will get use to the environment.
Here are a few more hints to smooth your transition.

In Chapter 3 we outlined some strategies for effective use of M-files, es-
pecially in the realm of debugging. Many of the techniques we described are

A Warning

99

unnecessary in the M-book mode. For example, the commands pause and
keyboard serve no purpose. In addition the UP- and DOWN-ARROW keys on the
keyboard cannot be used as they are in a Command Window. Those keys cause
your cursor to travel in the Word screen rather than to scroll through previous
input commands. For navigating in the M-book, you will likely find the scroll
bar and the mouse to be more useful than the arrow keys.

You may want to run script or function M-files in an M-book. You still must
take care of path business as you do in a Command Window. But assuming you
have done so, M-files are executed in an M-book exactly as in a Command Win-
dow. You invoke them simply by typing their name and pressing CTRL+ENTER.
The outputs they generate, both intermediate and final, are determined as
before. In particular, semicolons at ends of lines are important; the command
echo works as before; and so do loops. One thing that does not work so well
is the command more. We have found that, even if more on is executed, help
commands that run on for more than a page do not come out staggered in an
M-book. Thus you may want to bring MATLAB to the foreground and enter
your help requests in the Command Window.

Another standard MATLAB feature that does not work so well in M-books is
the . . . construct for continuing a long command entry on a second line. Word
automatically converts three dots into a single special ellipsis character and
so confuses MATLAB. There are two ways around this difficulty. Either do not
use ellipses (rather simply continue typing and allow Word to wrap as usual —
the command will be interpreted properly when passed to MATLAB) or turn off
the “Auto Correct” feature of Word that converts the three dots into an ellipsis.
This is most easily done by typing ctrl+z after the three dots. Alternatively,
open Tools : Auto Correct... and change the settings that appear there.

One final comment is in order. Another reason to bring MATLAB to the
foreground is if you want to use the Current Directory browser, Workspace
browser, or Editor/Debugger. The relevant icons on the tool bar or buttons on
the menu bar can only be found in the MATLAB Desktop, not in the Word
screen. However, you can also type pathtool, workspace, or edit directly
into the M-book, followed by ctrl+enter of course.

A Warning

The ellipsis difficulty described in the last section is not an isolated difficulty.
The various kinds of automatic formatting that Word carries out can truly
confuse MATLAB. Several such instances that we find particularly annoying
are: fractions (1/2 is converted to a single character V 2 representing one-half);

Chapter 6: M-Books

the character combination a construct often used when specifying the rows
of a matrix, which Word converts to a “smiley face”©; and various dashes that
wreak havoc with MATLAB’s attempts to interpret an ordinary hyphen as a
minus sign. Examine these in Tools : Auto Correct... and, if you use M-books
regularly, consider turning them off.

A more insidious problem is the following. If you cut and paste character
strings into an input cell, the characters in the original font may be converted
into something you don’t anticipate in the Courier input cell. Mysterious and
unfathomable error messages upon execution are a tip-off to this problem. In
general, you should not copy cells for evaluation unless it is from a cell that
has already been evaluated successfully — it is safer to type in the line anew.

Finally, we have seen instances in which a cell, for no discernible reason,
fails to evaluate. If this happens, try typing CTRL+ENTER again. If that fails, you
may have to delete and retype the cell. We have also occasionally experienced
the following problem: Reevaluation of a cell causes its output to appear in an
unpredictable place elsewhere in the M-book — sometimes even obliterating
unrelated output in that locale. If that happens, click on the Undo button on
the Word tool bar, retype the input cell before evaluating, and delete the old
input cell.

Chapter 7

MATLAB Programming

Every time you create an M-file, you are writing a computer program using
the MATLAB programming language. You can do quite a lot in MATLAB
using no more than the most basic programming techniques that we have
already introduced. In particular, we discussed simple loops (using for) and
a rudimentary approach to debugging in Chapter 3. In this chapter, we will
cover some further programming commands and techniques that are useful
for attacking more complicated problems with MATLAB. If you are already
familiar with another programming language, much of this material will be
quite easy for you to pick up!

/ Many MATLAB commands are themselves M-files, which you can examine
using type or edit (for example, enter type isprime to see the M-file for
the command isprime). You can learn a lot about MATLAB programming
techniques by inspecting the built-in M-files.

Branching

For many user-defined functions, you can use a function M-file that executes
the same sequence of commands for each input. However, one often wants a
function to perform a different sequence of commands in different cases, de-
pending on the input. You can accomplish this with a branching command, and
as in many other programming languages, branching in MATLAB is usually
done with the command if, which we will discuss now. Later we will describe
the other main branching command, switch.

101

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Chapter 7: MATLAB Programming

Branching with if

For a simple illustration of branching with if, consider the following function
M-file absval .m, which computes the absolute value of a real number:

function y = absval (x)
if x >= 0
y = x;

else

y = -x;

end

The first line of this M-file states that the function has a single input x and
a single output y. If the input x is nonnegative, the if statement is deter-
mined by MATLAB to be true. Then the command between the if and the
else statements is executed to set y equal to x, while MATLAB skips the
command between the else and end statements. However, if x is negative,
then MATLAB skips to the else statement and executes the succeeding com-
mand, setting y equal to -x. As with a for loop, the indentation of commands
above is optional; it is helpful to the human reader and is done automatically
by MATLAB’s built-in Editor/Debugger.

/ Most of the examples in this chapter will give peculiar results if their input
is of a different type than intended. The M-file absval . m is designed only
for scalar real inputs x, not for complex numbers or vectors. If x is complex
for instance, then x >= 0 checks only if the real part of x is nonnegative,
and the output y will be complex in either case. MATLAB has a built-in
function abs that works correctly for vectors of complex numbers.

In general, if must be followed on the same line by an expression that
MATLAB will test to be true or false; see the section below on Logical Expres-
sions for a discussion of allowable expressions and how they are evaluated.
After some intervening commands, there must be (as with for) a correspond-
ing end statement. In between, there may be one or more el seif state-
ments (see below) and/or an else statement (as above). If the test is true,
MATLAB executes all commands between the if statement and the first
elseif, else, or end statement and then skips all other commands un-
til after the end statement. If the test is false, MATLAB skips to the first
elseif, else, or end statement and proceeds from there, making a new test
in the case of an elseif statement. In the example below, we reformulate
absval . m so that no commands are necessary if the test is false, eliminating
the need for an else statement.

Branching

103

function y = absval (x)
y = x;
if y < 0

y = -y;

end

The el seif statement is useful if there are more than two alternatives
and they can be distinguished by a sequence of true/false tests. It is essen-
tially equivalent to an else statement followed immediately by a nested if
statement. In the example below, we use elsei f in an M-file signum . m, which
evaluates the mathematical function

[ 1 x > 0,
sgn(x) =| 0 x = 0,

[-1 x < 0 .

(Again, MATLAB has a built-in function sign that performs this function for
more general inputs than we consider here.)

function y = signum (x)
if x > 0
y = 1;

elseif x == 0
y = 0;
else

y = -1;

end

Here if the input x is positive, then the output y is set to 1 and all commands
from the elseif statement to the end statement are skipped. (In particular,
the test in the elseif statement is not performed.) If x is not positive, then
MATLAB skips to the elseif statement and tests to see if x equals 0 . If so, y is
set to 0; otherwise y is set to - 1. Notice that MATLAB requires a double equal
sign = = to test for equality; a single equal sign is reserved for the assignment
of values to variables.

/ Like for and the other programming commands you will encounter, if and
its associated commands can be used in the Command Window. Doing so can
be useful for practice with these commands, but they are intended mainly for
use in M-files. In our discussion of branching, we consider primarily the case
of function M-files; branching is less often used in script M-files.

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Chapter 7: MATLAB Programming

Logical Expressions

In the examples above, we used relational operators such as >=, >, and ==
to form a logical expression, and we instructed MATLAB to choose between
different commands according to whether the expression is true or false. Type
help relop to see all of the available relational operators. Some of these
operators, such as & (AND) and | (OR), can be used to form logical expressions
that are more complicated than those that simply compare two numbers. For
example, the expression (x > 0) | (y > 0 ) will be true if x or y (or both)

is positive, and false if neither is positive. In this particular example, the
parentheses are not necessary, but generally compound logical expressions
like this are both easier to read and less prone to errors if parentheses are
used to avoid ambiguities.

Thus far in our discussion of branching, we have only considered expressions
that can be evaluated as true or false. While such expressions are sufficient
for many purposes, you can also follow if or elseif with any expression
that MATLAB can evaluate numerically. In fact, MATLAB makes almost no
distinction between logical expressions and ordinary numerical expressions.
Consider what happens if you type a logical expression by itself in the Com-
mand Window:

» 2 > 3

0

When evaluating a logical expression, MATLAB assigns it a value of 0 (for
FALSE) or 1 (for TRUE). Thus if you type 2 < 3, the answer is 1. The rela-
tional operators are treated by MATLAB like arithmetic operators, inasmuch
as their output is numeric.

/ MATLAB makes a subtle distinction between the output of relational
operators and ordinary numbers. For example, if you type whos after the
command above, you will see that ans is a logical array. We will give an
example of how this feature can be used shortly. Type help logical for
more information.

Here is another example:

» 2 | 3

ans

Branching

105

The OR operator | gives the answer 0 if both operands are zero and 1 other-
wise. Thus while the output of relational operators is always 0 or 1, any
nonzero input to operators such as & (AND), | (OR), and - (NOT) is regarded
by MATLAB to be true, while only 0 is regarded to be false.

If the inputs to a relational operator are vectors or matrices rather than
scalars, then as for arithmetic operations such as + and . *, the operation is
done term-by-term and the output is an array of zeros and ones. Here are some
examples:

» [2 3] < [3 2]

ans =

1 0

>> x = -2 :2; x >= 0

ans =

0 0 111

In the second case, x is compared term-by-term to the scalar 0. Type help
relop or more information.

You can use the fact that the output of a relational operator is a logical array
to select the elements of an array that meet a certain condition. For example,
the expression x (x >= 0) yields a vector consisting of only the nonnegative
elements of x (or more precisely, those with nonzero real part). So, if x = -2:2
as above,

>> x (x >= 0)

ans =

0 12

If a logical array is used to choose elements from another array, the two arrays
must have the same size. The elements corresponding to the ones in the logical
array are selected while the elements corresponding to the zeros are not. In
the example above, the result is the same as if we had typed x (3 : 5) , but in
this case 3 : 5 is an ordinary numerical array specifying the numerical indices
of the elements to choose.

Next, we discuss how if and el seif decide whether an expression is true
or false. For an expression that evaluates to a scalar real number, the criterion
is the same as described above — namely, a nonzero number is treated as true
while 0 is treated as false. However, for complex numbers only the real part
is considered. Thus, in an if or el seif statement, any number with nonzero

Chapter 7: MATLAB Programming

real part is treated as true, while numbers with zero real part are treated as
false. Furthermore, if the expression evaluates to a vector or matrix, an i f
or el seif statement must still result in a single true-or-false decision. The
convention MATLAB uses is that all elements must be true (i.e., all elements
must have nonzero real part) for an expression to be treated as true. If any
element has zero real part, then the expression is treated as false.

You can manipulate the way branching is done with vector input by in-
verting tests with ~ and using the commands any and all. For example, the
statements if x == 0; . . . ; end will execute a block of commands (rep-
resented here by • • •) when all the elements of x are zero; if you would like
to execute a block of commands when any of the elements of x is zero you
could use the form if x ~= 0; else; end. Here -= is the relational

operator for “does not equal”, so the test fails when any element of x is zero,
and execution skips past the else statement. You can achieve the same effect
in a more straightforward manner using any, which outputs true when any
element of an array is nonzero: if any (x == 0) ; . . . ; end (remember
that if any element of x is zero, the corresponding element of x == 0 is
nonzero). Likewise all outputs true when all elements of an array are
nonzero.

Here is a series of examples to illustrate some of the features of logical
expressions and branching that we have just described. Suppose you want to
create a function M-file that computes the following function:

f( s _ J sin(x)/x x ^ 0,
nX) ~\ 1 x = 0.

You could construct the M-file as follows:

function y = f (x)

if x == 0
y = 1;

else

y = sin (x) /x;

end

This will work fine if the input x is a scalar, but not if x is a vector or matrix.
Of course you could change / to . / in the second definition of y, and change
the first definition to make y the same size as x. But if x has both zero and
nonzero elements, then MATLAB will declare the i f statement to be false and
use the second definition. Then some of the entries in the output array y will
be NaN, “not a number,” because 0/0 is an indeterminate form.

Branching

107

One way to make this M-file work for vectors and matrices is to use a loop
to evaluate the function element-by-element, with an i f statement inside the
loop:

function y = f (x)
y = ones (size (x) ) ;
for n = 1 :prod (size (x) )
if x (n) -= 0

y (n) = sin (x (n) ) /x (n) ;

end

end

In the M-file above, we first create the eventual output y as an array of ones
with the same size as the input x. Here we use size (x) to determine the
number of rows and columns of x; recall that MATLAB treats a scalar or a
vector as an array with one row and/or one column. Then prod (size (x) )
yields the number of elements in x. So in the for statement n varies from 1
to this number. For each element x (n) , we check to see if it is nonzero, and
if so we redefine the corresponding element y (n) accordingly. (If x (n) equals
0, there is no need to redefine y (n) since we defined it initially to be 1.)

/ We just used an important but subtle feature of MATLAB, namely that

each element of a matrix can be referred to with a single index; for example,
if x is a 3 x 2 array then its elements can be enumerated as x ( 1 ) , x ( 2 ) , . . . ,
x ( 6 ) . In this way, we avoided using a loop within a loop. Similarly, we could
use length (x ( : ) ) in place of prod (size (x) ) to count the total number of
entries in x. However, one has to be careful. If we had not predefined y to have
the same size as x, but rather used an else statement inside the loop to let
y (n) be 1 when x (n) is 0, then y would have ended up a 1 x 6 array rather
than a 3 x 2 array. We then could have used the command y = reshape (y,
size (x) ) at the end of the M-file to make y have the same shape
as x. However, even if the shape of the output array is not important, it is
generally best to predefine an array of the appropriate size before computing
it element-by-element in a loop, because the loop will then run faster.

Next, consider the following modification of the M-file above:

function y = f (x)
if x -= 0

y = sin (x) ./x;
return

end

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Chapter 7: MATLAB Programming

y = ones (size (x) ) ;
for n = 1 :prod (size (x) )
if x (n) ~= 0

y(n) = sin (x (n) ) /x (n) ;

end

end

Above the loop we added a block of four lines whose purpose is to make the
M-file run faster if all the elements of the input x are nonzero. The difference
in running time can be significant (more than a factor of 10) if x has a large
number of elements. Here is how the new block of four lines works. The first i f
statement will be true provided all the elements of x are nonzero. In this case,
we define the output y using MATLAB’s vector operations, which are generally
much more efficient than running a loop. Then we use the command return
to stop execution of the M-file without running any further commands. (The
use of return here is a matter of style; we could instead have indented all of
the remaining commands and put them between else and end statements.)
If, however, x has some zero elements, then the if statement is false and the
M-file skips ahead to the commands after the next end statement.

Often you can avoid the use of loops and branching commands entirely by
using logical arrays. Here is another function M-file that performs the same
task as in the previous examples; it has the advantage of being more concise
and more efficient to run than the previous M-files, since it avoids a loop in
all cases:

function y = f (x)
y = ones (size (x) ) ;
n = (x ~= 0);
y (n) = sin(x(n))./x(n);

Here n is a logical array of the same size as x with a 1 in each place where x has
a nonzero element and zeros elsewhere. Thus the line that defines y (n) only
redefines the elements of y corresponding to nonzero values of x and leaves
the other elements equal to 1. If you try each of these M-files with an array of
about 100,000 elements, you will see the advantage of avoiding a loop!

Branching with switch

The other main branching command is switch. It allows you to branch among
several cases just as easily as between two cases, though the cases must be de-
scribed through equalities rather than inequalities. Here is a simple example,
which distinguishes between three cases for the input:

More about Loops

109

function y = count (x)
switch x
case 1

y = ' one ' ;
case 2

y = ' two ' ;
otherwise

y = 'many' ;

end

Here the switch statement evaluates the input x and then execution of the
M-file skips to whichever case statement has the same value. Thus if the
input x equals 1, then the output y is set to be the string ' one ' , while if x is
2 , then y is set to ' two ' . In each case, once MATLAB encounters another case
statement or since an otherwise statement, it skips to the end statement,
so that at most one case is executed. If no match is found among the case
statements, then MATLAB skips to the (optional) otherwise statement, or
else to the end statement. In the example above, because of the otherwise
statement, the output is 'many' if the input is not 1 or 2.

Unlike if, the command switch does not allow vector expressions, but it
does allow strings. Type help switch to see an example using strings. This
feature can be useful if you want to design a function M-file that uses a string
input argument to select among several different variants of a program you
write.

y Though strings cannot be compared with relational operators such as ==
(unless they happen to have the same length), you can compare strings in an
if or elseif statement by using the command strcmp. Type help strcmp
to see how this command works; for an example of its use in conjunction
with if and elseif, enter type hold.

More about Loops

In Chapter 3 we introduced the command for, which begins a loop — a
sequence of commands to be executed multiple times. When you use for,
you effectively specify the number of times to run the loop in advance (though
this number may depend for instance on the input to a function M-file). Some-
times you may want to keep running the commands in a loop until a certain
condition is met, without deciding in advance on the number of iterations. In
MATLAB, the command that allows you to do so is while.

no

Chapter 7: MATLAB Programming

^ Using while, one can easily end up accidentally creating an “infinite
loop”, one that will keep running indefinitely because the condition
you set is never met. Remember that you can generally interrupt
the execution of such a loop by typing CTRL+C; otherwise, you may
have to shut down MATLAB.

Open-Ended Loops

Here is a simple example of a script M-file that uses while to numerically

sum the infinite series l/l 4 + 1/2 4 + 1/3 4 H , stopping only when the terms

become so small (compared to the machine precision) that the numerical sum
stops changing:

n = 1;

oldsum = -1;
newsum = 0;
while newsum > oldsum
oldsum = newsum;
newsum = newsum + n~(-4);
n = n + 1;

end

newsum

Here we initialize newsum to 0 and n to 1, and in the loop we successively
add n~ (-4) to newsum, add 1 to n, and repeat. The purpose of the variable
oldsum is to keep track of how much newsum changes from one iteration
to the next. Each time MATLAB reaches the end of the loop, it starts over
again at the while statement. If newsum exceeds oldsum, the expression in
the while statement is true, and the loop is executed again. But the first
time the expression is false, which will happen when newsum and oldsum are
equal, MATLAB skips to the end statement and executes the next line, which
displays the final value of newsum (the result is 1.0823 to five significant
digits). The initial value of - 1 that we gave to oldsum is somewhat arbitrary,
but it must be negative so that the first time the while statement is executed,
the expression therein is true; if we set oldsum to 0 initially, then MATLAB
would skip to the end statement without ever running the commands in the
loop.

/ Even though you can construct an M-file like the one above without deciding
exactly how many times to run the loop, it may be useful to consider roughly
how many times it will need to run. Since the floating point computations on

More about Loops

111

most computers are accurate to about 16 decimal digits, the loop above
should run until n~ ( -4) is about 10 ~ ( - 16 ) , that is, until n is about 10 "4.
Thus the computation will take very little time on most computers. However,
if the exponent were 2 and not 4, the computation would take about 10~8
operations, which would take a long time on most (current) computers —
long enough to make it wiser for you to find a more efficient way to sum the
series, for example using symsum if you have the Symbolic Math Toolbox!

Though we have classified it here as a looping command, while also has
features of a branching command. Indeed, the types of expressions allowed
and the method of evaluation for a while statement are exactly the same as
for an if statement. See the section Logical Expressions above for a
discussion of the possible expressions you can put in a while statement.

Breaking from a Loop

Sometimes you may want MATLAB to jump out of a for loop prematurely,
for example if a certain condition is met. Or, in a while loop, there may be an
auxiliary condition that you want to check in addition to the main condition
in the while statement. Inside either type of loop, you can use the command
break to tell MATLAB to stop running the loop and skip to the next line after
the end of the loop. The command break is generally used in conjunction with
an i f statement. The following script M-file computes the same sum as in the
previous example, except that it places an explicit upper limit on the number
of iterations:

newsum = 0;

for n = 1:100000

oldsum = newsum;
newsum = newsum + n~(-4);
if newsum == oldsum
break

end

end newsum

In this example, the loop stops after n reaches 100000 or when the variable
newsum stops changing, whichever comes first. Notice that break ignores
the end statement associated with if and skips ahead past the nearest end
statement associated with a loop command, in this case for.

112 Chapter 7: MATLAB Programming

Other Programming Commands

In this section we describe several more advanced programming commands
and techniques.

Subfunctions

In addition to appearing on the first line of a function M-file, the command
function can be used later in the M-file to define an auxiliary function, or
subfunction, which can be used anywhere within the M-file but will not be
accessible directly from the command line. For example, the following M-file
sums the cube roots of a vector x of real numbers:

function y = sumcuberoots (x)
y = sum(cuberoot (x) ) ;

% Subfunction starts here.

function z = cuberoot(x)
z = sign (x) . *abs (x) . ~ (1/3) ;

Here the subfunction cuberoot takes the cube root of x element-by-element,
but it cannot be used from the command line unless placed in a separate M-file.
You can only use subfunctions in a function M-file, not in a script M-file. For
examples of the use of subfunctions, you can examine many of MATLAB’s built-
in function M-files. For example, type ezplot will display three different
subfunctions.

Commands for Parsing Input and Output

You may have noticed that many MATLAB functions allow you to vary the
type and/or the number of arguments you give as input to the function. You
can use the commands nargin, nargout, varargin, and varargout in your
own M-files to handle variable numbers of input and/or output arguments,
whereas to treat different types of input arguments differently you can use
commands such as isnumeric and ischar.

When a function M-file is executed, the functions nargin and nargout re-
port respectively the number of input and output arguments that were speci-
fied on the command line. To illustrate the use of nargin, consider the follow-
ing M-file add . m that adds either 2 or 3 inputs:

function s = add(x, y, z)
if nargin < 2

Other Programming Commands

113

error ('At least two input arguments are required.')

end

if nargin == 2
s = x + y;

else

s = x + y + z;

end

First the M-file checks to see if fewer than 2 input arguments were given, and
if so it prints an error message and quits. (See the next section for more about
error and related commands.) Since MATLAB automatically checks to see if
there are more arguments than specified on the first line of the M-file, there is
no need to do so within the M-file. If the M-file reaches the second i f statement
in the M-file above, we know there are either 2 or 3 input arguments; the if
statement selects the proper course of action in either case. If you type, for
instance, add (4,5) at the command line, then within the M-file, x is set to
4, y is set to 5, and z is left undefined; thus it is important to use nargin to
avoid referring to z in cases where it is undefined.

To allow a greater number of possible inputs to add . m, we could add ad-
ditional arguments on the first line of the M-file and add more cases for
nargin. A better way to do this is to use the specially named input argument
varargin:

function s = add (varargin)

s = sum( [varargin{ : }] ) ;

In this example, all of the input arguments are assigned to the cell array
varargin. The expression varargin{ : } returns a comma-separated list of
the input arguments. In the example above, we convert this list to a vector by
enclosing it in square brackets, forming suitable input for sum.

The sample M-files above assume their input arguments are numeric and
will attempt to add them even if they are not. This may be desirable in some
cases; for instance, both M-files above will correctly add a mixture of numeric
and symbolic inputs. However, if some of the input arguments are strings,
the result will be either an essentially meaningless numerical answer or an
error message that may be difficult to decipher. MATLAB has a number of
test functions that you can use to make an M-file treat different types of input
arguments differently — either to perform different calculations or to produce
a helpful error message if an input is of an unexpected type. For a list of
some of these test functions, look up the commands beginning with is in the
Programming Commands section of the Glossary.

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Chapter 7: MATLAB Programming

As an example, here we use isnumeric in the M-file add.m to print an
error message if any of the inputs are not numeric:

function s = add (varargin)
if -isnumeric ( [varargin{ :}] )

error (' Inputs must be floating point numbers.')

end

s = sum( [varargin{ : }] ) ;

When a function M-file allows multiple output arguments, then if fewer out-
put arguments are specified when the function is called, the remaining outputs
are simply not assigned. Recall that if no output arguments are explicitly spec-
ified on the command line, then a single output is returned and assigned to
the variable ans. For example, consider the following M-file rectangular . m
that changes coordinates from polar to rectangular:

function [x, y] = rectangular (r, theta)
x = r. *cos (theta) ;
y = r. *sin (theta) ;

If you type rectangular (2 , 1) at the command line, then the answer will
be just the x coordinate of the point with polar coordinates (2, 1). The following
modification to rectangular . m adjusts the output in this case to be a complex
number x + iy containing both coordinates:

function [x, y] = rectangular (r, theta)
x = r. *cos (theta) ;
y = r. *sin (theta) ;
if nargout < 2
x = x + i*y;

end

See the online help for varargout and the functions described above for ad-
ditional information and examples.

User Input and Screen Output

In the previous section we used error to print a message to the screen and
then terminate execution of an M-file. You can also print messages to the
screen without stopping execution of the M-file with disp or warning. Not
surprisingly, warning is intended to be used for warning messages, when
the M-file detects a problem that might affect the validity of its result but is
not necessarily serious. You can suppress warning messages, either from the

Other Programming Commands 1 15

command prompt or within an M-file, with the command warning of f. There
are several other options for how MATLAB should handle warning messages;
type help warning for details.

In Chapter 4 we used disp to display the output of a command without
printing the “ans =” line. You can also use disp to display informational
messages on the screen while an M-file is running, or to combine numerical
output with a message on the same line. For example, the commands

x = 2 + 2; disp (['The answer is ' num2str(x)

will set x equal to 4 and then print The answer is 4 .

MATLAB also has several commands to solicit input from the user run-
ning an M-file. At the end of Chapter 3 we discussed three of them: pause,
keyboard, and input. Briefly, pause simply pauses execution of an M-file
until the user hits a key, while keyboard both pauses and gives the user a
prompt to use like the regular command line. Typing return continues ex-
ecuting the M-file. Lastly, input displays a message and allows the user to
enter input for the program on a single line. For example, in a program that
makes successive approximations to an answer until some accuracy goal is
met, you could add the following lines to be executed after a large number of
steps have been taken:

answer = input ([' Algorithm is converging slowly; ...
'continue (yes/no)? '], 's');

if -isequal (answer, 'yes')
return

end

Here the second argument ' s ' to input directs MATLAB not to evaluate
the answer typed by the user, just to assign it as a character string to the
variable answer. We use isequal to compare the answer to the string ' yes '
because == can only be used to compare arrays (in this case strings) of the
same length. In this case we decided that if the user types anything but the
full word yes, the M-file should terminate. Other approaches would be to
only compare the first letter answer (1) to 'y' , to stop only if the answer is
' no ' , etc.

If a figure window is open, you can use ginput to get the coordinates of a
point that the user selects with the mouse. As an example, the following M-file
prints an “X” where the user clicks:

function xmarksthespot

if isempty (get (0, ' CurrentFigure' ) )

116

Chapter 7: MATLAB Programming

error ('No current figure.')

end

flag = -ishold;

if flag

hold on

end

disp(' Click on the point where you want to plot an X.')

[x, y] = ginput (1) ;

plot (x, y, 'xk')

if flag

hold off

end

First the M-file checks to see if there is a current figure window. If so, it
proceeds to set the variable flag to 1 if hold off is in effect and 0 if hold
on is in effect. The reason for this is that we need hold on in effect to plot
an “X” without erasing the figure, but afterward we want to restore the figure
window to whichever state it was in before the M-file was executed. The M-file
then displays a message telling the user what to do, gets the coordinates of the
point selected with ginput (1) , and plots a black “X” at those coordinates.
The argument 1 to ginput means to get the coordinates of a single point;
using ginput with no input argument would collect coordinates of several
points, stopping only when the user presses the ENTER key.

In the next chapter we describe how to create a GUI (Graphical User Inter-
face) within MATLAB to allow more sophisticated user interaction.

Evaluation

The commands eval and feval allow you to run a command that is stored
in a string as if you had typed the string on the command line. If the entire
command you want to run is contained in a string str, then you can exe-
cute it with eval (str) . For example, typing eval ( ' cos (1) ' ) will produce
the same result as typing cos (1) . Generally eval is used in an M-file that
uses variables to form a string containing a command; see the online help for
examples.

You can use feval on a function handle or on a string containing the name
of a function you want to execute. For example, typing feval ( ' atan2 ' , 1 ,
0) or feval (@atan2 , 1 , 0) is equivalent to typing atan2 (1, 0). Often
feval is used to allow the user of an M-file to input the name of a function
to use in a computation. The following M-file iterate . m takes the name of a

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117

function and an initial value and iterates the function a specified number of
times:

function final = iterate (func, init, num)
final = init;
for k = l:num

final = feval(func, final);

end

Typing iterate (' cos ' , 1, 2) yields the numerical value of cos(cos(l)),
while iterate ( ' cos ' , 1 , 100 ) yields an approximation to the real num-
ber x for which cos(x) = x. (Think about it!) Most MATLAB commands that
take a function name argument use feval, and as with all these commands,
if you give the name of an inline function to feval, you should not enclose it
in quotes.

Debugging

In Chapter 3 we discussed some rudimentary debugging procedures. One
suggestion was to insert the command keyboard into an M-file, for instance
right before the line where an error occurs, so that you can examine the
Workspace of the M-file at that point in its execution. A more effective and flex-
ible way to do this kind of debugging is to use dbstop and related commands.
With dbstop you can set a breakpoint in an M-file in a number of ways, for
example, at a specific line number, or whenever an error occurs. Type help
dbstop for a list of available options.

When a breakpoint is reached, a prompt beginning with the letter K will
appear in the Command Window, just as if keyboard were inserted in the
M-file at the breakpoint. In addition, the location of the breakpoint is high-
lighted with an arrow in the Editor/Debugger (which is opened automatically
if you were not already editing the M-file). At this point you can examine in
the Command Window the variables used in the M-file, set another breakpoint
with dbstop, clear breakpoints with dbclear, etc. If you are ready to continue
running the M-file, type dbcont to continue or dbstep to step through the file
line-by-line. You can also stop execution of the M-file and return immediately
to the usual command prompt with dbquit.

You can also perform all the command-line functions that we described
in this section with the mouse and/or keyboard shortcuts in the
Editor / Debugger See the section Debugging Techniques in Chapter 11 for
more about debugging commands and features of the Editor /Debugger.

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Chapter 7: MATLAB Programming

Interacting with the Operating System

This section is somewhat advanced, as is the following chapter. On a first
reading, you might want to skip ahead to Chapter 9.

Calling External Programs

MATLAB allows you to run other programs on your computer from its com-
mand line. If you want to enter UNIX or DOS file manipulation commands,
you can use this feature as a convenience to avoid opening a separate window.
Or you may want to use MATLAB to graph the output of a program writ-
ten in a language such as FORTRAN or C. For large-scale computations, you
may wish to combine routines written in another programming language with
routines you write in MATLAB.

The simplest way to run an external program is to type an exclamation point
at the beginning of a line, followed by the operating system command you want
to run. For example, typing ! dir on a Windows system or ! Is - 1 on a UNIX
system will generate a more detailed listing of the files in the current working
directory than the MATLAB command dir. In Chapter 3 we described dir and
other MATLAB commands, such as cd, delete, pwd, and type, that mimic
similar commands from the operating system. However, for certain operations
(such as renaming a file) you may need to run an appropriate command from
the operating system.

/ If you use the operating system interface in an M-file that you want to run
on either a Windows or UNIX system, you should use the test functions
ispc and/or isunix to set off the appropriate commands for each type of
system, for example, i f isunix; . ..; else; . ..; end. If you need to
distinguish between different versions of UNIX (Linux, Solaris, etc.), you can
use computer instead of isunix.

The output from an operating system command preceded by ! can only be
displayed to the screen. To assign the output of an operating system com-
mand to a variable, you must use dos or unix. Though each is only docu-
mented to work for its respective operating system, in current versions of MAT-
LAB they work interchangeably. For example, if you type [stat, data] =
dos ( ' myprog 0.5 1000'), the program myprog will be run with command
line arguments 0.5 and 1000 and its “standard output” (which would nor-
mally appear on the screen) will be saved as a string in the variable data. (The
variable stat will contain the exit status of the program you run, normally 0

interacting with the Operating System

119

if the program runs without error.) If the output of your program consists only
of numbers, then str2num(data) will yield a row vector containing those
numbers. You can also use sscanf to extract numbers from the string data;
type help sscanf for details.

^ A program you run with ! , dos, or unix must be in the current
directory or elsewhere in the path your system searches for
executable files; the MATLAB path will not be searched.

If you are creating a program that will require extensive communication
between MATLAB and an external FORTRAN or C program, then compiling
the external program as a MEX file will be more efficient than using dos or
unix. To do so, you must write some special instructions into the external
program and compile the program from within MATLAB using the command
mex. This will result in a file with the extension . mex that you can run from
within MATLAB just as you would run an M-file. The advantage is that a
compiled program will generally run much faster than an M-file, especially
when loops are involved.

The instructions you need to write into your program to compile it with mex
are described in MATLAB : Using MATLAB : External Interfaces/API in
the Help Browser and in the “MEX, API, & Compilers” section of the web site

http : / /www . mathworks . com/ support/tech-notes

Look for the page entitled “Is there a tutorial for creating MEX-files with
emphasis on C MEX-files?” The instructions depend to some extent on whether
your program is written in FORTRAN or C, but they are not hard to learn if
you already know one of these languages. MATLAB version 6 also provides the
MATLAB Java Interface, which enables you to create and access Java objects
from within MATLAB.

File Input and Output

In Chapter 3 we discussed how to use save and load to transfer variables
between the Workspace and a disk file. By default the variables are written and
read in MATLAB’s own binary format, which is signified by the file extension
.mat. You can also read and write text files, which can be useful for sharing
data with other programs. With save you type -ascii at the end of the line
to save numbers as text rounded to 8 digits, or -ascii -double for 16-digit
accuracy. With load the data are assumed to be in text format if the file name
does not end in . mat. This provides an alternative to importing data with dos

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or unix in case you have previously run an external program and saved the
results in a file.

/ MATLAB 6 also offers an interactive tool called the Import Wizard to read
data from files (or the system clipboard) in different formats; to start it type
ui import (optionally followed by a file name) or select File : Import
Data....

For more control over file input and output — for example to annotate
numeric output with text — you can use fopen, fprintf, and related com-
mands. MATLAB also has commands to read and write graphics and sound
files. Type help iofun for an overview of input and output functions.

Chapter 8

SIMULINK and GUIs

In this chapter we describe SIMULINK, a MATLAB accessory for simulat-
ing dynamical processes, and GUIDE, a built-in tool for creating your own
graphical user interfaces. These brief introductions are not comprehensive,
but together with the online documentation they should be enough to get you
started.

SIMULINK

If you want to learn about SIMULINK in depth, you can read the massive PDF
document SIMULINK: Dynamic System Simulation for MATLAB that comes
with the software. Here we give a brief introduction for the casual user who
wants to get going with SIMULINK quickly. You start SIMULINK by double-
clicking on SIMULINK in the Launch Pad, by clicking on the SIMULINK
button on the MATLAB Desktop tool bar, or simply by typing simulink in
the Command Window. This opens the SIMULINK library window, which is
shown for UNIX systems in Figure 8-1. On Windows systems, you see instead
the SIMULINK Library Browser, shown in Figure 8-2.

To begin to use SIMULINK, click New : Model from the File menu. This
opens a blank model window. You create a SIMULINK model by copying units,
called blocks, from the various SIMULINK libraries into the model window.
We will explain how to use this procedure to model the homogeneous linear
ordinary differential equation u" + 2 u! + 5u = 0, which represents a damped
harmonic oscillator.

First we have to figure out how to represent the equation in a way that
SIMULINK can understand. One way to do this is as follows. Since the time
variable is continuous, we start by opening the “Continuous” library, in UNIX

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Chapter 8: SIMULINK and GUIs

Library: simulink3

■ [j Simulink Library Browser

D G? -W Find |

Continuous: simulink3/Continuous

1 0 M Simulink

|k| Continuous 1

JN

^1 Disc,ete

±tj Functions 8, Tables

| fej Math 0

Nonlinear

Jt

Discrete

Functions 1

ik) Signals & Systems 1 ®

Sinks

Sources g

* -
■r *

Math

g ffej Simulink Extras

fj|! Stateflow g

n

Nonlinear

ji r

Signals 8. S

1

it'"

Sinks

vtv

■

2ii

Ready

Figure 8-2: The SIMULINK Library Browser.

SIMULINK

123

Figure 8-3: The Continuous Library.

by double-clicking on the third icon from the left in Figure 8-1, or in Windows
either by clicking on the EH to the left of the “Continuous” icon at the top
right of Figure 8-2, or else by clicking on the small icon to the left of the
word “Continuous” in the left panel of the SIMULINK Library Browser. When
opened, the “Continuous” library looks like Figure 8-3.

Notice that u and u' are obtained from u! and u" (respectively) by integrating.
Therefore, drag two copies of the Integrator block into the model window, and
line them up with the mouse. Relabel them (by positioning the mouse at the
end of the text under the block, hitting the backspace key a few times to erase
what you don’t want, and typing something new in its place) to read u! and u.
Note that each Integrator block has an input port and an output port. Align
the output port of the u! Integrator with the input port of the u Integrator
and join them with an arrow, using the left button on the mouse. Your model
window should now look like this:

{THU

U' U

This models the fact that u is obtained by integration from u' . Now the
differential equation can be rewritten u" = —(5 u + 2 u'), and u! is obtained
by integration from u". So we want to add other blocks to implement these
relationships. For this purpose we add three Gain blocks, which implement

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multiplication by a constant, and one Sum block, used for addition. These
are all chosen from the “Math” library (fourth from the right in Figure 8-1, or
fourth from the top in Figure 8-2). Hooking them up the same way we did with
the Integrator blocks gives a model window that looks something like this:

We need to go back and edit the properties of the Gain blocks, to change
the constants by which they multiply from the default of 1 to 5 (in “Gain”),
—1 (in “Gainl”), and 2 (in “Gain2”). To do this, double-click on each Gain
block in turn. A Block Parameters box will open in which you can change the
Gain parameter to whatever you need. Next, we need to send a!, the output
of the first Integrator block, to the input port of block “Gain2”. This presents
a problem, since an Integrator block only has one output port and it’s already
connected to the next Integrator block. So we need to introduce a branch line.
Position the mouse in the middle of the arrow connecting the two Integrators,
hold down the CTRL key with one hand, simultaneously push down the left
mouse button with the other hand, and drag the mouse around to the input
port of the block entitled “Gain2”. At this point we’re almost done; we just
need a block for viewing the output. Open up the “Sinks” library and drag a
copy of the Scope block into the model window. Hook this up with a branch
line (again using the CTRL key) to the line connecting the second Integrator
and the Gain block. At this point you might want to relabel some more of the
blocks (by editing the text under each block), and also label some of the arrows
(by double-clicking on the arrow shaft to open a little box in which you can
type a label). We end up with the model shown in Figure 8-4.

Now we’re ready to run our simulation. First, it might be a good idea to save
the model, using Save as... from the File menu. One might choose to give it
the name li e OD . (MATLAB automatically adds the file extension . 1.)

To see what is happening during the simulation, double-click on the Scope
block to open an “oscilloscope” that will plot u as a function of t. Of course
one needs to set initial conditions also; this can be done by double-clicking on

SIMULINK

125

the Integrator blocks and changing the line of the Block Parameters box that
reads “Initial condition”. For example, suppose we set the initial condition for
u' (in the first Integrator block) to 5 and the condition for u (in the second
Integrator block) to 1. In other words, we are solving the system

u" + 2 u! + 5u = 0,
u(0) = %
u'(O) = 5,

which happens to have the exact solution
u(t) = 3e~* sin(2£) + e~ 1 cos(2£).

/ Your first instinct might be to rely on the Derivative block, rather than the
Integrator block, in simulating differential equations. But this has two
drawbacks: It is harder to put in the initial conditions, and also numerical
differentiation is much less stable than numerical integration.

Now go to the Simulation menu and hit Start. You should see in the Scope
window something like Figure 8-5. This of course is simply the graph of the
function 3e~* sin(2£) + e~ f cos(2 1). (By the way, you might need to change the
scale on the vertical axis of the Scope window. Clicking on the “binoculars” icon
does an “automatic” rescale, and right-clicking on the vertical axis opens an
Axes Properties... menu that enables you to manually select the minimum
and maximum values of the dependent variable.) It is easy to go back and
change some of the parameters and rerun the simulation again.

Finally, suppose one now wants to study the inhomogeneous equation for
“forced oscillations,” u" + 2 u’ + 5 u = g{t), where g is a specified “forcing” term.

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Chapter 8: SIMULINK and GUIs

0 2 4 6 8 10

Time offset : 0

Figure 8-5: Scope Output.

For this, all we have to do is add another block to the model from the “Sources”
library. Click on the shaft of the arrow at the top of the model going into the
first Integrator and use Cut from the Edit menu to remove it. Then drag in
another “Sum” block before the first Integrator and input a suitable source to
one input port of the “Sum” block. For example, if g{t ) is to represent “noise,”
drag the Band-Limited White Noise block from the “Sources” library into the
model and hook everything up as shown in Figure 8-6.

The output from this revised model (with the default values of 0.1 for the
noise power and 0.1 for the noise sample time) looks like Figure 8-7. The effect
of noise on the system is clearly visible from the simulation.

Figure 8-6: Model for the Inhomogeneous Equation.

Graphical User Interfaces (GUIs)

127

Figure 8-7: Scope Output for the Inhomogeneous
Equation.

Graphical User Interfaces (GUIs)

With MATLAB you can create your own Graphical User Interface, or GUI,
which consists of a Figure window containing menus, buttons, text, graphics,
etc., that a user can manipulate interactively with the mouse and keyboard.
There are two main steps in creating a GUI: One is designing its layout, and
the other is writing callback functions that perform the desired operations
when the user selects different features.

GUI Layout and GUIDE

Specifying the location and properties of different objects in a GUI can be done
with commands such as uicontrol, uimenu, and uicontextmenu in an M-
file. MATLAB also provides an interactive tool (a GUI itself !) called GUIDE
that greatly simplifies the task of building a GUI. We will describe here how
to get started writing GUIs with the MATLAB 6 version of GUIDE, which has
been significantly enhanced over previous versions.

/ One possible drawback of GUIDE is that it equips your GUI with commands
that are new in MATLAB 6 and it saves the layout of the GUI in a binary
. i file. If your goal is to create a robust GUI that many different users can

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use with different versions of MATLAB, you may still be better off writing
the GUI from scratch as an M-file.

To open GUIDE, select File:New:GUI from the Desktop menu bar or type
guide in the Command Window. If this is the first time you have run GUIDE,
you will next see a window that encourages you to click on “View GUIDE
Application Options dialog”. We recommend that you do so to see what your
options are, but leave the settings as is for now. After you click “OK”, the
Layout Editor will appear, containing a large white area with a grid. As with
most MATLAB windows, the Layout Editor has a tool bar with shortcuts to
many of the menu functions we describe below.

You can start building a GUI by clicking on one of the buttons to the left of
the grid, then moving to a desired location in the grid, and clicking again to
place an object on the grid. To see what type of object each button corresponds
to, move the mouse over the button but don’t click; soon a yellow box with
the name of the button will appear. Once you have placed an object on the
grid, you can click and drag (hold down the left mouse button and move the
mouse) on the middle of the object to move it or click and drag on a corner to
resize the object. After you have placed several objects, you can select multiple
objects by clicking and dragging on the background grid to enclose them with
a rectangle. Then you can move the objects as a block with the mouse, or align
them by selecting Align Objects from the Layout menu.

To change properties of an object such as its color, the text within it, etc.,
you must open the Property Inspector window. To do so, you can double-click
on an object, or choose Property Inspector from the Tools menu and then
select the object you want to alter with the left mouse button. You can leave
the Property Inspector open throughout your GUIDE session and go back
and forth between it and the Layout Editor. Let’s consider an example that
illustrates several of the more important properties.

Figure 8-8 shows an example of what the Layout Editor window looks like
after several objects have been placed and their properties adjusted. The
purpose of this sample GUI is to allow the user to type a MATLAB plot-
ting command, see the result appear in the same window, and modify the
graph in a few ways. Let us describe how we created the objects that make up
the GUI.

The boxes on the top row, as well as the one labeled “Set axis scaling:”, are
Static Text boxes, which the user of the GUI will not be allowed to manipulate.
To create each of them, we first clicked on the “Static Text” button — the
one to the right of the grid labeled “txt” — and then clicked in the grid where
we wanted to add the text. Next, to set the text for the box we opened the

Graphical User Interfaces (GUIs)

129

Property Inspector and clicked on the square button next to “String”, which
opens a new window in which to change the default text. Finally, we resized
each box according to the length of its text.

The buttons labeled “Plot it!”, “Change axis limits”, and “Clear Figure” are
all Push Button objects, created using the button to the left of the grid labeled
“OK”. To make these buttons all the same size, we first created one of them
and then after sizing it, we duplicated it (twice) by clicking the right mouse
button on the existing object and selecting Duplicate. We then moved each
new Push Button to a different position and changed its text in the same way
as we did for the Static Text boxes.

The blank box near the top of the grid is an Edit Text box, which allows the
user to enter text. We created it with the button to the left of the grid labeled
“edit” and then cleared its default text in the same way that we changed text
before. Below the Edit Text box is a large Axes box, created with the button
containing a small graph, and in the lower right the button labeled “Hold is
OFF” is a Toggle Button, created with the button labeled “tgl”. For toggling

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Chapter 8: SIMULINK and GUIs

(on-off) commands you could also use a Radio Button or a Checkbox, denoted
respectively by the buttons with a dot and a check mark in them. Finally, the
box on the right that says “equal” is a Popup Menu — we’ll let you find its
button in the Layout Editor since it is hard to describe! Popup Menus and
Listbox objects allow you to let the user choose among several options.

We moved, resized, and in most cases changed the properties of each object
similarly to the way we described above. In the case of the Popup Menu, after
we selected the “String” button in the Property Inspector, we entered into the
window that appeared three words on three separate lines: equal, normal,
and square. Using multiple lines is necessary to give the user multiple choices
in a Popup Menu or Listbox object.

/ In addition to populating your GUI with the objects we described above, you
can create a menu bar for it using the Menu Editor, which you can open by
selecting Edit Menubar from the Layout menu. You can also use the Menu
Editor to create a context menu for an object; this is a menu that appears
when you click the right mouse button on the object. See the online
documentation for GUIDE to learn how to use the Menu Editor.

We also gave our GUI a title, which will appear in the titlebar of its window,
as follows. We clicked on the grid in the Layout Editor to select the entire GUI
(as opposed to an object within it) and went to the Property Inspector. There
we changed the text to the right of “Name” from “Untitled” to “Simple Plot
GUI”.

Saving and Running a GUI

To save a GUI, select Save As... from the File menu. Type a file name for your
GUI without any extension; for the GUI described above we chose lo i.
Saving creates two files, an M-file and a binary file with extension . i , so
in our case the resulting files were named lo i . and lo i . i .
When you save a GUI for the first time, the M-file for the GUI will appear in
a separate Editor/Debugger window. We will describe how and why to modify
this M-file in the next section.

^ The instructions in this and the following section assume the default
settings of the Application Options, which you may have inspected
upon starting GUIDE, as described above. Otherwise, you can access
them from the Tools menu. We assume in particular that “Generate
.fig file and .m file”, “Generate callback function prototypes”, and
“Application allows only one instance to run” are selected.

Graphical User Interfaces (GUIs)

131

Figure 8-9: A Simple GUI.

Once saved, you can run the GUI from the Command Window by typing its
name, in our case plotgui, whether or not GUIDE is running. Both the . i
file and the . file must be in your current directory or MATLAB path. You
can also run it from the Layout Editor by typing CTRL+T or selecting Activate
Figure from the Tools menu. A copy of the GUI will appear in a separate
window, without all the surrounding menus and buttons of the Layout Editor.
(If you have added new objects since the last time you saved or activated
the GUI, the M-file associated to the GUI will also be brought to the front.)
Figure 8-9 shows how the GUI we created above looks when activated.

Notice that the appearance of the GUI is slightly different than in the
GUIDE window; in particular, the font size may differ. For this reason you
may have to go back to the GUIDE window after activating a GUI and resize
some objects accordingly. The changes you make will not immediately appear
in the active GUI; to see their effect you must activate the GUI again.

The objects you create in the Layout Editor are inert within that window —
you can’t type text in the Edit Text box, you can’t see the additional options by
clicking on the Popup Menu, etc. But in an activated GUI window, objects such
as Toggle Buttons and Popup Menus will respond to mouse clicks. However,
they will not actually perform any functions until you write a callback function
for each of them.

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GUI Callback Functions

When you are ready to create a callback function for a given object, click
the right mouse button on the object and select Edit Callback. The M-file
associated with the GUI will be brought to the front in an Editor/Debugger
window, with the cursor in a block of lines like the ones below. (If you haven’t
yet saved the GUI, you will be prompted to do so first, so that GUIDE knows
what name to give the M-file.)

function varargout = pushbuttonl-Callback (h, eventdata, handles, varargin)

% Stub for Callback of the uicontrol handles .pushbuttonl .
disp ( 'pushbuttonl Callback not implemented yet.')

% end pushbuttonl-Callback

In this case we have assumed that the object you selected was the first Push
Button that you created in the Layout Editor; the string “pushbuttonl” above
is its default tag. (Another way to find the tag for a given object is to select
it and look next to “Tag” in the Property Inspector.) All you need to do now
to bring this Push Button to life is to replace the di sp command line in the
template shown above with the commands that you want performed when the
user clicks on the button. Of course you also need to save the M-file, which you
can do in the usual way from the Editor/Debugger, or by activating the GUI
from the Layout Editor. Each time you save or activate a GUI, a block of four
lines like the ones above is automatically added to the GUI’s M-file for any
new objects or menu items that you have added to the GUI and that should
have callback functions.

In the example plotgui from the previous section, there is one case where
we used an existing MATLAB command as a callback function. For the Push
Button labeled “Change axis limits”, we simply entered axlimdlg into its
callback function in lo i. . This command opens a dialog box that allows
a user to type new values for the ranges of the x and y axes. MATLAB has a
number of dialog boxes that you can use either as callback functions or in an
ordinary M-file. For example, you can use inputdlg in place of input. Type
help uitools for information on the available dialog boxes.

For the Popup Menu on the right side of the GUI, we put the following lines
into its callback function template:

switch get (h, 'Value')
case 1

axis equal
case 2

axis normal

Graphical User Interfaces (GUIs)

133

case 3

axis square
end

Each time the user of the GUI selects an item from a Popup Menu, MATLAB
sets the “Value” property of the object to the line number selected and runs
the associated callback function. As we described in Chapter 5, you can use
get to retrieve the current setting of a property of a graphics object. When
you use the callback templates provided by GUIDE as we have described,
the variable h will contain the handle (the required first argument of get
and set) for the associated object. (If you are using another method to write
callback functions, you can use the MATLAB command gcbo in place of
h.) For our sample GUI, line 1 of the Popup Menu says “equal”, and if the
user selects line 1, the callback function above runs axis equal; line 2 says
“normal”; etc.

/ You may have noticed that in Figure 8-9 the Popup Menu says “normal”
rather than “equal” as in Figure 8-8; that’s because we set its “Value”
property to 2 when we created the GUI, using the Property Inspector. In this
way you can make the default selection something other than the first item
in a Popup Menu or Listbox.

For the Push Button labeled “Plot it!”, we wrote the following callback
function:

set (handles . figurel , ' HandleVisibility' , 'callback')
eval (get (handles . editl, ' String' ) )

Here handles . figurel and handles . editl are the handles for the en-
tire GUI window and for the Edit Text box, respectively. Again these vari-
ables are provided by the callback templates in GUIDE, and if you do not
use this feature you can generate the appropriate handles with gcbf and
findobj (gcbf, 'Tag', 'editl' ) Respectively. The second line of the call-
back function above uses get to find the text in the Edit Text box and then
runs the corresponding command with eval. The first line uses set to make
the GUI window accessible to graphics commands used within callback func-
tions; if we did not do this, a plotting command run by the second line would
open a separate figure window.

/ Another way to enable plotting within a GUI window is to select

Application Options from the Tools menu in the Layout Editor, and
within the window that appears change “Command-line accessibility” to
“On”. This has the possible drawback of allowing plotting commands the

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Chapter 8: SIMULINK and GUIs

user types in the Command Window to affect the GUI window. A safer
approach is to set “Command-line accessibility” to “User-specified”, click on
the grid in the Layout Editor to select the entire GUI, go to the Property
Inspector, and change “Handle Visibility” to “callback”. This would eliminate
the need to select this property with set in each of the callback functions
above and below that run graphics commands.

Here is our callback function for the Push Button labeled “Clear figure”:
set (handles . editl, 'String', ")

set (handles . figurel, ' HandleVisibility' , 'callback')
cla reset

The first line clears the text in the Edit Text box and the last line clears the
Axes box in the GUI window. (If your GUI contains more than one Axes box,
you can use axes to select the one you want to manipulate in each of your
callback functions.)

We used the following callback function for the Toggle Button labeled “Hold
is OFF”:

set (handles . figurel, 'HandleVisibility', 'callback')
if get(h, 'Value')
hold on

set (h, 'String', 'Hold is ON');
else

hold off

set (h, 'String', 'Hold is OFF');

end

We get the “Value” property of the Toggle Button in the same way as in the
the Popup Menu callback function above, but for a Toggle Button this value
is either 0 if the button is “out” (the default) or 1 if the button is pressed “in”.
(Radio Buttons and Checkboxes also have a “Value” property of either 0 or 1.)
When the user first presses the Toggle Button, the callback function above
runs hold on and resets the string displayed on the Toggle Button to reflect
the change. The next time the user presses the button, these operations are
reversed.

/ We can also associate a callback function with the Edit Text box; this

function will be run each time the user presses the ENTER key after typing
text in the box. The callback function eval (get (h, ' String' ) ) will run
the command just typed, providing an alternative to (or making superfluous)
the “Plot it!” button.

Graphical User Interfaces (GUIs)

135

Finally, if you create a GUI with an Axes box like we did, you may notice that
GUIDE puts in the GUI’s M-file a template like a callback template but labeled
“ButtondownFcn” instead. When the user clicks in an Axes object, this type
of function is called rather than a callback function, but within the template
you can write the function just as you would write a callback function. You
can also associate such a function with an object that already has a callback
function by clicking the right mouse button on the object in the Layout Editor
and selecting Edit ButtondownFcn. This function will be run when the
user clicks the right mouse button (as opposed to the left mouse button for the
callback function). You can associate functions with several other types of user
events as well; to learn more, see the online documentation, or experiment by
clicking the right mouse button on various objects and on the grid behind them
in the Layout Editor.

Chapter 9

Applications

In this chapter, we present examples showing you how to apply MATLAB
to problems in several different disciplines. Each example is presented as a
MATLAB M-book. These M-books are illustrations of the kinds of polished,
integrated, interactive documents that you can create with MATLAB, as aug-
mented by the Word interface. The M-books are:

• Illuminating a Room

• Mortgage Payments

• Monte Carlo Simulation

• Population Dynamics

• Linear Economic Models

• Linear Programming

• The 360° Pendulum

• Numerical Solution of the Heat Equation

• A Model of Traffic Flow

We have not explained all the MATLAB commands that we use; you can
learn about the new commands from the online help. SIMULINK is used in
A Model of Traffic Flow and as an optional accessory in Population Dynamics
and Numerical Solution of the Heat Equation. Running the M-book on Linear
Programming also requires an M-file found (in slightly different forms) in the
SIMULINK and Optimization toolboxes.

The M-books require different levels of mathematical background and ex-
pertise in other subjects. Illuminating a Room, Mortgage Payments, and
Population Dynamics use only high school mathematics. Monte Carlo Simula-
tion uses some probability and statistics; Linear Economic Models and Linear
Programming, some linear algebra; The 360° Pendulum, some ordinary dif-
ferential equations; Numerical Solution of the Heat Equation, some partial

136

Illuminating a Room

137

differential equations; and A Model of Traffic Flow, differential equations, lin-
ear algebra, and familiarity with the function e z for z a complex number. Even
if you don’t have the background for a particular example, you should be able
to learn something about MATLAB from the M-book.

Illuminating a Room

Suppose we need to decide where to put light fixtures on the ceiling of a
room, measuring 10 meters by 4 meters by 3 meters high, in order to best
illuminate it. For aesthetic reasons, we are asked to use a small number of
incandescent bulbs. We want the bulbs to total a maximum of 300 watts. For
a given number of bulbs, how should they be placed to maximize the
intensity of the light in the darkest part of the room? We also would like to
see how much improvement there is in going from one 300-watt bulb to two
150-watt bulbs to three 100-watt bulbs, and so on. To keep things simple, we
assume that there is no furniture in the room and that the light reflected
from the walls is insignificant compared with the direct light from the
bulbs.

One 300-Watt Bulb

If there is only one bulb, then we want to put the bulb in the center of the
ceiling. Let’s picture how well the floor is illuminated. We introduce
coordinates x running from 0 to 10 in the long direction of the room and y
running from 0 to 4 in the short direction. The intensity at a given point,
measured in watts per square meter, is the power of the bulb, 300, divided by
47 t times the square of the distance from the bulb. Since the bulb is 3 meters
above the point (5, 2) on the floor, we can express the intensity at a point
(x, y) on the floor as follows:

syms x y; ilium = 300/ (4*pi* ( (x - 5) ~2 + (y - 2) ~2 + 3~2))

ilium =

75/pi/ ( (x-5) ~2+ (y-2) ~2+9)

We can use ezcontourf to plot this expression over the entire floor. We
use colormap to arrange for a color gradation that helps us to see the

138

Chapter?: Applications

illumination. (See the online help for graph3d for more colormap options.)
ezcontourf (ilium, [0 10 0 4]); colormap (gray) ;
axis equal tight

The darkest parts of the floor are the corners. Let us find the intensity of the
light at the corners, and at the center of the room.

subs(illum, {x, y}, {0, 0})
subs(illum, {x, y}, {5, 2})

ans =

0.6282
ans =

2.6526

The center of the room, at floor level, is about 4 times as bright as the
corners when there is only one bulb on the ceiling. Our objective is to light
the room more uniformly using more bulbs with the same total amount of
power. Before proceeding to deal with multiple bulbs, we observe that the
use of ezcontourf is somewhat confining, as it does not allow us to control
the number of contours in our pictures. Such control will be helpful in seeing
the light intensity; therefore we shall plot numerically rather than
symbolically; that is, we shall use contourf instead of ezcontourf.

Two 150-Watt Bulbs

In this case we need to decide where to put the two bulbs. Common sense
tells us to arrange the bulbs symmetrically along a line down the center of

Illuminating a Room

139

the room in the long direction, that is, along the line y = 2. Define a function
that gives the intensity of light at a point (x, y) on the floor due to a 150-watt
bulb at a position ( d , 2) on the ceiling.

Iight2 = inline (vectorize (' 150/ (4*pi* ( (x - d) ~2 + (y - 2) ~2 +

3~2))'), 'x', 'y', 'd')

light2 =

Inline function:

light2 (x, y , d) = 150 ./ (4 . *pi . * ( (x - d) . -2 + (y - 2) . -2 +

3/2) )

Let’s get an idea of the illumination pattern if we put one light at d = 3
and the other at d = 7. We specify the drawing of 20 contours in this and the
following plots.

[X,Y] = meshgrid (0 : 0 . 1 : 10 , 0:0. 1:4); contourf (light2 (X, Y, 3)

+ light2 (X, Y, 7), 20); axis equal tight

The floor is more evenly lit than with one bulb, but it looks as if the bulbs
are closer together than they should be. If we move the bulbs further apart,
the center of the room will get dimmer but the corners will get brigher. Let’s
try changing the location of the lights to d = 2 and d = 8.

contourf (light2 (X, Y, 2) + light2 (X, Y, 8), 20);
axis equal tight

Chapter?: Applications

This is an improvement. The corners are still the darkest spots of the
room, though the light intensity along the walls toward the middle of the
room (near x = 5) is diminishing as we move the bulbs further apart. To
better illuminate the darkest spots we should keep moving the bulbs apart.
Let’s try lights at d = 1 and d = 9.

contourf (light2 (X, Y, 1) + light2 (X, Y, 9), 20);
axis equal tight

Looking along the long walls, the room is now darker toward the middle than
at the corners. This indicates that we have spread the lights too far apart.

We could proceed with further contour plots, but instead let’s be
systematic about finding the best position for the lights. In general, we can
put one light at x = d and the other symmetrically at x = 10 — d for d
between 0 and 5. Judging from the examples above, the darkest spots will be

Illuminating a Room

141

either at the corners or at the midpoints of the two long walls. By symmetry,
the intensity will be the same at all four corners, so let’s graph the intensity
at one of the corners (0, 0) as a function of d.

d = 0:0. 1:5; plot(d, light2(0, 0, d) + light2(0, 0, 10 - d) )

As expected, the smaller d is, the brighter the corners are. In contrast, the
graph for the intensity at the midpoint (5, 0) of a long wall (again by
symmetry it does not matter which of the two long walls we choose) should
grow as d increases toward 5.

plot (d, light2 (5 , 0, d) + light2(5, 0, 10 - d) )

We are after the value of d for which the lower of the two numbers on the
above graphs (corresponding to the darkest spot in the room) is as high as
possible. We can find this value by showing both curves on one graph.

142

Chapter?: Applications

hold on; plot(d, light2(0, 0, d) + light2(0, 0, 10 - d) ) ;
hold off

The optimal value of d is at the point of intersection, near 1.4, with
minimum intensity a little under 1. To get the optimum value of d, we find
exactly where the two curves intersect.

syms d; eqn = inline (char (light2 (0, 0, d) + light2(0, 0, 10 -
d) - light2 (5, 0, d) - light2(5, 0, 10 - d) ) )

eqn =

Inline function:

eqn (d) = 75/2/pi/ (d"2+13) +75/2/pi/ ( (-10+d) "2+13) -
75/2/pi/ ( (5-d) "2+13) -75/2/pi/ ( (-5+d) "2+13)

f zero (eqn, [0 5])

1.4410

So the lights should be placed about 1.44 meters from the short walls. For
this configuration, the approximate intensity at the darkest spots on the
floor is as follows:

light2 (0, 0, 1.441) + light2(0, 0, 10 - 1.441)

ans =

0.9301

Illuminating a Room

143

The darkest spots in the room have intensity around 0.93, as opposed to 0.63
for a single bulb. This represents an improvement of about 50%.

Three 100-Watt Bulbs

We redefine the intensity function for 100-watt bulbs:

light3 = inline (vectorize (' 100/ (4*pi* ( (x - d) ~2 + (y - 2) "2 +
3~2) ) ' ) , 'x', 'y', 'd')

light 3 =

Inline function:

light3 (x,y,d) = 100 . / (4 . *pi .*((x - d) . ~2 + (y - 2). "2 +

3 . ~2 ) )

Assume we put one bulb at the center of the room and place the other two
symmetrically as before. Here we show the illumination of the floor when the
off-center bulbs are one meter from the short walls.

[X, Y] = meshgrid (0:0. 1:10, 0 : 0 . 1 : 4) ; contourf (light3 (X, Y, 1)
+ light3 (X, Y, 5) + light3 (X, Y, 9), 20);
axis equal tight

It appears that we should put the bulbs even closer to the walls. (This may
not please everyone’s aesthetics!) Let d be the distance of the bulbs from the
short walls. We define a function giving the intensity at position x along a
long wall and then graph the intensity as a function of d for several values
ofx.

144

Chapter?: Applications

d = 0 : 0.1:5;
for x = 0: 0.5:5

plot (d, light3 (x, 0, d) + light3 (x, 0, 5) + ...
Iight3 (x, 0, 10 - d) )
hold on

end

hold off

We know that for d near 5, the intensity will be increasing as x increases
from 0 to 5, so the bottom curve corresponds to x = 0 and the top curve to
x = 5. Notice that the x = 0 curve is the lowest one for all d, and it rises as d
decreases. Thus d = 0 maximizes the intensity of the darkest spots in the
room, which are the corners (corresponding to x = 0). There the intensity is
as follows:

light3 (0, 0, 0) + light3(0, 0, 5) + light3(0, 0, 10)

0.8920

This is surprising; we do worse than with two bulbs. In going from two
bulbs to three, with a decrease in wattage per bulb, we are forced to move
wattage away from the ends of the room and bring it back to the center. We
could probably improve on the two-bulb scenario if we used brighter bulbs at
the ends of the room and a dimmer bulb in the center, or if we used four
75-watt bulbs. But our results so far indicate that the amount to be gained in
going to more than two bulbs is likely to be small compared with the amount
we gained by going from one bulb to two.

Mortgage Payments

145

Mortgage Payments

We want to understand the relationships among the mortgage payment rate
of a fixed rate mortgage, the principal (the amount borrowed), the annual
interest rate, and the period of the loan. We are going to assume (as is
usually the case in the United States) that payments are made monthly,
even though the interest rate is given as an annual rate. Let’s define

peryear = 1/12; percent = 1/100;

So the number of payments on a 30-year loan is
30*12

ans =

360

and an annual percentage rate of 8% comes out to a monthly rate of
8*percent*peryear

ans =

0.0067

Now consider what happens with each monthly payment. Some of the
payment is applied to interest on the outstanding principal amount, P, and
some of the payment is applied to reduce the principal owed. The total
amount, R, of the monthly payment remains constant over the life of the
loan. So if J denotes the monthly interest rate, we have R = J* P + (amount
applied to principal), and the new principal after the payment is applied is

P + J*P-R = P*(l + J)-R = P*m-R ,

where m = 1 + J. So a table of the amount of the principal still outstanding
after n payments is tabulated as follows for a loan of initial amount A, for n
from 0 to 6:

syms m J P R A
P = A;

for n = 0:6,

disp ( [n, P]),

P = simplify ( -R + P*m) ;

end

146

Chapter?: Applications

[ 0, A]

[ 1, -R+A*m]

[ 2, -R-m*R+A*nT2]

[ 3, -R-m*R-m~2*R+A*m / '3]

[ 4, -R-m*R-m~2*R-m~3*R+A*m / '4]

[ 5, -R-m*R-nT2*R-m~3*R-

m~4*R+A*m~5]

[ 6, -R-m*R-nT2*R-m~3*R-

m~4*R-m / '5*R+A*m~6]

We can write this in a simpler way by noticing that P = A * m n + (terms
divisible by R). For example, with n = 7 we have

factor (p - A*m~7)

-R* ( l+m+m''2+m~3+m / '4+m~5+m~6 )

But the quantity inside the parentheses is the sum of a geometric series

i fe m ~ 1

So we see that the principal after n payments can be written as
P = A* m n — R* ( m n — l)/(m — 1).

Now we can solve for the monthly payment amount R under the assumption
that the loan is paid off in N installments, that is, P is reduced to 0 after N
payments:

syms N; solve (A*nTN - R* (m^N - l)/(m - 1), R)

ans =

A*m~N* (m-1) / (nTN-1)

R = subs (ans, m, J + 1)

R=

A* (J+l) / 'N*J/ ( (J+l) "N-l)

For example, with an initial loan amount A = $150,000 and a loan lifetime
of 30 years (360 payments), we get the following table of payment amounts
as a function of annual interest rate:

Mortgage Payments

147

format bank; disp(' Interest Rate
for rate = 1:10,

dlsp ( [rate, double (subs (R, [A, N,

rate*percent*peryear] ) ) ] )
end

Payment' )

J] , [150000, 360,...

Interest Rate

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00
9.00

10.00

Payment
482.46
554.43
632.41
716 . 12
805.23
899.33
997.95
1100.65
1206 . 93
1316.36

Note the use of format bank to write the floating point numbers with two
digits after the decimal point.

There’s another way to understand these calculations that’s a little slicker
and that uses MATLAB’s linear algebra capability. Namely, we can write the
fundamental equation

Pnew = Paid * m — R
in matrix from as

Vnew = £u old ,

where v is the column vector ( ^ ) and B is the square matrix

We can check this using matrix multiplication:
syms RP; B= [m-R; 0 1]; v =

ans =

[ m*P-R]
[ 1 ]

[P; 1] ; B*v

148

Chapter?: Applications

which agrees with the formula we had above. (Note the use of syms to reset
R and P to undefined symbolic quantities.) Thus the column vector [P; 1]
resulting after n payments can be computed by left-multiplying the starting
vector [A; 1] by the matrix B n . Assuming m > 1, that is, a positive rate of
interest, the calculation

[eigenvectors, diagonalform] = eig(B)

eigenvectors =

[ 1 , 1 ]

[ 0, (m-1) /R]

diagonalform =

[ m, 0]

[ 0 , 1 ]

shows us that the matrix B has eigenvalues m and 1, and corresponding
eigenvectors [1; 0] and [1; ( m — 1 )/R] = [1; J/R]. Now we can write the vector
[A; 1] as a linear combination of the eigenvectors: [A; 1] = x[l; 0] + y[l; J/R].
We can solve for the coefficients:

[x, y] = solve ('A = x*l + y*l' , '1 = x*0 + y* J/R' )

x =

(A* J-R) /J

y =

R/J
and so

[A; 1] = (A - ( R/J )) * [1; 0] + (R/J) * [1; J/R]

and

B n ■ [ A; 1] = (A - (R/J)) *m n * [1; 0] + (R/J) * [1; J/R] .

Therefore the principal remaining after n payments is

P = ((A * J — R) *m n + R)/J = A*m n — R* (m n — 1 )/J.

This is the same result we obtained earlier.

To conclude, let’s determine the amount of money A one can afford to
borrow as a function of what one can afford to pay as the monthly
payment R. We simply solve for A in the equation that P = 0 after N
payments.

Monte Carlo Simulation

149

solve (A*m~N - R*(m~N - l)/(m - 1), A)

ans =

R* (nTN-1) / (nTN) / (m-1)

For example, if one is shopping for a house and can afford to pay $1500 per
month for a 30-year fixed-rate mortgage, the maximum loan amount as a
function of the interest rate is given by

disp(' Interest Rate Loan Amt.')
for rate = 1:10,

disp([rate, double (subs (ans, [R, N, m] , [1500, 360,...

1 + rate*percent*peryear] ) ) ] )
end

Interest Rate Loan Amt.

1.00 466360.60

2.00 405822.77

3.00 355784.07

4.00 314191.86

5.00 279422.43

6.00 250187.42

7.00 225461.35

8.00 204425.24

9.00 186422.80

10.00 170926.23

Monte Carlo Simulation

In order to make statistical predictions about the long-term results of a
random process, it is often useful to do a simulation based on one’s
understanding of the underlying probabilities. This procedure is referred to
as the Monte Carlo method.

As an example, consider a casino game in which a player bets against the
house and the house wins 51% of the time. The question is: How many
games have to be played before the house is reasonably sure of coming out
ahead? This scenario is common enough that mathematicians long ago
figured out very precisely what the statistics are, but here we want to
illustrate how to get a good idea of what can happen in practice without
having to absorb a lot of mathematics.

150

Chapter?: Applications

First we construct an expression that computes the net revenue to the
house for a single game, based on a random number chosen between 0 and 1
by the MATLAB function rand. If the random number is less than or equal
to 0.51, the house wins one betting unit, whereas if the number exceeds 0.51,
the house loses one unit. (In a high-stakes game, each bet may be worth
$1000 or more. Thus it is important for the casino to know how bad a losing
streak it may have to weather to turn a profit — so that it doesn’t go
bankrupt first!) Here is an expression that returns 1 if the output of rand is
less than 0.51 and —1 if the output of rand is greater than 0.51 (it will also
return 0 if the output of rand is exactly 0.51, but this is extremely unlikely):

revenue = sign (0.51 - rand)

revenue =

-1

In the game simulated above, the house lost. To simulate several games at
once, say 10 games, we can generate a vector of 10 random numbers with the
command rand (1, 10) and then apply the same operation.

revenues = sign(0.51 - rand(l, 10))

revenues =

1 - 11 - 1 - 111 - 11-1

In this case the house won 5 times and lost 5 times, for a net profit of 0 units.
For a larger number of games, say 100, we can let MATLAB sum the revenue
from the individual bets as follows:

profit = sum (sign (0 . 51 - rand (1, 100)))

profit =

-4

For this trial, the house had a net loss of 4 units after 100 games. On
average, every 100 games the house should win 51 times and the player(s)
should win 49 times, so the house should make a profit of 2 units (on
average). Let’s see what happens in a few trial runs.

profits = sum(sign (0 . 51 - rand(100, 10)))

profits =

14 -12 6 2 -4 0 -10 12 0 12

Monte Carlo Simulation

151

We see that the net profit can fluctuate significantly from one set of 100
games to the next, and there is a sizable probability that the house has lost
money after 100 games. To get an idea of how the net profit is likely to be
distributed in general, we can repeat the experiment a large number of times
and make a histogram of the results. The following function computes the
net profits for k different trials of n games each:

profits = inline (' sum(sign (0 . 51 - rand(n, k) ) ) ' , 'n', 'k')

profits =

Inline function:

profits (n,k) = sum (sign (0 . 51 - randfn, k) ) )

What this function does is to generate annxi matrix of random
numbers and then perform the same operations as above on each entry of
the matrix to obtain a matrix with entries 1 for bets the house won and —1
for bets it lost. Finally it sums the columns of the matrix to obtain a row
vector of k elements, each of which represents the total profit from a
column of n bets.

Now we make a histogram of the output of profits using n = 100 and
k = 100. Theoretically the house could win or lose up to 100 units, but in
practice we find that the outcomes are almost always within 30 or so of 0.
Thus we let the bins of the histogram range from —40 to 40 in increments of
2 (since the net profit is always even after 100 bets).

hist (profits (100, 100), -40:2:40); axis tight

Chapter?: Applications

The histogram confirms our impression that there is a wide variation in the
outcomes after 100 games. The house is about as likely to have lost money
as to have profited. However, the distribution shown above is irregular
enough to indicate that we really should run more trials to see a better
approximation to the actual distribution. Let’s try 1000 trials.

hist (profits (100, 1000), -40:2:40); axis tight

According to the Central Limit Theorem, when both n and k are large, the
histogram should be shaped like a “bell curve”, and we begin to see this
shape emerging above. Let’s move on to 10,000 trials.

hist (profits (100, 10000), -40:2:40); axis tight

Monte Carlo Simulation 1 53

Here we see very clearly the shape of a bell curve. Though we haven’t gained
that much in terms of knowing how likely the house is to be behind after 100
games, and how large its net loss is likely to be in that case, we do gain
confidence that our results after 1000 trials are a good depiction of the
distribution of possible outcomes.

Now we consider the net profit after 1000 games. We expect on average
the house to win 510 games and the player(s) to win 490, for a net profit of 20
units. Again we start with just 100 trials.

hist (profits (1000, 100), -100:10:150); axis tight

Though the range of observed values for the profit after 1000 games is
larger than the range for 100 games, the range of possible values is 10 times
as large, so that relatively speaking the outcomes are closer together than
before. This reflects the theoretical principle (also a consequence of the
Central Limit Theorem) that the average “spread” of outcomes after a large
number of trials should be proportional to the square root of n, the number of
games played in each trial. This is important for the casino, since if the
spread were proportional to n, then the casino could never be too sure of
making a profit. When we increase n by a factor of 10, the spread should only
increase by a factor of VTO, or a little more than 3.

Note that after 1000 games, the house is definitely more likely to be ahead
than behind. However, the chances of being behind are still sizable. Let’s
repeat with 1000 trials to be more certain of our results.

hist (profits (1000, 1000), -100:10:150); axis tight

154

Chapter?: Applications

We see the bell curve shape emerging again. Though it is unlikely, the
chances are not insignificant that the house is behind by more than 50 units
after 1000 games. If each unit is worth $1000, then we might advise the
casino to have at least $100,000 cash on hand to be prepared for this
possibility. Maybe even that is not enough — to see we would have to
experiment further.

Finally, let’s see what happens after 10,000 games. We expect on average
the house to be ahead by 200 units at this point, and based on our earlier
discussion the range of values we use to make the histogram need only go up
by a factor of 3 or so from the previous case. Even 100 trials will take a while
to run now, but we have to start somewhere.

hist (profits (10000, 100), -200:25:600); axis tight

Monte Carlo Simulation

155

It seems that turning a profit after 10,000 games is highly likely, although
with only 100 trials we do not get such a good idea of the worst-case
scenario. Though it will take a good bit of time, we should certainly do
1000 trials or more if we are considering putting our money into such a
venture.

hist (prof its (10000 , 1000), -200:25:600); axis tight
??? Error using ==> inlineeval

Error in inline expression ==> sum(sign (0 . 51 - rand(n, k) ) )
??? Error using ==> -

Out of memory. Type HELP MEMORY for your options.

Error in ==>

C:\MATLABR12\toolbox\matlab\funfun\@inline\subsref .m
On line 25 ==> INLINE_OUT_ = inlineeval (INLINE_INPUTS_,
INLINE_OB J_ . inputExpr , INLINE_OBJ_. expr) ;

This error message illustrates a potential hazard in using MATLAB’s vector
and matrix operations in place of a loop: In this case the matrix rand (n, k)
generated within the profits function must fit in the memory of the
computer. Since n is 10,000 and k is 1000 in our most recent attempt to run
this function, we requested a matrix of 10,000,000 random numbers. Each
floating point number in MATLAB takes up 8 bytes of memory, so the matrix
would have required 80MB to store, which is too much for some computers.
Since k represents a number of trials that can be done independently, a
solution to the memory problem is to break the 1000 trials into 10 groups
of 100, using a loop to run 100 trials 10 times and assemble the
results.

profitvec = [] ;
for i = 1:10

profitvec = [profitvec profits (10000, 100)];
end

hist (prof itvec, -200:25:600); axis tight

156

Chapter?: Applications

Though the chances of a loss after 10,000 games is quite small, the
possibility cannot be ignored, and we might judge that the house should not
rule out being behind at some point by 100 or more units. However, the
overall upward trend seems clear, and we may expect that after 100,000
games the casino is overwhelmingly likely to have made a profit. Based on
our previous observations of the growth of the spread of outcomes, we expect
that most of the time the net profit will be within 1000 of the expected value
of 2000. We show the results of 10 trials of 100,000 games below.

profits (100000, 10)

ans =

Columns 1 through 6

2294 1946

2078

Columns 7 through 10

1984 1552

2652 2630 1872

2138 1852

Population Dynamics

We are going to look at two models for population growth of a species. The
first is a standard exponential growth and decay model that describes quite
well the population of a species becoming extinct, or the short-term behavior
of a population growing in an unchecked fashion. The second, more realistic

Population Dynamics

157

model, describes the growth of a species subject to constraints of space, food
supply, competitors, and predators.

Exponential Growth and Decay

We assume that the species starts with an initial population Pq. The
population after n times units is denoted P n . Suppose that in each time
interval, the population increases or decreases by a fixed proportion of its
value at the begining of the interval. Thus P„ = P„_i + rP„_ i, n > 1. The
constant r represents the difference between the birth rate and the death
rate. The population increases if r is positive, decreases if r is negative, and
remains fixed if r = 0.

Here is a simple M-file that will compute the population at stage n, given
the population at the previous stage and the rate r:

function X = itseq(f, Xinit, n, r)

% computing an iterative sequence of values
X = zeros (n +1, 1);

X(l) = Xinit;
for i = l:n

X(i + 1) = f (X(i) , r) ;
end

In fact, this is a simple program for computing iteratively the values of a
sequence a n = f(a n - 1 ), n> 1, provided you have previously entered the
formula for the function /"and the initial value of the sequence a®. Note the
extra parameter r built into the algorithm.

Now let’s use the program to compute two populations at five-year
intervals for different values of r:

r = 0.1; Xinit = 100; f = inline('x*(l + r) ' , 'x', 'r');

X = itseq(f , Xinit, 100, r) ;
format long; X (1:5: 101)

ans =

1.0e+006 *

0.00010000000000

0.00016105100000

0.00025937424601

0.00041772481694

158

Chapter?: Applications

0.00067274999493
0.00108347059434
0 . 00174494022689
0 . 00281024368481
0.00452592555682
0.00728904836851
0.01173908528797
0 . 01890591424713
0.03044816395414
0 . 04903707252979
0.07897469567994
0.12718953713951
0.20484002145855
0.32989690295921
0.53130226118483
0.85566760466078
1.37806123398224

r = -0.1; X = itseq(f, Xinit, 100, r) ;

X (1 ; 5 : 101)

1.0e+002 *
1.00000000000000
0.59049000000000
0.34867844010000
0.20589113209465
0.12157665459057
0.07178979876919
0.04239115827522
0.02503155504993
0.01478088294143
0.00872796356809
0.00515377520732
0.00304325272217
0.00179701029991
0.00106111661200
0.00062657874822
0.00036998848504
0.00021847450053

Population Dynamics

159

0 . 00012900700782
0 . 00007617734805
0 . 00004498196225
0 . 00002656139889

In the first case, the population is growing rapidly; in the second, it is
decaying rapidly. In fact, it is clear from the model that, for any n, the
quotient P n+ i/P n = (1 + r), and therefore it follows that P n = Po(l + r) n , n > 0.
This accounts for the expression exponential growth and decay. The model
predicts a population growth without bound (for growing populations) and is
therefore not realistic. Our next model allows for a check on the population
caused by limited space, limited food supply, competitors and predators.

Logistic Growth

The previous model assumes that the relative change in population is
constant, that is,

(P n+ 1 -P n )/Pn = r.

Now let’s build in a term that holds down the growth, namely

(Pn + 1 - Pn)/Pn =T- uP n .

We shall simplify matters by assuming that u = 1 + r, so that our recursion
relation becomes

Pn + 1 = uP n {l - P n ),

where u is a positive constant. In this model, the population P is constrained
to lie between 0 and 1, and should be interpreted as a percentage of a
maximum possible population in the environment in question. So let us set
up the function we will use in the iterative procedure:

clear f; f = inline (' u*x* (1 - x) ' , 'x', 'u');

Now let’s compute a few examples; and use plot to display the results.

u = 0.5; Xinit = 0.5; X = itseq(f, Xinit, 20, u) ; plot (X)

160

Chapter?: Applications

u = 1; X = itseq ( f , Xinit, 20, u) ; plot (X)

u = 1.5; X = itseq (f, Xinit, 20, u) ; plot (X)

Population Dynamics

161

u = 3.4; X

itseq(f, Xinit, 20, u) ; plot (X)

In the first computation, we have used our iterative program to compute
the population density for 20 time intervals, assuming a logistic growth
constant u = 0.5 and an initial population density of 50%. The population
seems to be dying out. In the remaining examples, we kept the initial
population density at 50%; the only thing we varied was the logistic growth
constant. In the second example, with a growth constant u = 1, once again
the population is dying out — although more slowly. In the third example,
with a growth constant of 1.5 the population seems to be stabilizing at
33.3...%. Finally, in the last example, with a constant of 3.4 the population
seems to oscillate between densities of approximately 45% and 84%.

These examples illustrate the remarkable features of the logistic
population dynamics model. This model has been studied for more than 150
years, with its origins lying in an analysis by the Belgian mathematician
Pierre Verhulst. Here are some of the facts associated with this model. We
will corroborate some of them with MATLAB. In particular, we shall use bar
as well as plot to display some of the data.

(1) The Logistic Constant Cannot Be Larger Than 4

For the model to work, the output at any point must be between 0 and 1. But
the parabola ux( 1 — x), for 0 < x < 1, has its maximum height when x = 1/2,
where its value is a/4. To keep that number between 0 and 1, we must
restrict u to be at most 4. Here is what happens if u is bigger than 4:

162

Chapter?: Applications

u = 4.5; Xinit = 0.9; X = itseq(f, Xinit, 10, u)

X =

1 . 0 e +072 *

0.00000000000000

0.00000000000000

0.00000000000000

- 0.00000000000000

- 0.00000000000000

- 0.00000000000000

- 0.00000000000000

- 0.00000000000000

- 0.00000000000000

- 0.00000000000000

- 3.49103403458070

(2) If 0 < u < 1 , the Population Density Tends to Zero for Any
Initial Configuration

X = itseq(f , 0.99, 100, 0.8); X(101)

ans =

1 . 93 9524 0246913 87 e - 012

X = itseq(f , 0.75, 20, 1) ;
bar (X)

Population Dynamics

163

(3) If 1 < u < 3, the Population Will Stabilize at Density 1-1/ufor
Any Initial Density Other Than Zero

The third of the original four examples corroborates the assertion (with
u = 1.5 and 1 — 1 /u= 1/3). In the following examples, we set u = 2, 2.5, and
3, so that 1 — 1/u equals 0.5, 0.6, and 0.666..., respectively. The convergence
in the last computation is rather slow (as one might expect from a boundary
case — or bifurcation point).

X = itseq(f , 0.25, 100, 2); X(101)

ans =

0.50000000000000

X = itseq(f , 0.75, 100, 2); X(101)

ans =

0.50000000000000

X = itseq(f , 0.5, 20, 2.5) ;
plot (X)

X = itseq (f , 0.75, 100, 3);
bar (X) ; axis([0 100 0 0.8])

164

Chapter?: Applications

(4) If 3 < u < 3.56994 . . . , Then There Is a Periodic Cycle

The theory is quite subtle. For a fuller explanation, the reader may consult
Encounters with Chaos, by Denny Gulick, McGraw-Hill, 1992, Section 1.5. In
fact there is a sequence

Uq = 3 < Ml = 1 + V6 < «2 < U 3 < . . . < 4,

such that between uq and u\ there is a cycle of period 2, between u\ and U 2
there is a cycle of period 4, and in general, between uk and Uk + 1 there is a
cycle of period 2 k+1 . One also knows that, at least for small k, one has the
approximation Uk + 1 ~ 1 + -/3 + «*.. So

ul = 1 + sqrt(6)

ul =

3.44948974278318

u2approx = 1 + sqrt(3 + ul)

u2approx =

3.53958456106175

This explains the oscillatory behavior we saw in the last of the original four
examples (with uo < u = 3.4 < u\). Here is the behavior for u\ < u = 3.5 < U 2 .
The command bar is particularly effective here for spotting the cycle of
order 4.

X = itseq(f , 0.75, 100, 3.5);
bar (X) ; axis([0 100 0 0.9])

Population Dynamics

165

(5) There Is a Value u < 4 Beyond Which Chaos Ensues

It is possible to prove that the sequence Uk tends to a limit u^. The value of
Uoo, sometimes called the Feigenbaum parameter, is aproximately 3.56994... .
Let’s see what happens if we use a value of u between the Feigenbaum
parameter and 4.

X = itseq(f , 0.75, 100, 3.7); plot (X)

This is an example of what mathematicians call a chaotic phenomenon! It
is not random — the sequence was generated by a precise, fixed
mathematical procedure — but the results manifest no predictible pattern.
Chaotic phenomena are unpredictable, but with modern methods (including
computer analysis), mathematicians have been able to identify certain
patterns of behavior in chaotic phenomena. For example, the last figure

166

Chapter?: Applications

suggests the possibility of unstable periodic cycles and other recurring
phenomena. Indeed a great deal of information is known. The
aforementioned book by Gulick is a fine reference, as well as the source of an
excellent bibliography on the subject.

Rerunning the Model with SIMULINK

The logistic growth model that we have been exploring lends itself
particularly well to simulation using SIMULINK. Here is a simple
SIMULINK model that corresponds to the above calculations:

Let’s briefly explain how this works. If you ignore the Discrete Pulse
Generator block and the Sum block in the lower left for a moment, this
model implements the equation

x at next time = ux( 1 — x) at old time,

which is the equation for the logistic model. The Scope block displays a plot
of x as a function of (discrete) time. However, we need somehow to build in
the initial condition for x. The simplest way to do this is as illustrated here:
We add to the right-hand side a discrete pulse that is the initial value of x at
time t = 0 and is 0 thereafter. Since the model is discrete, you can achieve
this by setting the period of the Discrete Pulse Generator block to something
longer than the length of the simulation, and setting the width of the pulse

Population Dynamics

167

to 1 and the amplitude of the pulse to the initial value of x. The outputs
from the model in the two interesting cases of u = 3.4 and u = 3.7 are shown
here:

Output with u = 3.7

In the first case of u = 3.4, the periodic behavior is clearly visible. However,
when u = 3.7, we get chaotic behavior.

168

Chapter?: Applications

Linear Economic Models

MATLAB’s linear algebra capabilities make it a good vehicle for studying
linear economic models, sometimes called Leontief models (after their
primary developer, Nobel Prize-winning economist Wassily Leontief) or
input-output models. We will give a few examples. The simplest such model
is the linear exchange model or closed Leontief model of an economy. This
model supposes that an economy is divided into, say, n sectors, such as
agriculture, manufacturing, service, consumers, etc. Each sector receives
input from the various sectors (including itself) and produces an output,
which is divided among the various sectors. (For example, agriculture
produces food for home consumption and for export, but also seeds and new
livestock, which are reinvested in the agricultural sector, as well as
chemicals that may be used by the manufacturing sector, and so on.) The
meaning of a closed model is that total production is equal to total
consumption. The economy is in equilibrium when each sector of the
economy (at least) breaks even. For this to happen, the prices of the various
outputs have to be adjusted by market forces. Let % denote the fraction of
the output of the yth sector consumed by the tth sector. Then the % are the
entries of a square matrix, called the exchange matrix A, each of whose
columns sums to 1. Let pt be the price of the output of the tth sector of the
economy. Since each sector is to at least break even, p, cannot be smaller
than the value of the inputs consumed by the tth sector, or in other words,

Pi > YjfyPJ-
j

But summing over i and using the fact that £)• a;/ = 1, we see that both sides
must be equal. In matrix language, that means that (7 — A)p = 0, where p is
the column vector of prices. Thus p is an eigenvector of A for the eigenvalue
1, and the theory of stochastic matrices implies (assuming that A is
irreducible, meaning that there is no proper subset E of the sectors of the
economy such that outputs from E all stay inside E) that p is uniquely
determined up to a scalar factor. In other words, a closed irreducible linear
economy has an essentially unique equilibrium state. For example, if we
have

A = [.3, .1, .05, .2; .1, .2, .3, .3; .3, .5, .2, .3; .3,

.2, .45, .2]

Linear Economic Models

169

then as required,

sum (A)

ans =

0.3000

0.1000

0.3000

0.3000

0.1000

0.2000

0.5000

0.2000

0.0500

0.3000

0.2000

0.4500

0.2000

0.3000

0.3000

0.2000

1111
That is, all the columns sum to 1, and

[V, D] = eig(A) ; D ( 1 , 1)
p = V ( : , 1)

ans =

0.2739

0.4768

0.6133

0.5669

shows that 1 is an eigenvalue of A with price eigenvector p as shown.

Somewhat more realistic is the (static, linear) open Leontief model of an
economy, which takes labor, consumption, etc., into account. Let’s illustrate
with an example. The following cell inputs an actual input-output
transactions table for the economy of the United Kingdom in 1963. (This
table is taken from Input-Output Analysis and its Applications by
R. O’Connor and E. W. Henry, Hafner Press, New York, 1975.) Tables such
as this one can be obtained from official government statistics. The table T
is a 10 x 9 matrix. Units are millions of British pounds. The rows represent
respectively, agriculture, industry, services, total inter-industry, imports,
sales by final buyers, indirect taxes, wages and profits, total primary inputs,
and total inputs. The columns represent, respectively, agriculture, industry,
services, total inter-industry, consumption, capital formation, exports, total
final demand, and output. Thus outputs from each sector can be read off
along a row, and inputs into a sector can be read off along a column.

170

Chapter?: Applications

T = [277, 444, 14, 735, 1123, 35, 51, 1209, 1944; ...

587, 11148, 1884, 13619, 8174, 4497, 3934, 16605, 30224; ...

236, 2915, 1572, 4723, 11657, 430, 1452, 13539, 18262; ...
1100, 14507, 3470, 19077, 20954, 4962, 5437, 31353, 50430; ...

133, 2844, 676, 3653, 1770, 250, 273, 2293, 5946; ...

3, 134, 42, 179, -90, -177, 88, -179, 0; ...

-246, 499, 442, 695, 2675, 100, 17, 2792, 3487; ...

954, 12240, 13632, 26826, 0, 0, 0, 0, 26826; ...

844, 15717, 14792, 31353, 4355, 173, 378, 4906, 36259; ...
1944, 30224, 18262, 50430, 25309, 5135, 5815, 36259, 86689];

A few features of this matrix are apparent from the following:

T (4 , :) - T ( 1 , ;) - T(2, ;) - T(3, :)

T (9 , :) - T (5, :) - T(6, ;) - T(7, ;) - T(8, ;)

T (10 , ;) - T (4 , :) - T(9, ;)

T (10 , 1:4) - T (1 : 4 , 9) '

000000000
000000000
000000000
0 0 0 0

Thus the 4th row, which summarizes inter-industry inputs, is the sum of the
first three rows; the 9th row, which summarizes “primary inputs,” is the sum
of rows 5 through 8; the 10th row, total inputs, is the sum of rows 4 and 9,
and the first four entries of the last row agree with the first four entries of
the last column (meaning that all output from the industrial sectors is
accounted for). Also we have

(T ( : ,

, 4)

- T ( : ,

, 1)

- T ( : ,

r 2) -

T ( : ,

, 3))

(T ( : ,

r 8)

- T ( : ,

, 5)

- T ( : ,

r 6) -

T ( : ,

, 7))

(T ( : ,

r 9)

- T ( : ,

, 4)

- T ( : ,

, 8))'

Linear Economic Models

171

0000000000

0000000000

0000000000

So the 4th column, representing total inter-industry output, is the sum of
columns 1 through 3; the 8th column, representing total “final demand,” is
the sum of columns 5 through 7 ; and the 9th column, representing total
output, is the sum of columns 4 and 8. The matrix A of inter-industry
technical coefficients is obtained by dividing the columns of T corresponding
to industrial sectors (in our case there are three of these) by the
corresponding total inputs. Thus we have

A = [T ( : , 1) /T (10 , 1), T ( : , 2)/T(10, 2), T(:, 3)/T(10, 3)]

A =

0.1425 0.0147 0.0008

0.3020 0.3688 0.1032

0.1214 0.0964 0.0861

0.5658 0.4800 0.1900

0.0684 0.0941 0.0370

0.0015 0.0044 0.0023

- 0.1265 0.0165 0.0242

0.4907 0.4050 0.7465

0.4342 0.5200 0.8100

1.0000 1.0000 1.0000

Here the square upper block (the first three rows) is most important, so we
make the replacement

A = A (1 : 3 , :)

A =

0.1425 0.0147 0.0008

0.3020 0.3688 0.1032

0.1214 0.0964 0.0861

If the vector Y represents total final demand for the various industrial
sectors, and the vector X represents total outputs for these sectors, then the

172

Chapter?: Applications

fact that the last column of T is the sum of columns 4 (total inter-industry
outputs) and 8 (total final demand) translates into the matrix equation
X = AX + Y, or Y = (1 — A)X. Let’s check this:

Y = T (1 : 3 , 8); X = T(l:3, 9); Y - (eye(3) - A) *X

0

0

0

Now one can do various numerical experiments. For example, what would
be the effect on output of an increase of £10 billion (10,000 in the units of
our problem) in final demand for industrial output, with no corresponding
increase in demand for services or for agricultural products? Since the
economy is assumed to be linear, the change AX in X is obtained by solving
the linear equation AY = (1 — A) AX, and

deltaX = (eye (3) - A) \ [0; 10000; 0]

deltaX =

1.0e+004 *

0.0280

1.6265

0.1754

Thus agricultural output would increase by £280 million, industrial output
would increase by £16.265 billion, and service output would increase by
£1.754 billion. We can illustrate the result of doing this for similar increases
in demand for the other sectors with the following pie charts:

deltaXl = (eye (3 ) - A) \ [10000; 0; 0] ;
deltaX2 = (eye (3 ) - A) \ [0; 0; 10000];

subplotd, 3, 1), pie(deltaXl, {'Agr.', 'Ind.', 'Serv.'}) ,
subplotd, 3, 2), pie(deltaX, {'Agr.', 'Ind.', 'Serv.'}),
title (' Effect of increases in demand for each of the 3
sectors', ' FontSize' , 18) ,

subplotd, 3, 3), pie(deltaX2, {'Agr.', 'Ind.', 'Serv.'});

Linear Programming

173

€>© ^

Linear Programming

MATLAB is ideally suited to handle linear programming problems. These
are problems in which you have a quantity, depending linearly on several
variables, that you want to maximize or minimize subject to several
constraints that are expressed as linear inequalities in the same variables. If
the number of variables and the number of constraints are small, then there
are numerous mathematical techniques for solving a linear progra mmin g
problem — indeed these techniques are often taught in high school or
university courses in finite mathematics. But sometimes these numbers are
high, or even if low, the constants in the linear inequalities or the object
expression for the quantity to be optimized may be numerically
complicated — in which case a software package like MATLAB is required to
effect a solution. We shall illustrate the method of linear programming by
means of a simple example, giving a combination graphical-numerical
solution, and then solve both a slightly and a substantially more complicated
problem.

Suppose a farmer has 75 acres on which to plant two crops: wheat and
barley. To produce these crops, it costs the farmer (for seed, fertilizer, etc.)
$120 per acre for the wheat and $210 per acre for the barley. The farmer has
$15,000 available for expenses. But after the harvest, the farmer must store
the crops while awaiting favorable market conditions. The farmer has

174

Chapter?: Applications

storage space for 4,000 bushels. Each acre yields an average of 110 bushels
of wheat or 30 bushels of barley. If the net profit per bushel of wheat (after
all expenses have been subtracted) is $1.30 and for barley is $2.00, how
should the farmer plant the 75 acres to maximize profit?

We begin by formulating the problem mathematically. First we express
the objective, that is, the profit, and the constraints algebraically, then we
graph them, and lastly we arrive at the solution by graphical inspection and
a minor arithmetic calculation.

Let x denote the number of acres allotted to wheat and y the number of
acres allotted to barley. Then the expression to be maximized, that is, the
profit, is clearly

P = (110X1.30)* + (30)(2.00)y = 143x + 60y.

There are three constraint inequalities, specified by the limits on expenses,
storage, and acreage. They are respectively

12 Ox + 210y < 15,000
llOx + 30y < 4,000

x +y < 75.

Strictly speaking there are two more constraint inequalities forced by the
fact that the farmer cannot plant a negative number of acres, namely,

x > 0,y > 0.

Next we graph the regions specified by the constraints. The last two say
that we only need to consider the first quadrant in the x-y plane. Here’s a
graph delineating the triangular region in the first quadrant determined by
the first inequality.

X = 0:125;

Y1 = (15000 - 120 . *X) . /210 ;

area (X, Yl)

Linear Programming

175

Now let’s put in the other two constraint inequalities.

Y2 = max ( (4000 - 110.*X)./30, 0) ;

Y3 = max (75 - X, 0) ;

Ytop = min ( [Y1 ; Y2 ; Y3] ) ;
area(X, Ytop)

It’s a little hard to see the polygonal boundary of the region clearly. Let’s
hone in a bit.

area (X, Ytop); axis([0 40 40 75])

176

Chapter?: Applications

Now let’s superimpose on top of this picture a contour plot of the objective
function P.

hold on

[U V] = meshgrid(0:40, 40:75);
contour (U, V, 143. *U + 60. *V); hold off

It seems apparent that the maximum value of P will occur on the level curve
(that is, level line) that passes through the vertex of the polygon that lies
near (23, 53). In fact we can compute

[x, y] = solve ( ' x + y = 75', '110*x + 30*y = 4000')

x =

175/8

Linear Programming

177

y =

425/8

doublet [x, y] )

ans =

21.8750 53.1250

The acreage that results in the maximum profit is 21.875 for wheat and
53.125 for barley. In that case the profit is

P = 143*x + 60*y

50525/8

format bank; double (P)

ans =

6315.63

that is, $6,315.63.

This problem illustrates and is governed by the Fundamental Theorem of
Linear Programming, stated here in two variables:

A linear expression ax + by, defined over a closed bounded convex set
S whose sides are line segments, takes on its maximum value at a
vertex of S and its minimum value at a vertex of S. If S is unbounded,
there may or may not be an optimum value, but if there is, it occurs at a
vertex. (A convex set is one for which any line segment joining two
points of the set lies entirely inside the set.)

In fact the SIMULINK toolbox has a built-in function, simlp, that
implements the solution of a linear programming problem. The optimization
toolbox has an almost identical function called 1 inprog. You can learn
about either one from the online help. We will use simlp on the above
problem. After that we will use it to solve two more complicated problems
involving more variables and constraints. Here is the beginning of the
output from help simlp:

SIMLP Helper function for GETXO; solves linear programming

problem.

178

Chapter?: Applications

X=SIMLP ( f , A, b) solves the linear programming problem:
min f 'x subject to: Ax <= b

So

f = [-143 -60] ;

A = [120 210; 110 30; 1 1; -1 0; 0 -1] ;
b = [15000; 4000; 75; 0; 0] ;

format short; simlp(f. A, b)

ans =

21.8750

53.1250

This is the same answer we obtained before. Note that we entered the
negative of the coefficient vector for the objective function P because simlp
searches for a minimum rather than a maximum. Note also that the
nonnegativity constraints are accounted for in the last two rows of A and b.

Well, we could have done this problem by hand. But suppose that the
farmer is dealing with a third crop, say corn, and that the corresponding
data are

cost per acre $150.75

yield per acre 125 bushels

profit per bushel $1.56.

If we denote the number of acres allotted to corn by 2 , then the objective
function becomes

P = (110X1.30)* + (30X2.00)3/ + (125X1.56) = 143x + 60y + 1952,
and the constraint inequalities are

120x + 210y + 150.752 < 15,000
llOx + 30y + 125 2 < 4,000
x +y +2 < 75
x > 0,y > 0,2 > 0.

The problem is solved with simlp as follows:

clear f A b; f = [-143 -60 -195];

A = [120 210 150.75; 110 30 125; 1 1 1;...

Linear Programming

179

-1 0 0; 0 -1 0; 0 0 -1];

b = [15000; 4000; 75; 0; 0; 0] ;

simlp(f. A, b)

ans =

0 . 0000
56.5789
18.4211

So the farmer should ditch the wheat and plant 56.5789 acres of barley and
18.4211 acres of corn.

There is no practical limit on the number of variables and constraints that
MATLAB can handle — certainly none that the relatively unsophisticated
user will encounter. Indeed, in many true applications of the technique of
linear programming, one needs to deal with many variables and constraints.
The solution of such a problem by hand is not feasible, and software such as
MATLAB is crucial to success. For example, in the farming problem with
which we have been working, one could have more than two or three crops.
(Think agribusiness instead of family farmer.) And one could have
constraints that arise from other things besides expenses, storage, and
acreage limitations, for example:

• Availability of seed. This might lead to constraint inequalities such as

Xj < k.

• Personal preferences. Thus the farmer’s spouse might have a preference
for one variety or group of varieties over another, and insist on a
corresponding planting, thus leading to constraint inequalities such as

Xi < Xj or xi + x 2 > X 3 .

• Government subsidies. It may take a moment’s reflection on the
reader’s part, but this could lead to inequalities such as xj > k.

Below is a sequence of commands that solves exactly such a problem. You
should be able to recognize the objective expression and the constraints from
the data that are entered. But as an aid, you might answer the following
questions:

• How many crops are under consideration?

• What are the corresponding expenses? How much money is available
for expenses?

• What are the yields in each case? What is the storage capacity?

• How many acres are available?

180

Chapter?: Applications

• What crops are constrained by seed limitations? To what extent?

• What about preferences?

• What are the minimum acreages for each crop?
clear f A b

f = [-110*1.3 -30*2.0 -125*1.56 -75*1.8 -95*. 95 -100*2.25 -
50*1.35] ;

A = [120 210 150.75 115 186 140 85; 110 30 125 75 95 100 50;
1111111 ; 1000000 ; 0010000 ; 0000010 ;

1 -1 0 0 0 0 0 ; 0 0 1 0 -2 0 0 ; 0 0 0 -1 0 -1 1 ;

-1 0 0 0 0 0 0; 0 -1 0 0 0 0 0; 0 0 -1 0 0 0 0;

0 0 0 -1 0 0 0; 0 0 0 0 -1 0 0; 0 0 0 0 0 -1 0;

0 0 0 0 0 0 - 1 ];

b = [55000;40000;400;100;50;250;0;0; 0; -10; -10; -10;

- 10 ; - 20 ; - 20 ; - 20 ] ;

slmlp(f. A, b)

ans =

10.0000

10.0000

40.0000
45.6522

20.0000

250.0000

20.0000

Note that despite the complexity of the problem, MATLAB solves it almost
instantaneously. It seems the farmer should bet the farm on crop number 6.
We suggest you alter the expense and/or the storage limit in the problem and
see what effect that has on the answer.

The 360° Pendulum

Normally we think of a pendulum as a weight suspended by a flexible string
or cable, so that it may swing back and forth. Another type of pendulum
consists of a weight attached by a light (but inflexible) rod to an axle, so that

The 360° Pendulum

181

it can swing through larger angles, even making a 360° rotation if given
enough velocity.

Though it is not precisely correct in practice, we often assume that the
magnitude of the frictional forces that eventually slow the pendulum to a
halt is proportional to the velocity of the pendulum. Assume also that the
length of the pendulum is 1 meter, the weight at the end of the pendulum
has mass 1 kg, and the coefficient of friction is 0.5. In that case, the
equations of motion for the pendulum are

x'(t) = y{t), y'it) = — 0.5y(£) — 9.81 sin(x(£)),
where t represents time in seconds, x represents the angle of the pendulum
from the vertical in radians (so that x = 0 is the rest position), y represents
the velocity of the pendulum in radians per second, and 9.81 is
approximately the acceleration due to gravity in meters per second squared.
Here is a phase portrait of the solution with initial position x(0) = 0 and
initial velocity y(0) = 5. This is a graph of x versus y as a function of t, on the
time interval 0 < t < 20.

g = inline (' [x (2) ; -0.5*x(2) - 9 . 81*sin (x (1) ) ] ' , ' t' , 'x');
[t, xa] = ode45 (g, [0 20], [0 5]);

plot(xa(:, 1), xa(:, 2))

Recall that the x coordinate corresponds to the angle of the pendulum and
the y coordinate corresponds to its velocity. Starting at (0, 5), as t increases
we follow the curve as it spirals clockwise toward (0, 0). The angle oscillates
back and forth, but with each swing it gets smaller until the pendulum is
virtually at rest by the time t = 20. Meanwhile the velocity oscillates as well,
taking its maximum value during each oscillation when the pendulum is in

182

Chapter?: Applications

the middle of its swing (the angle is near zero) and crossing zero when the
pendulum is at the end of its swing.

Next we increase the initial velocity to 10.

[t, xa] = ode45 (g, [0 20], [0 10]);

plot(xa(:, 1), xa ( : , 2))

This time the angle increases to over 14 radians before the curve spirals in to
a point near (12.5, 0). More precisely, it spirals toward (4 tt, 0), because 4jr
radians represents the same position for the pendulum as 0 radians does.
The pendulum has swung overhead and made two complete revolutions
before beginning its damped oscillation toward its rest position. The velocity
at first decreases but then rises after the angle passes through n, as the
pendulum passes the upright position and gains momentum. The pendulum
has just enough momentum to swing through the upright position once more
at the angle 3n.

Now suppose we want to find, to within 0.1, the minimum initial velocity
required to make the pendulum, starting from its rest position, swing
overhead once. It will be useful to be able to see the solutions corresponding
to several different initial velocities on one graph.

First we consider the integer velocities 5 to 10.

hold on
for a = 5:10

[t, xa] = ode45 (g, [0 20], [0 a]);

plot(xa(:, 1), xa(:, 2))
end

hold off

The 360° Pendulum

183

Initial velocities 5, 6, 7 are not large enough for the angle to increase past n,
but initial velocities 8, 9, 10 are enough to make the pendulum swing over-
head. Let’s see what happens between 7 and 8.

hold on

for a = 7. 0:0. 2:8.0

[t, xa] = ode45(g, [0 20], [0 a]);

plot(xa(:, 1), xa(:, 2))
end

hold off

We see that the cutoff is somewhere between 7.2 and 7.4. Let’s make one
more refinement.

hold on

for a = 7.2:0.05:7.4

[t, xa] = ode45(g, [0 20], [0 a]);

184

Chapter?: Applications

plot(xa(:, 1), xa ( : , 2))
end

hold off

We conclude that the minimum velocity needed is somewhere between 7.25
and 7.3.

Numerical Solution of the
Heat Equation

In this section we will use MATLAB to numerically solve the heat equation
(also known as the diffusion equation), a partial differential equation that
describes many physical processes including conductive heat flow or the
diffusion of an impurity in a motionless fluid. You can picture the process of
diffusion as a drop of dye spreading in a glass of water. (To a certain extent
you could also picture cream in a cup of coffee, but in that case the mixing is
generally complicated by the fluid motion caused by pouring the cream into
the coffee and is further accelerated by stirring the coffee.) The dye consists
of a large number of individual particles, each of which repeatedly bounces
off of the surrounding water molecules, following an essentially random
path. There are so many dye particles that their individual random motions
form an essentially deterministic overall pattern as the dye spreads evenly
in all directions (we ignore here the possible effect of gravity). In a similar
way, you can imagine heat energy spreading through random interactions of
nearby particles.

In a three-dimensional medium, the heat equation is
du _ / 3 2 u d 2 u 3 2 w\

3 * = \ d^ 2 + dy 2+ ~d^)'

Numerical Solution of the Heat Equation

185

Here u is a function of t, x, y, and 2 that represents the temperature, or
concentration of impurity in the case of diffusion, at time t at position (x, y, z )
in the medium. The constant k depends on the materials involved; it is
called the thermal conductivity in the case of heat flow and the diffusion
coefficient in the case of diffusion. To simplify matters, let us assume that the
medium is instead one-dimensional. This could represent diffusion in a thin
water-filled tube or heat flow in a thin insulated rod or wire; let us think
primarily of the case of heat flow. Then the partial differential equation
becomes

3 u _ 3 2 u
3 t dx 2 ’

where u(x, t) is the temperature at time t a distance x along the wire.

A Finite Difference Solution

To solve this partial differential equation we need both initial conditions of
the form u{x, 0) = fix), where fix) gives the temperature distribution in the
wire at time 0, and boundary conditions at the endpoints of the wire; call
them x = a and x = b. We choose so-called Dirichlet boundary conditions
u(a, t) = T a and uib, t) = %, which correspond to the temperature being held
steady at values T a and % at the two endpoints. Though an exact solution is
available in this scenario, let us instead illustrate the numerical method of
finite differences.

To begin with, on the computer we can only keep track of the temperature
u at a discrete set of times and a discrete set of positions x. Let the times be
0,A£, 2A t, ... , NAt, and let the positions be a, a + Ax, ... ,a + JAx = b, and
let u'i = u{a + j At, nAt). Rewriting the partial differential equation in terms
of finite-difference approximations to the derivatives, we get

U T' ~ u j = k u h 2u j + u j-i
At Ax 2

(These are the simplest approximations we can use for the derivatives, and
this method can be refined by using more accurate approximations,
especially for the t derivative.) Thus if for a particular n, we know the values
of Uj for all j, we can solve the equation above to find u”j +1 for each j:

“f 1 = u j + ^ (“"+1 “ 2u ] + U J- 1) = s ( u j+i + u U) + (1 - 2s K

where s = kAt/{Ax) 2 . In other words, this equation tells us how to find the
temperature distribution at time step n + 1 given the temperature

Chapter?: Applications

distribution at time step n. (At the endpoints j = 0 and j = J, this equation
refers to temperatures outside the prescribed range for x, but at these points
we will ignore the equation above and apply the boundary conditions
instead.) We can interpret this equation as saying that the temperature at a
given location at the next time step is a weighted average of its temperature
and the temperatures of its neighbors at the current time step. In other
words, in time At, a given section of the wire of length Ax transfers to each
of its neighbors a portion s of its heat energy and keeps the remaining
portion 1 — 2s of its heat energy. Thus our numerical implementation of the
heat equation is a discretized version of the microscopic description of
diffusion we gave initially, that heat energy spreads due to random
interactions between nearby particles.

The following M-file, which we have named heat . m, iterates the
procedure described above:

function u = heat(k, x, t, init, bdry)

% solve the ID heat equation on the rectangle described by
% vectors x and t with u(x, t(l)) = init and Dirichlet
% boundary conditions

% u(x(l), t) = bdry(l), u(x(end), t) = bdry(2).

J = length (x);

N = length (t);
dx = mean (dif f (x) ) ;
dt = mean (dif f (t) ) ;
s = k*dt/dx~2 ;

u = zeros (N, J) ;
u(l, :) = init;

for n = 1 : N- 1

u (n+1, 2 ; J- 1) = s* (u (n, 3:J) + u(n, l:J-2)) +...

(1 - 2*s) *u (n, 2: J-l) ;
u (n+1, 1) = bdry (1) ;
u (n+1, J) = bdry (2) ;
end

The function heat takes as inputs the value of k, vectors of t and x values, a
vector init of initial values (which is assumed to have the same length as
x), and a vector bdry containing a pair of boundary values. Its output is a
matrix of u values. Notice that since indices of arrays in MATLAB must start
at 1, not 0, we have deviated slightly from our earlier notation by letting n=l

Numerical Solution of the Heat Equation

187

represent the initial time and j =1 represent the left endpoint. Notice also
that in the first line following the for statement, we compute an entire row
of u, except for the first and last values, in one line; each term is a vector of
length J-2, with the index j increased by 1 in the term u (n, 3 s J) and
decreased by 1 in the term u (n , 1 : J- 2 ) .

Let’s use the M-file above to solve the one-dimensional heat equation with
k = 2 on the interval —5 < x < 5 from time 0 to time 4, using boundary
temperatures 15 and 25, and initial temperature distribution of 15 for x < 0
and 25 for x > 0. You can imagine that two separate wires of length 5 with
different temperatures are joined at time 0 at position x = 0, and each of
their far ends remains in an environment that holds it at its initial
temperature. We must choose values for At and Ax; let’s try At = 0.1 and
Ax = 0.5, so that there are 41 values of t ranging from 0 to 4 and 21 values
of x ranging from —5 to 5.

tvals = linspace(0, 4, 41);
xvals = linspace(-5, 5, 21);
init = 20 + 5*sign (xvals) ;

uvals = heat(2, xvals, tvals, init, [15 25]);
surf (xvals, tvals, uvals)
xlabel x; ylabel t; zlabel u

188

Chapter?: Applications

Here we used surf to show the entire solution u{x, t). The output is clearly
unrealistic; notice the scale on the u axis! The numerical solution of partial
differential equations is fraught with dangers, and instability like that seen
above is a common problem with finite difference schemes. For many partial
differential equations a finite difference scheme will not work at all, but for
the heat equation and similar equations it will work well with proper choice
of At and Ax. One might be inclined to think that since our choice of Ax was
larger, it should be reduced, but in fact this would only make matters worse.
Ultimately the only parameter in the iteration we’re using is the constant s,
and one drawback of doing all the computations in an M-file as we did above
is that we do not automatically see the intermediate quantities it computes.
In this case we can easily calculate that s = 2(0.1)/(0.5) 2 = 0.8. Notice that
this implies that the coefficient 1 — 2s of vA- in the iteration above is negative.
Thus the “weighted average” we described before in our interpretation of the
iterative step is not a true average; each section of wire is transferring more
energy than it has at each time step!

The solution to the problem above is thus to reduce the time step At; for
instance, if we cut it in half, then s = 0.4, and all coefficients in the iteration
are positive.

tvals = linspace(0, 4, 81);

uvals = heat (2 , xvals, tvals, init, [15 25]);
surf (xvals, tvals, uvals)
xlabel x; ylabel t; zlabel u

Numerical Solution of the Heat Equation

189

This looks much better! As time increases, the temperature distribution
seems to approach a linear function of x. Indeed u(x, t) = 20 + x is
the limiting “steady state” for this problem; it satisfies the boundary
conditions and it yields 0 on both sides of the partial differential
equation.

Generally speaking, it is best to understand some of the theory of partial
differential equations before attempting a numerical solution like we have
done here. However, for this particular case at least, the simple rule of
thumb of keeping the coefficients of the iteration positive yields realistic
results. A theoretical examination of the stability of this finite difference
scheme for the one-dimensional heat equation shows that indeed any value
of s between 0 and 0.5 will work, and it suggests that the best value of At to
use for a given Ax is the one that makes s = 0.25. (See Partial Differential
Equations: An Introduction, by Walter A. Strauss, John Wiley and Sons,
1992.) Notice that while we can get more accurate results in this case by
reducing Ax, if we reduce it by a factor of 10 we must reduce At by a factor of
100 to compensate, making the computation take 1000 times as long and use
1000 times the memory!

The Case of Variable Conductivity

Earlier we mentioned that the problem we solved numerically could also be
solved analytically. The value of the numerical method is that it can be
applied to similar partial differential equations for which an exact solution
is not possible or at least not known. For example, consider the
one-dimensional heat equation with a variable coefficient, representing an
inhomogeneous material with varying thermal conductivity k(x).

du

dt

(« d S=* w S + *' w S-

For the first derivatives on the right-hand side, we use a symmetric finite
difference approximation, so that our discrete approximation to the partial
differential equations becomes

U T ~ U 1 = j, . U J + 1 ~ 2U ’j + U U kj + 1 - kj—\ U n j+ 1 ~

At j Ax 2 2Ax 2Ax

where kj = k(a + j Ax). Then the time iteration for this method is

uf 1 = Sj(u n j+ 1 + + (1 - 2s j) u n j + 0.25 (S /+1 - s,-_ i) (u n j+1 - w^i)>

190

Chapter?: Applications

where sj = kj At /(Ax) 2 . In the following M-file, which we called heatvc . m,
we modify our previous M-file to incorporate this iteration.

function u = heatvc (k, x, t, init, bdry)

% Solve the ID heat equation with variable coefficient k on
% the rectangle described by vectors x and t with
% u(x, t(l)) = init and Dirichlet boundary conditions
% u(x(l), t) = bdry(l), u(x(end), t) = bdry(2).

J = length (x);

N = length (t);
dx = mean (dif f (x) ) ;
dt = mean (dif f (t) ) ;
s = k*dt/dx~2 ;

u = zeros (N, J) ;
u (1, : ) = init;

for n = 1 :N-1

u (n+1, 2 : J-l) = s (2 ; J- 1) . * (u (n, 3:J) + u(n, l:J-2)) + ...

(1 - 2*s(2: J-l) ) .*u(n,2:J-l) + ...

0 . 25* (s (3 ; J) - s (Is J-2) ) .* (u(n, 3:J) - u(n, l:J-2));
u(n+l, 1) = bdry (1) ;
u(n+l, J) = bdry (2) ;
end

Notice that k is now assumed to be a vector with the same length as x and
that as a result so is s. This in turn requires that we use vectorized
multiplication in the main iteration, which we have now split into three
lines.

Let’s use this M-file to solve the one-dimensional variable-coefficient heat
equation with the same boundary and initial conditions as before, using
k(x) = 1 + (x/5) 2 . Since the maximum value of k is 2, we can use the same
values of At and Ax as before.

kvals = 1 + (xvals/5) . ~2 ;

uvals = heatvc (kvals, xvals, tvals, init, [15 25]);
surf (xvals, tvals, uvals)
xlabel x; ylabel t; zlabel u

Numerical Solution of the Heat Equation

191

In this case the limiting temperature distribution is not linear; it has a
steeper temperature gradient in the middle, where the thermal
conductivity is lower. Again one could find the exact form of this limiting
distribution, u(x, t) = 20(1 + (l/;r)arctan(x/5)), by setting the t derivative
to zero in the original equation and solving the resulting ordinary
differential equation.

You can use the method of finite differences to solve the heat equation
in two or three space dimensions as well. For this and other partial
differential equations with time and two space dimensions, you can also
use the PDE Toolbox, which implements the more sophisticated finite
element method.

A SIMULINK Solution

We can also solve the heat equation using SIMULINK. To do this we
continue to approximate the x derivatives with finite differences, but we
think of the equation as a vector-valued ordinary differential equation, with
t as the independent variable. SIMULINK solves the model using MATLAB’s
ODE solver, ode45. To illustrate how to do this, let’s take the same example
we started with, the case where k = 2 on the interval — 5 < x < 5 from time 0
to time 4, using boundary temperatures 15 and 25, and initial temperature
distribution of 15 for x < 0 and 25 for x > 0. We replace u(x, t) for fixed t by
the vector u of values of u(x, t), with, say, x = -5:5. Here there are 11

192

Chapter?: Applications

values of x at which we are sampling u, but since u(x, t ) is pre-determined at
the endpoints, we can take u to be a 9-dimensional vector, and we just tack
on the values at the endpoints when we’re done. Since we’re replacing
d 2 u/dx 2 by its finite difference approximation and we’ve taken Ax = 1 for
simplicity, our equation becomes the vector-valued ODE

9u ... ,

— = k(Au + c).

Here the right-hand side represents our approximation to k{d 2 u/dx 2 ). The
matrix A is

since we are replacing d 2 u/dx 2 at (n, t ) with u(n — 1, t) — 2 u(n, t ) + u(n + 1, t).
We represent this matrix in MATLAB’s notation by

-2*eye(9) + [zeros (8, 1) , eye (8) ; zeros (1, 9) ] +...

[zeros (8,1) , eye (8) ; zeros (1,9)] '

The vector c comes from the boundary conditions, and has 15 in its first
entry, 25 in its last entry, and Os in between. We represent it in MATLAB’s
notation as [15; zeros (7 , 1) ;25] The formula for c comes from the fact
that u (1) represents u{— 4, t), and d 2 u/dx 2 at this point is approximated by

u(— 5, t) — 2u(— 4, t) + u(— 3, t) = 15 — 2 u (1) + u (2) ,

and similarly at the other endpoint. Here’s a SIMULINK model representing
this equation:

Note that one needs to specify the initial conditions for u as Block
Parameters for the Integrator block, and that in the Block Parameters dialog

Numerical Solution of the Heat Equation

193

box for the Gain block, one needs to set the multiplication type to “Matrix”.
Since u (1) through u (4) represent u{x, t) at x = —4 through —1, and u (6)
through u (9) represent u{x, t) at x = 1 through 4, we take the initial value
of u to be [15*ones (4 , 1) ;20; 25*ones (4,1)]. (The value 20 is a
compromise at x = 0, since this is right in the middle of the regions where u
is 15 and 25.) The output from the model is displayed in the Scope block in
the form of graphs of the various entries of u as functions of t, but it’s more
useful to save the output to the MATLAB Workspace and then plot it with
surf. To do this, go to the menu item Simulation Parameters... in the
Simulation menu of the model. Under the Solver tab, set the stop time to
4.0 (since we are only going out to t = 4), and under the Workspace I/O tab,
check the “States” box under “Save to workspace”, like this:

After you run the model, you will find in your Workspace a 53 x 1 vector
tout, plus a 53 x 9 matrix uout. Each row of these arrays corresponds to a
single time step, and each column of uout corresponds to one value of x. But
remember that we have to add in the values of u at the endpoints as
additional columns in u. So we plot the data as follows:

u= [15*ones (length (tout) , 1) , uout, 25*ones (length (tout) , 1) ] ;
x = -5:5;

surf (x, tout, u)

xlabel('x'), ylabel('t'), zlabel('u')
title (' solution to heat equation in a rod')

194

Chapter?: Applications

Note how similar this is to the picture obtained before. We leave it to the
reader to modify the model for the case of variable heat conductivity.

Solution with pdepe

A new feature of MATLAB 6.0 is a built-in solver for partial differential
equations in one space dimension (as well as time t). To find out more about
it, read the online help on pdepe. The instructions for use of pdepe are quite
explicit but somewhat complicated. The method it uses is somewhat similar
to that used in the SIMULINK solution above; that is, it uses an ODE solver
in t and finite differences in x. The following M-file solves the second
problem above, the one with variable conductivity. Note the use of function
handles and subfunctions.

function heateqex2

% Solves a sample Dirichlet problem for the heat equation in a

% rod, this time with variable conductivity, 21 mesh points

m = 0; %This simply means geometry is linear.

x = linspace ( -5 , 5, 21) ;

t = linspace (0 , 4 , 81) ;

sol = pdepe (m,@pdex, @pdexic,@pdexbc,x, t) ;

% Extract the first solution component as u.

u = sol ( : , : , 1) ;

% A surface plot is often a good way to study a solution.

Numerical Solution of the Heat Equation

195

surf(x,t,u); title ([' Numerical solution',...
'computed with 21 mesh points in x.'])

xlabel('x'), ylabel('t'), zlabel('u')

% A solution profile can also be illuminating,
figure

plot (x,u (end, : ) )

title (' Solution at t = 4')

xlabel('x'), ylabel ( ' u (x, 4 ) ' )

%

function [c,f,s] = pdex (x, t,u,DuDx)
c = 1;

f = (1 + (x/5) . ~2)*DuDx;

% flux is variable conductivity times u_x
s = 0 ;

%

function uO = pdexic (x)

% initial condition at t = 0
uO = 20+5*sign (x) ;

%

function [pl,ql,pr,qr] = pdexbc (xl , ul , xr , ur, t)

% q's are zero since we have Dirichlet conditions
% pi = 0 at the left, pr = 0 at the right endpoint
pi = ul-15;
ql = 0;
pr = ur-25;
qr = 0;

Running it gives
heateqex2

196

Chapter?: Applications

Again the results are very similar to those obtained before.

A Model of Traffic Flow

Everyone has had the experience of sitting in a traffic jam, or of seeing cars
bunch up on a road for no apparent good reason. MATLAB and SIMULINK
are good tools for studying models of such behavior. Our analysis here will be
based on “follow-the-leader” theories of traffic flow, about which you can read
more in Kinetic Theory of Vehicular Traffic, by Ilya Prigogine and Robert
Herman, Elsevier, New York, 1971 or in The Theory of Road Traffic Flow, by
Winifred Ashton, Methuen, London, 1966. We will analyze here an extremely
simple model that already exhibits quite complicated behavior. We consider a
one-lane, one-way, circular road with a number of cars on it (a very primitive
model of, say, the Inner Loop of the Capital Beltway around Washington, DC,
since in very dense traffic, it is hard to change lanes and each lane behaves
like a one-lane road). Each driver slows down or speeds up on the basis of his
or her own speed, the speed of the car directly ahead, and the distance to the
car ahead. But human drivers have a finite reaction time. In other words, it
takes them a certain amount of time (usually about a second) to observe
what is going on around them and to press the gas pedal or the brake, as
appropriate. The standard “follow-the-leader” theory supposes that

u n (t + T) = X.(u n -i(t) - Unit)), (*)

where t is time; T is the reaction time; u n is the position of the nth car; and

A Model of Traffic Flow 1 97

the “sensitivity coefficient” A. may depend on u n -i(t) — u„(t), the spacing
between cars, and/or u n {t), the speed of the nth car. The idea behind this
equation is this. Drivers will tend to decelerate if they are going faster than
the car in front of them, or if they are close to the car in front of them, and
will tend to accelerate if they are going slower than the car in front of them.
In addition, drivers (especially in light traffic) may tend to speed up or slow
down depending on whether they are going slower or faster (respectively)
than a “reasonable” speed for the road (often, but not always, equal to the
posted speed limit). Since our road is circular, in this equation uq is
interpreted as un, where N is the total number of cars.

The simplest version of the model is the one where the “sensitivity
coefficient” A is a (positive) constant. Then we have a homogeneous linear
differential-difference equation with constant coefficients for the velocities
u n (t). Obviously there is a “steady state” solution when all the velocities are
equal and constant (i.e., traffic is flowing at a uniform speed), but what we
are interested in is the stability of the flow, or the question of what effect is
produced by small differences in the velocities of the cars. The solution of (*)
will be a superposition of exponential solutions of the form u n {t) = exp(at)r„,
where the v n s and a are (complex) constants, and the system will be
unstable if the velocities are unbounded; that is, there are any solutions
where the real part of a is positive. Using vector notation, we have

u{t) = exp(a t)v, ii(t + T) = a exp(a T) exp(a t)v.

Substituting back in (*), we get the equation

a exp(a T) exp(a t) v = A(S— I) exp(a t)v,

where

/o • •

• 0

1\

1 0 •

0

0 1 •

s=

1 0

l • •

0 1

0/

is the “shift” matrix that, when it multiplies a vector on the left, cyclically
permutes the entries of the vector. We can cancel the exp(a t) on each side to
get

aexp(a T)v = A(S— I)v, or {S— [1 + (a/A)exp(a T)\I}v = 0, (**)

198

Chapter?: Applications

which says that v is an eigenvector for S with eigenvalue 1 + (a/X)e aT . Since
the eigenvalues of S are the Mh roots of unity, which are evenly spaced
around the unit circle in the complex plane, and closely spaced together for
large N, there is potential instability whenever 1 + (a/X)e aT has absolute
value 1 for some a with positive real part: that is, whenever (aT / XT)e aT can
be of the form e ie — 1 for some aT with positive real part. Whether instability
occurs or not depends on the value of the product XT. We can see this by
plotting values of zexp(z) for 2 = aT = iy a complex number on the critical
line Re z = 0, and comparing with plots of XT(e ,f> — 1) for various values of
the parameter XT.

syms y; expand (i*y* (cos (y) + i*sin(y)))

i*y*cos (y) -y*sin(y)

ezplot ( -y*sin (y) , y*cos (y) , [-2*pi, 2*pi] ) ; hold on

theta = 0:0. 05*pi : 2*pi;

plot ( (1/2) * (cos (theta) - 1), (1/2) *sin (theta) , '-');

plot (cos (theta) - 1, sin(theta), ':')

plot (2* (cos (theta) - 1), 2*sin (theta) , '--');

title (' iye~{iy} and circles \lambda T (e''{i\theta} - 1) ' ) ;

hold off

A Model of Traffic Flow

199

Here the small solid circle corresponds to XT = 1/2, and we are just at the
limit of stability, since this circle does not cross the spiral produced by
z exp(z) for z a complex number on the critical line Re z = 0, though it “hugs”
the spiral closely. The dotted and dashed circles, corresponding to XT = 1 or
2, do cross the spiral, so they correspond to unstable traffic flow.

We can check these theoretical predictions with a simulation using
SIMULINK. We’ll give a picture of the SIMULINK model and then
explain it.

Here the subsystem, which corresponds to multiplication by S — I, looks like
this:

5

Here are some words of explanation. First, we are showing the model using
the options Wide nonscalar lines and Signal dimensions in the Format

200

Chapter?: Applications

menu of the SIMULINK model, to distinguish quantities that are vectors
from those that are scalars. The dimension 5 on most of the lines is
the value of N, the number of cars. Most of the model is like the example in
Chapter 8, except that our unknown function (called u), representing the car
positions, is vector-valued and not scalar-valued. The major exceptions are
these:

1. We need to incorporate the reaction-time delay, so we’ve inserted a
Transport Delay block from the Continuous block library.

2. The parameter X shows up as the value of the gain in the sensitivity
parameter Gain block in the upper right.

3. Plotting car positions by themselves is not terribly useful, since only the
relative positions matter. So before outputting the car positions to the
Scope block labeled “relative car positions,” we’ve subtracted off a
constant linear function (corresponding to uniform motion at the
average car speed) created by the Ramp block from the Sources block
library.

4. We’ve made use of the option in the Integrator blocks to input the initial
conditions, instead of having them built into the block. This makes the
logical structure a little clearer.

5. We’ve used the subsystem feature of SIMULINK. If you enclose a bunch of
blocks with the mouse and then click on “Create subsystem” in the
model’s Edit menu, SIMULINK will package them as a subsystem. This
is helpful if your model is large or if there is some combination of blocks
that you expect to use more than once. Our subsystem sends a vector v to
(S — I)v = Sv — v. A Sum block (with one of the signs changed to a — ) is
used for vector subtraction. To model the action of S, we’ve used the
Demux and Mux blocks from the Signals and Systems block library. The
Demux block, with “number of outputs” parameter set to [4, 1] , splits a
five-dimensional vector into a pair consisting of a four-dimensional vector
and a scalar (corresponding to the last car). Then we reverse the order
and put them back together with the Mux block, with “number of inputs”
parameter set to [1, 4],

Once the model is assembled, it can be run with various inputs. The
following pictures show the two scope windows with a set of conditions
corresponding to stable flow (though, to be honest, we’ve let two cars cross
through each other briefly!):

A Model of Traffic Flow

201

As you can see, the speeds fluctuate but eventually converge to a single
value, and the separations between cars eventually stabilize.

In contrast, if k is increased by changing the “sensitivity parameter” in the
Gain block in the upper right, say from 0.8 to 2.0, one gets this sort of output,
typical of instability:

202

Chapter 9: Applications

We encourage you to go back and tinker with the model (for instance using
a sensitivity parameter that is also inversely proportional to the spacing
between cars) and study the results. We should mention that the To
Workspace block in the lower right has been put in to make it possible to
create a movie of the moving cars. This block sends the car positions to a
variable called carpositions. This variable is what is called a structure
array. To make use of it, you can create a movie with the following script
M-file:

theta = 0:0.025*pi:2*pi;
for j = 1 : length (tout)

plot (cos (carpositions . signals .values (j , :)*2*pi), ...
sin (carpositions . signals .values (j , :)*2*pi), 'o');
axis ( [-1, 1, -1, 1]);

hold on; plot (cos (theta) , sin(theta), 'r'); hold off;
axis equal;

M ( j ) = getframe;
end

The idea here is that we have taken the circular road to have radius 1 (in
suitable units), so that the command plot (cos (theta) , sin (theta) , ' r' )
draws a red circle (representing the road) in each frame of the movie, and on
top of that the cars are shown with moving little circles. The vector tout is a
list of all the values of t at which the model computes the values of the vector

A Model of Traffic Flow

203

u{t), and at the yth time, the car positions are stored in the jth row of the
matrix carpositions . signals .values. Try the program!

We should mention here one fine point needed to create a realistic movie.
Namely, we need the values of tout to be equally spaced — otherwise the
cars will appear to be moving faster when the time steps are large and will
appear to be moving slower when the time steps are small. In its default
mode of operation, SIMULINK uses a variable-step differential equation
solver based on MATLAB’s command ode45, and so the entries of tout will
not be equally spaced. To fix this, open the Simulation Parameters...
dialog box using the Edit menu in the model window, choose the Solver tab,
and change the Output options box to read: Produce specified output
only, chosen to be something such as [0:0.5:20]. Then the model will output
the car positions only at multiples of t = 0.5, and the MATLAB program
above will produce a 41-frame movie.

Practice Set C

Developing Your
MATLAB Skills

Remarks. Problem 7 is a bit more advanced than the others. Problem 11a
requires the Symbolic Math Toolbox; the others do not. SIMULINK is needed
for Problems 12 and 13.

1. Captain Picard is hiding in a square arena, 50 meters on a side, which is
protected by a level-5 force field. Unfortunately, the Cardassians, who are
firing on the arena, have a death ray that can penetrate the force field.
The point of impact of the death ray is exposed to 10,000 illumatons of
lethal radiation. It requires only 50 illumatons to dispatch the Captain;
anything less has no effect. The amount of illumatons that arrive at point
(x, y) when the death ray strikes one meter above ground at point (xo, jo)
is governed by an inverse square law, namely

10,000

47r((x - x 0 ) 2 + {y - yo) 2 + 1) ’

The Cardassian sensors cannot locate Picard’s exact position, so they fire
at a random point in the arena.

(a) Use contour to display the arena after five random bursts of the
death ray. The half-life of the radiation is very short, so one can
assume it disappears almost immediately; only its initial burst has
any effect. Nevertheless include all five bursts in your picture, like
a time-lapse photo. Where in the arena do you think Captain Picard
should hide?

(b) Suppose Picard stands in the center of the arena. Moreover, suppose
the Cardassians fire the death ray 100 times, each shot landing at
a random point in the arena. Is Picard killed?

(c) Rerun the “experiment” in part (b) 100 times, and approximate the
probability that Captain Picard can survive an attack of 100 shots.

204

Practice Set C: Developing Your MATLAB Skills

205

(d) Redo part (c) but place the Captain halfway to one side (that is,
at x = 37.5, y = 25 if the coordinates of the arena are 0 < x < 50,

0 < y < 50).

(e) Redo the simulation with the Captain completely to one side, and
finally in a corner. What self-evident fact is reinforced for you?

2. Consider an account that has M dollars in it and pays monthly interest J.

Suppose beginning at a certain point an amount S is deposited monthly

and no withdrawals are made.

(a) Assume first that S = 0. Using the Mortgage Payments application in
Chapter 9 as a model, derive an equation relating J, M, the number
n of months elapsed, and the total T in the account after n months.
Assume that the interest is credited on the last day of the month
and that the total T is computed on the last day after the interest
is credited.

(b) Now assume that M = 0, that S is deposited on the first day of the
month, that as before interest is credited on the last day of the
month, and that the total T is computed on the last day after the
interest is credited. Once again, using the mortgage application as
a model, derive an equation relating J, S, the number n of months
elapsed, and the total T in the account after n months.

(c) By combining the last two models derive an equation relating all of
M, S, J, n, and T, now of course assuming there is an initial amount
in the account (M) as well as a monthly deposit (S).

(d) If the annual interest rate is 5%, and no monthly deposits are made,
how many years does it take to double your initial stash of money?
What if the annual interest rate is 10%?

(e) In this and the next part, there is no initial stash. Assume an annual
interest rate of 8%. How much do you have to deposit monthly to be
a millionaire in 35 years (a career)?

(f) If the interest rate remains as in (e) and you can only afford to
deposit $300 each month, how long do you have to work to retire a
millionaire?

(g) You hit the lottery and win $100,000. You have two choices: Take
the money, pay the taxes, and invest what’s left; or receive $100,000/
240 monthly for 20 years, depositing what’s left after taxes. Assume
a $100,000 windfall costs you $35,000 in federal and state taxes, but
that the smaller monthly payoff only causes a 20% tax liability. In
which way are you better off 20 years later? Assume a 5% annual
interest rate here.

(h) Banks pay roughly 5%, the stock market returns 8% on average over

206

Practice Set C: Developing Your MATLAB Skills

a 10-year period. So parts (e) and (f) relate more to investing than to
saving. But suppose the market in a 5-year period returns 13%, 15%,
—3%, 5%, and 10% in five successive years, and then repeats the
cycle. (Note that the [arithmetic] average is 8%, though a geometric
mean would be more relevant here.) Assume $50,000 is invested at
the start of a 5-year market period. How much does it grow to in
5 years? Now recompute four more times, assuming you enter the
cycle at the beginning of the second year, the third year, etc. Which
choice yields the best/worst results? Can you explain why? Compare
the results with a bank account paying 8%. Assume simple annual
interest. Redo the five investment computations, assuming $10,000
is invested at the start of each year. Again analyze the results.

3. In the late 1990s, Tony Gwynn had a lifetime batting average of .339. This
means that for every 1000 at bats he had 339 hits. (For this exercise, we
shall ignore walks, hit batsmen, sacrifices, and other plate appearances
that do not result in an official at bat.) In an average year he amassed 500
official at bats.

(a) Design a Monte Carlo simulation of a year in Tony’s career. Run it.
What is his batting average?

(b) Now simulate a 20-year career. Assume 500 official at bats every
year. What is his best batting average in his career? What is his
worst? What is his lifetime average?

(c) Now run the 20-year career simulation four more times. Answer the
questions in part (b) for each of the four simulations.

(d) Compute the average of the five lifetime averages you computed in
parts (b) and (c). What do you think would happen if you ran the
20-year simulation 100 times and took the average of the lifetime
averages for all 100 simulations?

The next four problems illustrate some basic MATLAB programming skills.

4. For a positive integer n, let A(n) be the n x n matrix with entries Oij =
1/0 + j — 1). For example,

The eigenvalues of A{ri) are all real numbers. Write a script M-file that
prints the largest eigenvalue of A(500), without any extraneous output.
{Hint: The M-file may take a while to run if you use a loop within a loop
to define A. Try to avoid this!)

Practice Set C: Developing Your MATLAB Skills

207

5. Write a script M-file that draws a bulls -eye pattern with a central circle
colored red, surrounded by alternating circular strips (annuli) of white
and black, say ten of each. Make sure the final display shows circles, not
ellipses. ( Hint : One way to color the region between two circles black is to
color the entire inside of the outer circle black and then color the inside of
the inner circle white.)

6. MATLAB has a function 1cm that finds the least common multiple of
two numbers. Write a function M-file mylcm.m that finds the least com-
mon multiple of an arbitrary number of positive integers, which may be
given as separate arguments or in a vector. For example, my lcm ( 4 , 5 ,
6 ) and mylcm( [4 5 6 ] ) should both produce the answer 60. The pro-
gram should produce a helpful error message if any of the inputs are not
positive integers. (Hint: For three numbers you could use lcm to find the
least common multiple m of the first two numbers and then use lcm again
to find the least common multiple of m and the third number. Your M-file
can generalize this approach.)

7. * Write a function M-file that takes as input a string containing the name
of a text file and produces a histogram of the number of occurrences of each
letter from A to Z in the file. Try to label the figure and axes as usefully as
you can.

8. Consider the following linear programming problem. Jane Doe is running
for County Commissioner. She wants to personally canvass voters in the
four main cities in the county: Gotham, Metropolis, Oz, and River City.
She needs to figure out how many residences (private homes, apartments,
etc.) to visit in each city. The constraints are as follows:

(i) She intends to leave a campaign pamphlet at each residence; she
only has 50,000 available.

(ii) The travel costs she incurs for each residence are: $0.50 in each of
Gotham and Metropolis, $1 in Oz, and $2 in River City; she has
$40,000 available.

(iii) The number of minutes (on average) that her visits to each resi-
dence require are: 2 minutes in Gotham, 3 minutes in Metropolis,

1 minute in Oz, and 4 minutes in River City; she has 300 hours
available.

(iv) Because of political profiles Jane knows that she should not visit any
more residences in Gotham than she does in Metropolis and that
however many residences she visits in Metropolis and Oz, the total
of the two should not exceed the number she visits in River City;

(v) Jane expects to receive, during her visits, on average, campaign
contributions of: one dollar from each residence in Gotham, a

208

Practice Set C: Developing Your MATLAB Skills

quarter from those in Metropolis, a half-dollar from the Oz resi-
dents, and three bucks from the folks in River City. She must raise
at least $10,000 from her entire canvass.

Jane’s goal is to maximize the number of supporters (those likely to
vote for her). She estimates that for each residence she visits in Gotham
the odds are 0.6 that she picks up a supporter, and the corresponding
probabilities in Metropolis, Oz, and River City are, respectively, 0.6, 0.5,
and 0.3.

(a) How many residences should she visit in each of the four cities?

(b) Suppose she can double the time she can allot to visits. Now what
is the profile for visits?

(c) But suppose that the extra time (in part (b» also mandates that she
double the contributions she receives. What is the profile now?

9. Consider the following linear programming problem. The famous football
coach Nerv Turnip is trying to decide how many hours to spend with each
component of his offensive unit during the coming week — that is, the
quarterback, the running backs, the receivers, and the linemen. The con-
straints are as follows:

(i) The number of hours available to Nerv during the week is 50.

(ii) Nerv figures he needs 20 points to win the next game. He estimates
that for each hour he spends with the quarterback, he can expect
a point return of 0.5. The corresponding numbers for the r unnin g
backs, receivers, and linemen are 0.3, 0.4, and 0.1, respectively.

(iii) In spite of their enormous size, the players have a relatively thin
skin. Each hour with the quarterback is likely to require Nerv to
criticize him once. The corresponding number of criticisms per hour
for the other three groups are 2 for running backs, 3 for receivers,
and 0.5 for linemen. Nerv figures he can only bleat out 75 criticisms
in a week before he loses control.

(iv) Finally, the players are prima donnas who engage in rivalries. Be-
cause of that, he must spend the exact same number of hours with
the running backs as he does with the receivers, at least as many
hours with the quarterback as he does with the runners and re-
ceivers combined, and at least as many hours with the receivers as
with the linemen.

Nerv figures he’s going to be fired at the end of the season regardless
of the outcome of the game, so his goal is to maximize his pleasure during
the week. (The team’s owner should only know.) He estimates that, on a
sliding scale from 0 to 1, he gets 0.2 units of personal satisfaction for each

Practice Set C: Developing Your MATLAB Skills

209

AAA

resistor

battery

diode

V

Figure C-l : A Nonlinear Electrical Circuit

hour with the quarterback. The corresponding numbers for the runners,
receivers and linemen are 0.4, 0.3, and 0.6, respectively.

(a) How many hours should Nerv spend with each group?

(b) Suppose he only needs 15 points to win; then how many?

(c) Finally suppose, despite needing only 15 points, that the troops are
getting restless and he can only dish out 7 0 criticisms this week. Is
Nerv getting the most out of his week?

10. This problem, suggested to us by our colleague Tom Antonsen, concerns
an electrical circuit, one of whose components does not behave linearly.
Consider the circuit in Figure C-l.

Unlike the resistor, the diode is a nonlinear element — it does not obey
Ohm’s Law. In fact its behavior is specified by the formula

i — Iq expiVo/Vr), (1)

where i is the current in the diode (which is the same as in the resistor by
Kirchhoff’s Current Law), Vd is the voltage across the diode, 7o is the leak-
age current of the diode, and Vy = kT/e, where k is Boltzmann’s constant,
T is the temperature of the diode, and e is the electrical charge.

By Ohm’s Law applied to the resistor, we also know that Vr = HI,
where Vr is the voltage across the resistor and R is its resistance. But by
Kirchhoff’s Voltage Law, we also have Vr = Vq — Vd. This gives a second

210 Practice Set C: Developing Your MATLAB Skills

equation relating the diode current and voltage, namely

i = (Vo- V d )/R. (2)

Note now that (2) says that i is a decreasing linear function of Vd with value
Vo/R when Vd is zero. At the same time (1) says that i is an exponentially
growing function of Vd starting out at To- Since typically, RIq < Vo, the two
resulting curves (for i as a function of Vd) must cross once. Eliminating i
from the two equations, we see that the voltage in the diode must satisfy
the transcendental equation

(Vo - V d )/R = Io exp(V D /Vr),
or

V D = V 0 -RI 0 exp(V D /V T ).

(a) Reasonable values for the electrical constants are: Vb = 1.5 volts,

R = 1000 ohms, Iq = 10 -5 amperes, and Vr = .0025 volts. Use f zero
to find the voltage Vd and current i in the circuit.

(b) In the remainder of the problem, we assume the voltage in the bat-
tery Vb and the resistance of the resistor R are unchanged. But
suppose we have some freedom to alter the electrical characteristics
of the diode. For example, suppose that fy is halved. What happens
to the voltage?

(c) Suppose instead of halving Io, we halve Vr. Then what is the effect
on Vd?

(d) Suppose both Io and Vr are cut in half. What then?

(e) Finally, we want to examine the behavior of the voltage if both Io and
Vr are decreased toward zero. For definitiveness, assume that we set
Io = 10 -5 w and Vr = .0025w, and let u -» 0. Specifically, compute the
solution for u = 10 - - 7 , j = 0, ... ,5. Then, display a loglog plot of
the solution values, for the voltage as a function of Io . What do you
conclude?

11. This problem is based on both the Population Dynamics and 360 ° Pendu-
lum applications from Chapter 9. The growth of a species was modeled in
the former by a difference equation. In this problem we will model pop-
ulation growth by a differential equation, akin to the second application
mentioned above. In fact we can give a differential equation model for the
logistic growth of a population x as a function of time t by the equation

x = x(l — x) = x — x‘

(3)

Practice Set C: Developing Your MATLAB Skills

211

where x denotes the derivative of x with respect to t. We think of x as
a fraction of some maximal possible population. One advantage of this
continuous model over the discrete model in Chapter 9 is that we can get
a “reading” of the population at any point in time (not just on integer
intervals).

(a) The differential equation (3) is solved in any beginning course in
ordinary differential equations, but you can do it easily with the
MATLAB command dsolve. (Look up the syntax via online help.)

(b) Now find the solution assuming an initial value xo = x(0) of x. Use
the values xo = 0, 0.25, . . . , 2.0. Graph the solutions and use your
picture to justify the statement: “Regardless of xo > 0, the solution
of (3) tends to the constant solution x(t) = 1 in the long term.”

The logistic model presumes two underlying features of population
growth: (i) that ideally the population expands at a rate proportional
to its current total (that is, exponential growth — this corresponds
to the x term on the right side of (3)) and (ii) because of interactions
between members of the species and natural limits to growth, unfet-
tered exponential growth is held in check by the logistic term, given
by the — x 2 expression in (3). Now assume there are two species
x(t) and y(t), competing for the same resources to survive. Then
there will be another negative term in the differential equation that
reflects the interaction between the species. The usual model pre-
sumes it to be proportional to the product of the two populations,
and the larger the constant of proportionality, the more severe the
interaction, as well as the resulting check on population growth.

(c) Here is a typical pair of differential equations that model the growth
in population of two competing species x{t) and y(t):

x(t) = x — x 2 — 0.5xy , . .

y(t) = y — y 2 — 0.5xy. ()

The command dsolve can solve many pairs of ordinary differential
equations — especially linear ones. But the mixture of quadratic
terms in (4) makes it unsolvable symbolically, and so we need to use a
numerical ODE solver as we did in the pendulum application. Using
the commands in that application as a template, graph numerical

212

Practice Set C: Developing Your MATLAB Skills

solution curves to the system (4) for initial data

x(0) = 0 : 1/12 : 13/12
3/(0) = 0 : 1/12 : 13/12.

( Hint : Use axis to limit your view to the square 0 < x, y < 13/12.)

(d) The picture you drew is called a phase portrait of the system. In-
terpret it. Explain the long-term behavior of any population distri-
bution that starts with only one species present. Relate it to part
(b). What happens in the long term if both populations are present
initially? Is there an initial population distribution that remains
undisturbed? What is it? Relate those numbers to the model (4).

(e) Now replace 0.5 in the model by 2; that is, consider the new model

x(t) = x-x 2 -2xy . .

y(t) =y-y 2 - 2 xy.

Draw the phase portrait. (Use the same initial data and viewing
square.) Answer the same questions as in part (d). Do you see a
special solution trajectory that emanates from near the origin and
proceeds to the special fixed point? And another trajectory from the
upper right to the fixed point? What happens to all population dis-
tributions that do not start on these trajectories?

(f) Explain why model (4) is called “peaceful coexistence” and model (5)
is called “doomsday.” Now explain heuristically why the coefficient
change from 0.5 to 2 converts coexistence into doomsday.

12. Build a SIMULINK model corresponding to the pendulum equation

x(t) = —0.5 x(t) — 9.81 sin(xfr)) (6)

from The 360° Pendulum in Chapter 9. You will need the Trigonometric
Function block from the Math library. Use your model to redraw some of
the phase portraits.

13. As you know, Galileo and Newton discovered that all bodies near the
earth’s surface fall with the same acceleration g due to gravity, approx-
imately 32.2 ft/sec 2 . However, real bodies are also subjected to forces due
to air resistance. If we take both gravity and air resistance into account,
a moving ball can be modeled by the differential equation

x = [0, - g ] - c ||x|| x. (7)

Here x, a function of the time t, is the vector giving the position of the ball
(the first coordinate is measured horizontally, the second one vertically),
x is the velocity vector of the ball, x is the acceleration of the ball, ||x||

Practice Set C: Developing Your MATLAB Skills

213

is the magnitude of the velocity, that is, the speed, and c is a constant
depending on the shape and mass of the ball and the density of the air.
(We are neglecting the lift force that comes from the ball’s rotation, which
can also play a major role in some situations, for instance in analyzing
the path of a curve ball, as well as forces due to wind currents.) For a
baseball, the constant c turns out to be approximately 0.0017, assuming
distances are measured in feet and time is measured in seconds. (See,
for example, Chapter 18, “Balls and Strikes and Home Runs,” in Towing
Icebergs, Falling Dominoes, and Other Adventures in Applied Mathematics,
by Robert Banks, Princeton University Press, 1998.) Build a SIMULINK
model corresponding to Equation (7), and use it to study the trajectory of
a batted baseball. Here are a few hints. Represent x, x, and x as vector
signals, joined by two Integrator blocks. The quantity x, according to (7),
should be computed from a Sum block with two vector inputs. One should
be a Constant block with the vector value [0, —32.2], representing gravity,
and the other should represent the drag term on the right of Equation
(7), computed from the value of x. You should be able to change one of the
parameters to study what happens both with and without air resistance
(the cases of c = 0.0017 and c = 0, respectively). Attach the output to an
XY Graph block, with the parameters x-min = 0, y-min = 0, x-max = 500,
y-max = 150, so that you can see the path of the ball out to a distance of
500 feet from home plate and up to a height of 150 feet.

(a) Let x(0) = [0, 4], x(0) = [80, 80]. (This corresponds to the ball start-
ing at t = 0 from home plate, 4 feet off the ground, with the hori-
zontal and vertical components of its velocity both equal to 80 ft/sec.
This corresponds to a speed off the bat of about 77 mph, which is not
unrealistic.) How far (approximately — you can read this off your
XY Graph output) will the ball travel before it hits the ground, both
with and without air resistance? About how long will it take the ball
to hit the ground, and how fast will the ball be traveling at that time
(again, both with and without air resistance)? (The last parts of the
question are relevant for outfielders.)

(b) Suppose a game is played in Denver, Colorado, where because of
thinning of the atmosphere due to the high altitude, c is only 0.0014.
How far will the ball travel now (given the same initial velocity as
in (a))?

(c) (This is not a MATLAB problem.) Estimate from a comparison of
your answers to (a) and (b) what effect altitude might have on the
team batting average of the Colorado Rockies.

Chapter 10

MATLAB and the
Internet

In this chapter, we discuss a number of interrelated subjects: how to use the
Internet to get additional help with MATLAB and to find MATLAB programs
for certain specific applications, how to disseminate MATLAB programs over
the Internet, and how to use MATLAB to prepare documents for posting on
the World Wide Web.

MATLAB Help on the Internet

For answers to a variety of questions about MATLAB, it pays to visit the web
site for The Math Works,

http : // www . mathworks . com

(In MATLAB 6, the Web menu on the Desktop menu bar can take you there
automatically.) Since files on this site are moved around periodically, we won’t
tell you precisely what is located where, but we will point out a few things
to look for. First, you can find complete documentation sets for MATLAB and
all the toolboxes. This is particularly useful if you didn’t install all the docu-
mentation locally in order to save space. Second, there are lists of frequently
asked questions about MATLAB, bug reports and bug fixes, etc. Third, there is
an index of MATLAB -based books (including this one), with descriptions and
ordering information. And finally, there are libraries of M-files, developed both
by The MathWorks and by various MATLAB users, which you can download
for free. These are especially useful if you need to do a standard sort of calcu-
lation for which there are established algorithms but for which MATLAB has
no built-in M-file; in all probability, someone has written an M-file for it and
made it available. You can also find M-files and MATLAB help elsewhere on
the Internet; a search on “MATLAB” will turn up dozens of MATLAB tutorials

214

Posting MATLAB Programs and Output

215

and help pages at all levels, many based at various universities. One of these
is the web site associated with this book,

http : / /www . math . umd . edu/ schol/matlab

where you can find nearly all of the MATLAB code used in this book.

Posting MATLAB Programs and Output

To post your own MATLAB programs or output on the Web, you have a number
of options, each with different advantages and disadvantages.

M-Files, M-Books, Reports, and HTML Files

First, since M-files (either script M-files or function M-files) are simply plain
text files, you can post them, as is, on a web site, for interested parties to down-
load. This is the simplest option, and if you’ve written a MATLAB program
that you’d like to share with the world, this is the way to do it. It’s more likely,
however, that you want to incorporate MATLAB graphics into a web page. If
this is the case, there are basically three options:

1. You can prepare your document as an M-book in Microsoft Word. After
debugging and executing your M-book, you have two options. You can
simply post the M-book on your web site, allowing viewers with a Word
installation to read it, and allowing viewers with both a Word and a
MATLAB installation to execute it. Or you can click on File : Save As...,
and when the dialog box appears, under “Save as type”, select “Web Page
(*.htm, *.html)”. This will store your entire document in HTML (HyperText
Markup Language) format for posting on the web, and will automatically
convert all of the graphics to the correct format. Once your web document
is created, you can modify it with any HTML editor (including Word itself).

2. If you’ve installed the MATLAB Report Generator, it can take your
MATLAB programs and convert them into an HTML report with embed-
ded graphics.

3. Finally, you can create your web document with your favorite HTML editor
and add links to your MATLAB graphics. For this to work, you need to save
your graphics in a convenient format. The simplest way to do this is to
select File : Export... in your figure window. Under “Save as type” in the
dialog box that appears, you can for instance select “JPEG images (*.jpg)”,
and the resulting JPEG file can be incorporated into the document with a

216

Chapter 10: MATLAB and the Internet

tag such as <img src=sphere . jpg>. If you are not happy with the size
of the resulting image, you can modify it with any image editor. (Almost
any PC these days comes with one; in UNIX you can use ImageMagick®,
xv®, or many other programs.) If you are planning to modify the image
before posting it, it may be preferable to have MATLAB store the figure in
TIFF format instead; that way no resolution is lost before you begin the
editing process.

/ If you are an advanced MATLAB user and you want to use MATLAB as an
engine to power an interactive web site, then you might want to purchase the
MATLAB Web Server, which is designed exactly for this purpose. You can see
samples of what it can do at

http : / /www . mathworks . com/products/demos/webserver/

Configuring Your Web Browser

In this section, we explain how to configure the most popular Web browsers
to display M-files in the M-file editor or to launch M-books automatically.

Microsoft Internet Explorer

If MATLAB and Word are installed on your Windows computer then Internet
Explorer® should automatically know how to open M-books. With M-files, it
may give you a choice of downloading the file or “opening” it; if you choose the
latter, it will appear in the M-file editor, a slightly stripped-down version of
the Editor/Debugger.

Netscape Navigator

The situation with Netscape Navigator® is slightly more complicated. If you
click on an M-file (with the . m extension), it will probably appear as a plain text
file. You can save the file and then open it if you wish with the M-file editor.
On a PC (but not in UNIX) you can open the M-file editor without launch-
ing MATLAB; look for it in the MATLAB group under Start : Programs,
or else look for the executable file meditor.exe (in MATLAB 5.3 and ear-
lier, Medit . exe). If you click on an M-book (with the . doc extension), your
browser will probably offer you a choice of opening it or saving it, unless
you have preconfigured Netscape to open it without prompting. (This
depends also on your security settings.) What program Netscape uses to open

Configuring Your Web Browser

217

a file is controlled by your Preferences. To make changes, select Edit : Prefer-
ences in the Netscape menu bar, find the Navigator section, and look for the
“Applications” subsection. You will see a panel that looks something like
Figure 10-1. (Its exact appearance depends on what version of Netscape you
are using and your operating system.) Look for the “Microsoft Word Document”
file type (with file extension . doc) and, if necessary, change the program used
to open such files. Typical choices would be Word or Wordpad® in Windows
and StarOffice® or PC File Viewer® in UNIX. Choices other than Word will
only allow you to view, not to execute, M-books.

Chapter 1 1

Troubleshooting

In this chapter, we offer advice for dealing with some common problems that
you may encounter. We also list and describe the most common mistakes that
MATLAB users make. Finally, we offer some simple but useful techniques for
debugging your M-files.

Common Problems

Problems manifest themselves in various ways: Totally unexpected or plainly
wrong output appears; MATLAB produces an error message (or at least a
warning); MATLAB refuses to process an input line; something that worked
earlier stops working; or, worst of all, the computer freezes. Fortunately, these
problems are often caused by several easily identifiable and correctable mis-
takes. What follows is a description of some common problems, together with
a presentation of likely causes, suggested solutions, and illustrative examples.
We also refer to places in the book where related issues are discussed.

Here is a list of the problems:

• wrong or unexpected output,

• syntax error,

• spelling error,

• error messages when plotting,

• a previously saved M-file evaluates differently, and

• computer won’t respond.

Wrong or Unexpected Output

There are many possible causes for this problem, but they are likely to be
among the following:

218

Common Problems

219

cause: Forgetting to clear or reset variables.

solution: Clear or initialize variables before using them, especially in a long ses-
sion.

tc ^ See Variables and Assignments in Chapter 2.
cause: Conflicting definitions.

solution: Do not use the same name for two different functions or variables, and
in particular, try not to overwrite the names of any of MATLAB’s built-in
functions.

You can accidentally mask one of MATLAB’s built-in M-files either with your
own M-file of the same name or with a variable (including, perhaps, an inline
function). When unexpected output occurs and you think this might be the
cause, it helps to use which to find out what M-file is actually being referenced.

Here is perhaps an extreme example.

EXAMPLE:

>> plot = gcf;

» x = -2:0. 1:2;

>> plot(x, x . ~2 )

Warning: Subscript indices must be integer values.

??? Index into matrix is negative or zero. See release
notes on changes to logical indices .

What’s wrong, of course, is that plot has been masked by a variable with the
same name. You could detect this with

>> which plot

plot is a variable.

If you type clear plot and execute the plot command again, the prob-
lem will go away and you’ll get a picture of the desired parabola. A more
subtle example could occur if you did this on purpose, not thinking you would
use plot, and then called some other graphics script M-file that uses it
indirectly.

cause: Not keeping track of ans.

solution: Assign variable names to any output that you intend to use.

If you decide at some point in a session that you wish to refer to prior output
that was unnamed, then give the output a name, and execute the command

220

Chapter 1 1 : Troubleshooting

again. (The UP-ARROW key or Command History window is useful for recalling
the command to edit it.) Do not rely on ans as it is likely to be overwritten
before you execute the command that references the prior output.

cause: Improper use of built-in functions.

solution: Always use the names of built-in functions exactly as MATLAB specifies
them; always enclose inputs in parentheses, not brackets and not braces;
always list the inputs in the required order.

B® 5 See Managing Variables and Online Help in Chapter 2.

cause: Inattention to precedence of arithmetic operations.

solution: Use parentheses liberally and correctly when entering arithmetic or
algebraic expressions.

EXAMPLE:

MATLAB, like any calculator, first exponentiates, then divides and multiplies,
and finally adds and subtracts, unless a different order is specified by using
parentheses. So if you attempt to compute 5 2/3 — 25/(2 * 3) by typing

» 5~2/3 - 25/2*3

-29.1667

the answer MATLAB produces is not what you intended because 5 is raised
to the power 2 before the division by 3, and 25 is divided by 2 before the
multiplication by 3. Here is the correct calculation:

» 5~ (2/3) - 25/ (2*3)

-1 . 2426

Syntax Error

cause: Mismatched parentheses, quote marks, braces, or brackets.

solution: Look carefully at the input line to find a missing or an extra delimiter.

MATLAB usually catches this kind of mistake. In addition, the MATLAB 6
Desktop automatically highlights matching delimiters as you type and
color-codes strings (expressions enclosed in single quotes) so that you can see

Common Problems

221

where they begin and end. In the Command Window of MATLAB 5 and earlier
versions, however, you have to hunt for matching delimiters by hand.

cause: Wrong delimiters: Using parentheses in place of brackets, or vice versa, and

so on.

solution: Remember the basic rules about delimiters in MATLAB.

Parentheses are used both for grouping arithmetic expressions and for enclo-
sing inputs to a MATLAB command, an M-file, or an inline function. They
are also used for referring to an entry in a matrix. Square brackets are used
for defining vectors or matrices. Single quote marks are used for defining
strings.

The following illustrates what can happen if you don’t follow these rules:

» X = -1: .01:1;

» X[l]

??? X[l]

I

Error: Missing operator, comma, or semicolon.

» A= (0,1,2)

??? A= (0,1,2)

I

Error: Error: ")" expected, found.

These examples are fairly straightforward to understand; in the first case,
x ( 1 ) was intended, and in the second case, A= [ 0 , 1 , 2 ] was intended. But
here’s a trickier example:

>> sin 3

ans =

0.6702

Here there’s no error message, but if one looks closely, one discovers that
MATLAB has printed out the sine of 51 radians, not of 3 radians!! The ex-
planation is as follows: Any time a MATLAB command is followed by a space
and then an argument to the command (as in the construct clear x), the
argument is always interpreted as a string. Thus MATLAB has inter-
preted 3 not as the number 3, but as the string ' 3' ! And sure enough, one
discovers:

222

Chapter 1 1 : Troubleshooting

>> char (51)

3

In other words, in MATLAB’s encoding scheme, the string ' 3 ' is stored as the
number 51, which is why sin 3 (or also sin ( ' 3 ' ) ) produces as output the
sine of 51 radians.

Braces or curly brackets are used less often than either parentheses or
square brackets and are usually not needed by beginners. Their main use
is with cell arrays. One example to keep in mind is that if you want an M-file
to take a variable number of inputs or produce a variable number of outputs,
then these are stored in the cell arrays varargin and varargout, and braces
are used to refer to the cells of these arrays. Similarly, case is sometimes used
with braces in the middle of a switch construct. If you want to construct a
vector of strings, then it has to be done with braces, since brackets when ap-
plied to strings are interpreted as concatenation.

EXAMPLE:

» {'a', 'b'}

'b'

'b']

ab

cause: Improper use of arithmetic symbols.

solution: When you encounter a syntax error, review your input line carefully for
mistakes in typing.

EXAMPLE:

If the user, intending to compute 2 times —4, inadvertently switches the
symbols, the result is

>> 2 - * 4

I

Error: Expected a variable, function, or constant,

found .

Common Problems

223

Here the vertical bar highlights the place where MATLAB believes the error
is located. In this case, the actual error is earlier in the input line.

Spelling Error

cause: Using uppercase instead of lowercase letters in MATLAB commands, or

misspelling the command.

solution: Fix the spelling.

For example, the UNIX version of MATLAB does not recognize Fzero or FZERO
(in spite of the convention that the help lines in MATLAB’s M-files always
refer to capitalized function names); the correct command is fzero.

EXAMPLE:

>> Fzero (inline ( 'x~2 - 3'), 1)

??? Undefined function or variable 'Fzero'.

» FZERO (inline ( ' x~2 - 3'), 1)

??? Undefined function or variable 'FZERO'.

>> text = help (' fzero' ) ; text (1:38)

ans =

FZERO Scalar nonlinear zero finding.

>> fzero (inline ('x"2 - 3'), 1)

ans =

1.7321

Error Messages When Plotting

cause: There are several possible explanations, but usually the problem is the wrong

type of input for the plotting command chosen.

solution: Carefully follow the examples in the help lines of the plotting command,
and pay attention to the error messages.

EXAMPLE:

» [X, Y] = meshgrid(-l: .1:1, -1:.1:1);
» mesh (X, Y, sqrt(l - X.~2 - Y.~2))

??? Error using ==> surface
X, Y, Z, and C cannot be complex.

224

Chapter 1 1 : Troubleshooting

Error in ==> /usr/matlabrl2/toolbox/matlab/graph3d/mesh.m
On line 68 ==> hh = surf ace (x, ' FaceColor' , fc EdgeColor' ,
'flat', ' FaceLighting' , 'none', ' EdgeLighting' , 'flat');

These error messages indicate that you have tried to plot the wrong kind of
object, and that’s why the figure window (Figure 11-1) is blank. What’s wrong
in this case is evident from the first error m essage. While you might think
you can plot the hemisphere z = y' l — x 2 — y 2 this way, there are points in the
domain — 1 < x, y < 1 where 1 — x 2 — y 2 is negative and thus the square root
is imaginary. But mesh can’t handle complex inputs; the coordinates need to
be real. One can get around this by redefining the function at the points where
it’s not real, like this:

>> [X, Y] = meshgrid(-l: .1:1, -1:.1:1);

>> mesh (X, Y, sqrt(max(l - X."2 - Y.~2, 0)))

The output is shown in Figure 11-2.

A Previously Saved M-File Evaluates Differently

One of the most frustrating problems you may encounter occurs when a
previously saved M-file, one that you are sure is in good shape, won’t eval-
uate or evaluates incorrectly, when opened in a new session.

Common Problems

225

cause: Change in the sequence of evaluation, or failure to clear variables.

cause: Differences between the Professional and Student Versions

EXAMPLE:

Some commands that work correctly in the Professional Version of MATLAB
may not work in the Student Version. Here is an example from MATLAB
Release 11:

>> syms p t

>> ezsurf (sin (p) *cos (t) , sin (p) *sin (t) , cos (p) ,

[0, pi, 0, 2*pi] ) ; axis equal

This correctly plots a sphere (using spherical coordinates) in the Professional
Version, but in the Student Version you get strange error messages such as
??? The 'maple' function is restricted in the
Student Edition.

Error in ==> C:\MATLAB_SRll\toolbox\symbolic\maplemex.dll
Error in ==> C:\MATLAB_SRll\toolbox\symbolic\maple.m
On line 116 ==> [result , status] = maplemex (statement) ;

Error in ==> C :\MATLAB_SRll\toolbox\symbolic\@sym\ezsurf .m
(symf ind)

On line 104 ==> vars = maple ( [ vars ' minus ' 'pi' ]);

226

Chapter 1 1 : Troubleshooting

Error in ==> C : \MATLAB_SRll\toolbox\symbolic\@sym\ezsurf . m
(makeinline)

On line 73 ==> vars = symfind(f);

Error in ==> C : \MATLAB_SRll\toolbox\symbolic\@sym\ezsurf . m
On line 60 ==> F = makeinline (f) ;

since ez sur f in the Student Version is not equipped to accept symbolic inputs;
it requires string inputs instead. You can easily fix this by typing

>> ezsurf ( ' sin (p) *cos (t) ' , ' sin (p) *sin (t) ' , 'cos(p)', •••

[0, pi, 0, 2*pi] ) ; axis equal

or else by using char to convert symbolic expressions to strings.

Computer Won’t Respond

cause: MATLAB is caught in a very large calculation, or some other calamity has

occurred that has caused it to fail to respond. Perhaps you are using an array
that is too large for your computer memory to handle.

solution: Abort the calculation with CTRL+C.

If overuse of computer memory is the problem, try to redo your calculation
using smaller arrays, for example, by using fewer grid points in a 3D plot,
or by breaking a large vectorized calculation into smaller pieces using a loop.
Clearing large arrays from your Workspace may help too.

EXAMPLE:

You’ll know it when you see it!

The Most Common Mistakes

The most common mistakes are all accounted for in the causes of the problems
described earlier. But to help you prevent these mistakes, we compile them
here in a single list to which you can refer periodically. Doing so will help you
to establish “good MATLAB habits”. The most common mistakes are

• forgetting to clear values,

• improperly using built-in functions,

• not paying attention to the order of precedence of arithmetic operations,

• improperly using arithmetic symbols,

• mismatching delimiters,

• using the wrong delimiters,

Debugging Techniques

227

• plotting the wrong kind of object, and

• using uppercase instead of lowercase letters in MATLAB commands, or
misspelling commands.

Debugging Techniques

Now that we have discussed the most common mistakes, it’s time to discuss
how to debug your M-files, and how to locate and fix those pesky problems
that don’t fit into the neat categories above.

If one of your M-files is not working the way you expected, perhaps the
easiest thing you can do to debug it is to insert the command keyboard some-
where in the middle. This temporarily suspends (but does not stop) execution
and returns command to the keyboard, where you are given a special prompt
with a K in it. You can execute whatever commands you want at this point
(for instance, to examine some of the variables). To return to execution of the
M-file, type return or dbcont, short for “debug continue.”

A more systematic way to debug M-files is to use the MATLAB M-file
debugger to insert “breakpoints” in the file. Usually you would do this with
the Breakpoints menu or with the “Set/clear breakpoint” icon at the top of
the Editor/Debugger window, but you can also do this from the command line
with the command dbstop. Once a breakpoint is inserted in the M-file, you
will see a little red dot next to the appropriate line in the Editor/Debugger. (An
example is illustrated in Figure 11-8 below.) Then when you call the M-file,
execution will stop at the breakpoint, and just as in the case of keyboard,
control will return to the Command Window, where you will be given a special
prompt with a K in it. Again, when you are ready to resume execution of the
M-file, type dbcont. When you are done with the debugging process, dbclear
“clears” the breakpoint from the M-file.

Let’s illustrate these techniques with a real example. Suppose you want
to construct a function M-file that takes as input two expressions f and g
(given either as symbolic expressions or as strings) and two numbers a and b,
plots the functions f and g between x = a and x = b, and shades the region in
between them. As a first try, you might start with the nine-line function M-file
shadecurves .m given as follows:

function shadecurves (f, g, a, b)

%SHADECURVES Draws the region between two curves
% SHADECURVES (f, g, a, b) takes strings or expressions f
% and g, interprets them as functions, plots them between
% x = a and x = b, and shades the region in between.

228

Chapter 1 1 : Troubleshooting

% Example: shadecurves ( ' sin (x) ' , '-sin(x)', 0, pi)

ffun = inline (vectorize (f) ) ; gfun = inline (vectorize (g) ) ;

xvals = a: (b - a)/50:b;

plot([xvals, xvals], [ffun (xvals) , gfun (xvals) ] )

Trying this M-file out with the example specified in the help lines, that is,
executing

>> shadecurves (' sin (x) ' , '-sin(x)', 0, pi)

or

>> syms x; shadecurves (sin (x) , -sin(x), 0, pi)

gives the output shown in Figure 11-3.

This is not really what we wanted; the figure we seek is shown in Figure 11-4.
To begin to determine what went wrong, let’s try a different example, say
>> shadecurves ('x~2' , ' sqrt (x) ' , 0, 1)

>> axis square

or

>> syms x; shadecurves (x~2 , sqrt (x) , 0, 1)

>> axis square

Now we get the output shown in Figure 11-5.

Debugging Techniques

229

Figure 11-4

Figure 1 1-5

230

Chapter 1 1 : Troubleshooting

It’s not too hard to figure out why our regions aren’t shaded; that’s be-
cause we used plot (which plots curves) instead of patch (which plots filled
patches). So that suggests we should try changing the last line of the
M-file to

patch( [xvals, xvals] , [f fun (xvals) , gfun (xvals) ] )

That gives the error message

??? Error using ==> patch
Not enough input arguments.

Error in ==> shadecurves . m

On line 9 ==> patch ( [xvals , xvals], [f fun (xvals) ,
gfun (xvals) ] )

So we go back and try
>> help patch

to see if we can get the syntax right. The help lines indicate that patch
requires a third argument, the color (in RGB coordinates) with which our
patch is to be filled. So we change our final line to, for instance,

patch ( [xvals, xvals] , [f fun (xvals) , gfun (xvals) ] , [.2, 0,-8])

That gives us now as output to shadecurves (x~2 , sqrt (x) , 0, 1) ;
axis square the picture shown in Figure 11-6.

That’s better, but still not quite right, because we can see a mysterious
diagonal line down the middle. Not only that, but if we try

>> syms x; shadecurves (x~2 , x~4, -1.5, 1.5)

we now get the bizarre picture shown in Figure 11-7.

There aren’t a lot of lines in the M-file, and lines 7 and 8 seem OK, so
the problem must be with the last line. We need to reread the online help for
patch. It indicates that patch draws a filled 2D polygon defined by the vectors
X and Y, which are its first two inputs. A way to see how this is working is to
change the “50” in line 9 of the M-file to something much smaller, say 5, and
then insert a breakpoint in the M-file before line 9. At this point, our M-file
in the Editor/Debugger window now looks like Figure 11-8. Note the large dot
to the left of the last line, indicating the breakpoint. When we run the M-file
with the same input, we now obtain in the Command Window a K>> prompt.
At this point, it is logical to try to list the coordinates of the points that are
the vertices of our filled polygon, so we try

Debugging Techniques

231

Figure 11-7

232

Chapter 1 1 : Troubleshooting

file Edit Text Debug Breakpoints Window .Help

iHe. m nr> *

% SHADE CORVES Draws the region between two curves
5 SHADECURVES (f, g, a, b) takes strings or egressions f
S and g, interprets than as functions, plots than between
t x=a and x=n . and shades the region in between.

5 Example: shadecurves ('sin(x) ' , '-sin(x)', 0, pi)
ffun=inline (vectorize (f ) ) ; gfun=inline (vectorize (g) ) ;
xvals=a: (b-a)/5:bi

patch ( [ xvals , xvals ] , Iffun(xvals) ,gfun(xvals) ] , [ .2,0, .8] )

Figure 11-8: The Editor/Debugger.

K>> [[xvals, xvals]', [f fun (xvals) , gfun (xvals) ]' ]

ans =

- 1.5000 2.2500
- 0.9000 0.8100
- 0.3000 0.0900
0.3000 0.0900
0.9000 0.8100
1.5000 2.2500
- 1.5000 5.0625
- 0.9000 0.6561
- 0.3000 0.0081
0.3000 0.0081
0.9000 0.6561
1.5000 5.0625

If we now type

K>> dbcont

we see in the figure window what is shown in Figure 11-9 below.

Finally we can understand what is going on; MATLAB has “connected the
dots” using the points whose coordinates were just printed out, in the or-
der it encountered them. In particular, MATLAB has drawn a line from the
point (1.5, 2.25) to the point (—1.5, 5.0625). This is not what we wanted; we
wanted MATLAB to join the point (1.5, 2.25) on the curve y = x 2 to the point
(1.5, 5.0625) on the curve y = x 4 . We can fix this by reversing the order of the

Debugging Techniques

233

Figure 1 1-9

x coordinates at which we evaluate the second function g. So letting slavx
denote xvals reversed, we correct our M-file to read

function shadecurves (f , g, a, b)

%SHADECURVES Draws the region between two curves
% SHADECURVES (f, g, a, b) takes strings or expressions f
% and g, interprets them as functions, plots them between
% x = a and x = b, and shades the region in between.

% Example: shadecurves (' sin (x) ' , '-sin(x)', 0, pi)

ffun = inline (vectorize (f) ) ; gfun = inline (vectorize (g) ) ;

xvals = a: (b - a)/50:b; slavx = b:(a - b)/50:a;

patch ( [xvals, slavx] , [f fun(xvals) ,gfun(slavx) ] , [.2,0,. 8])

Now it works properly. Sample output from this M-file is shown in Figure 11-4.
Try it out on the other examples we have discussed, or on others of your choice.

Solutions to the
Practice Sets

Practice Set A

■ Problem 1

(a)

1111 - 345

ans =

766

(b)

format long; [exp(14), 382801*pi]

1.0e+006 *

1.20260428416478 1.20260480938683

The second number is bigger.

(c)

[2709/1024, 10583/4000, 2024/765, sqrt(7)]

ans =

2.64550781250000 2.64575000000000 2.64575163398693

2 . 64575131106459

The third, that is, 2024/765, is the best approximation.

235

236

Solutions to the Practice Sets

■ Problem 2

(a)

cosh (0 .1)

1.00500416805580

(b)

log (2)

0.69314718055995

(C)

atan (1/2 )

ans =

0.46364760900081

format short

■ Problem 3

[x, y, z] = solve('3*x + 4*y + 5*z = 2', '2*x - 3*y + 7*z = -
1', 'x - 6*y + z = 3', 'x' # 'y' , 'z')

241/92

y =

-21/92

-91/92

Now we’ll check the answer.

A = [3, 4, 5; 2, -3, 7; 1, -6, 1]; A* [x; y; z]

[ 2 ]

Practice Set A

237

[ -i]

[ 3]

It checks!

■ Problem 4

[x, y, z] = solve ( ' 3*x - 9*y + 8*z = 2', '2*x - 3*y + 7*z = -
1', 'x - 6*y + z = 3', 'x', 'y', 'z')

39/5*y+22/5

y =
y

-9/5*y-7/5

We get a one-parameter family of answers depending on the variable y. In
fact the three planes determined by the three linear equations are not
independent, because the first equation is the sum of the second and third.
The locus of points that satisfy the three equations is not a point, the
intersection of three independent planes, but rather a line, the intersection
of two distinct planes. Once again we check.

B = [3, -9, 8; 2, -3, 7; 1, -6, 1]; B* [x; y; z]

[

[

[

2 ]

-1]

3]

■ Problem 5

syms x y; factor (x~4 - y~4)

(x-y) * (x+y) * (x A 2+y A 2)

238

Solutions to the Practice Sets

■ Problem 6

(a)

simplify (1/ (1 + 1/(1 + 1/x) ) )

ans =

(x+1) / (2*x+l)

(b)

simplify (cos (x) *2 - sin(x)*2)

ans =

2 * COS (x) *2-1

Let’s try simple instead.

[better, how] = simple (cos (x) *2 - sin(x)*2)

better =
cos ( 2 *x)
how =
combine

■ Problem 7

3*3 01

4 . 1067e + 143

sym ( ' 3 ' ) *301

4106744371757651279739780821462649478993910868760123094144405

7023510699153249722978140061846706682416475145332179398212844

0538198297087323698003

But note the following:

sym ('3*301' )

Practice Set A

239

3*301

This does not work because sym, by itself, does not cause an evaluation.

■ Problem 8

(a)

solve ('8*x +3=0', 'x')

-3/8

(b)

vpa(ans, 15)

ans =

- .375000000000000

(C)

syms p q; solve ('x~3 + p*x + q = O', 'x')

ans =

[

1/6* ( - 108*q+12* ( 12 *p*3 + 81*q*2 ) * (1/2) ) * (1/3) -2*p/ (-
108*q+12* ( 12 *p*3+81*q*2 ) * (1/2) ) * (1/3) ]

[ -1/12* (-108*q+12* ( 12*p*3+81*q*2 ) * (1/2) ) * (1/3) +p/ (-
108*q+12* ( 12 *p*3+81*q*2 ) * (1/2) ) * (1/3 ) +l/2*i*3* (1/2) * (1/6* (-
108*q+12* ( 12 *p*3+81*q*2 ) * (1/2) ) * (1/3) +2 *p/ (-
108*q+12* ( 12 *p*3 + 81*q*2 ) * ( 1/2 ) ) * ( 1/3 ) ) ]

[ -1/12* (-108*q+12* ( 12*p*3+81*q*2 ) * (1/2) ) * (1/3) +p/ (-
108*q+12* ( 12 *p*3 + 81*q*2 ) * (1/2) ) * (1/3) -l/2*i*3* (1/2) * (1/6* (-
108*q+12* ( 12 *p*3+81*q*2 ) * (1/2) ) * (1/3) +2 *p/ (-
108*q+12* ( 12 *p*3 + 81*q*2 ) * ( 1/2 ) ) * (1/3 ) ) ]

(d)

ezplot ( ' exp (x) ' ) ; hold

ezplot('8*x - 4'); hold off

240

Solutions to the Practice Sets

f zero (inline (

ans =

0.7700

f zero (inline (

ans =

2.9929

■ Problem 9

(a)

ezplot ( 'x~3 - x' , [-4 4] )

'exp(x) - 8*x + 4' ) , 1)

'exp(x) - 8*x + 4')/ 3)

Practice Set A

241

(b)

ezplot ( ' sin (l/x~2) ' , [-2 2])

X = -2:0. 1:2;

plot (X, sin (1 . /X. ~2) )

Warning: Divide by zero.

This picture is incomplete. Let’s see what happens if we refine the mesh.

X = -2:0.01:2; plot (X, sin (1 . /X. ~2) )

Warning: Divide by zero.

242

Solutions to the Practice Sets

Because of the wild oscillation near x = 0, neither plot nor ezplot
gives a totally accurate graph of the function.

(c)

ezplot (' tan (x/2) ' , [-pi pi]); axis([-pi, pi, -10, 10])

(d)

X = -2:0.05:2;

plot (X, exp ( -X . ~2 ) , X, X . "4 - X.~2)

Practice Set A

243

■ Problem 10

Let’s plot 2* and x 4 and look for points of intersection. We plot them first
with ezplot just to get a feel for the graph.

ezplot ( 'x~4 ' ) ; hold on; ezplot (' 2 ~x' ) ; hold off

Note the large vertical range. We learn from the plot that there are no points
of intersection between 2 and 6 or —6 and —2; but there are apparently two
points of intersection between —2 and 2. Let’s change to plot now and focus
on the interval between —2 and 2. We’ll plot the monomial dashed.

X = -2:0. 1:2; plot(X, 2 . 'X) ; hold on; plot (X, X.~4,
hold off

244

Solutions to the Practice Sets

We see that there are points of intersection near —0.9 and 1.2. Are there any
other points of intersection? To the left of 0, 2 X is always less than 1, whereas
x 4 goes to infinity as x goes to — oo. However, both x 4 and 2 X go to infinity as
x goes to oo, so the graphs may cross again to the right of 6. Let’s check.

X = 6:0.1:20; plot(X, 2 . ~X) ; hold on; plot(X, X.~4,
hold off

We see that they do cross again, near x = 16. If you know a little calculus,
you can show that the graphs never cross again (by taking logarithms, for
example), so we have found all the points of intersection. Now let’s use
f zero to find these points of intersection numerically. This command looks
for a solution near a given starting point. To find the three different points of

Practice Set A

245

intersection we will have to use three different starting points. The graphical
analysis above suggests appropriate starting points.

rl = f zero (inline (' 2":
r2 = f zero (inline (' 2":
r3 = f zero (inline (' 2":

:~4 ' ) , -0.9)
:~4'), 1.2)

rl =

-0.8613
r2 =

1.2396
r3 =

16

Let’s check that these “solutions” satisfy the equation.

subs('2~x ■
subs('2~x ■
subs('2~x ■

rl)
r2 )
r3)

2 . 2204e- 016

-8 . 8818e- 016

So rl and r2 very nearly satisfy the equation, and r3 satisfies it exactly. It is
easily seen that 16 is a solution. It is also interesting to try solve on this
equation.

symroots = solve ( ' 2

4 = O')

symroots =

[ -4*lambertw(-l/4*log(2) ) /log(2) ]

[ -4*lambertw ( -l/4*i*log (2) ) /log (2) ]

246

Solutions to the Practice Sets

[ -4*lambertw (l/4*log (2 ) ) /log (2 ) ]

[ -4*lambertw(l/4*i*log(2) ) /log(2) ]

In fact we get the three real solutions already found and two complex solutions,

doublet symr oo t s )

1.2396

16.0000

-0.1609

-0.8613

-0.1609

0 . 9591i
0 . 9591i

Only the real solutions correspond to points where the graphs intersect.

Practice Set B

■ Problem 1

(a)

[X, Y] = meshgr id ( - 1 : 0 . 1 : 1 , -1:0. 1:1); contour(X, Y, 3*Y +
Y.~3 - X . ~3 , 'k')

[X, Y] = meshgrid (-10:0.1:10, -10:0.1:10); contour(X, Y, 3*Y
+ Y.~3 - X.~3, 'k')

Practice Set B

247

Here is a plot with more level curves.

[X, Y] = meshgrid(-10:0.1:10, - 10 : 0 . 1 : 10) ; contour (X, Y, 3*Y
+ Y.~3 - X.~3, 30, 'k')

(b)

Now we plot the level curve through 5.

[X, Y] = meshgrid ( - 5 : 0 . 1 : 5 , -5: 0.1: 5); contour (X, Y, 3 . *Y +
Y.~3 - X.~3, [5 5] , 'k')

248

Solutions to the Practice Sets

We note that 1, 1) = 0, so the appropriate command to plot the
level curve of /through the point (1, 1) is

[X, Y] = meshgrid(0:0.1:2, 0:0. 1:2); contour(X, Y, Y.*log(X)
+ X . *log (Y) , [00], 'k' )

Warning: Log of zero.

Warning: Log of zero.

■ Problem 2

We find the derivatives of the given functions:

syms x r

Practice Set B

249

dif f (6*x~3 - 5*x~2 + 2*x - 3, x)

ans =

18*x A 2-10*x+2

diff ( (2*x - 1) / (x~2 + 1) , x)

ans =

2/ (x*2+l) -2* (2*x-l) / (x A 2+l) a 2*:

simplify (ans)

ans =

-2* (x a 2-1-x) / ( x A 2+1) *2

diff (sin (3*x~2 + 2) , x)

6*cos (3*x A 2+2) *

diff (asin(2*x + 3) , x)

ans =

1/ (-2-x A 2-3*x) * (1/2)

diff (sqrt(l + x~4 ) , x)

ans =

2/ (l+x A 4) A (1/2 ) *x a 3

diff (x~r , x)

diff (atan(x~2 +1), x)

250

Solutions to the Practice Sets

ans =

2*x/ (1 + (x a 2+1) a 2)

■ Problem 3

We compute the following integrals.

(a)

int(cos(x), x, 0, pi/2)

ans =

1

(b)

int (x*sin (x~2) , x)

ans =

- 1 /2 * COS (x A 2)

To check the indefinite integral, we just differentiate.

diff (-cos(x~2)/2, x)

ans =
x*sin (x a 2 )

(C)

int (sin (3*x) *sqrt (1 - cos(3*x)), x)

ans =

2/9* (1-cos (3*x) ) A (3/2)

diff (ans, x)

ans =

sin (3*x) * (1-cos (3*x) ) ^ (1/2)

(d)

int (x~2*sqrt (x +4), x)

ans =

2/7* (x+4 ) A (7/2) -16/5* (x+4) A (5/2) +32/3* (x+4) A (3/2)

Practice Set B

251

dif f (ans, x)

(x+4) * (5/2) -8* (x+4 ) A (3/2) +16* (x+4) A (1/2)

simplify (ans)

x*2 * (x+4) * (1/2)

(e)

int (exp ( -x~2 ) , x, -Inf, Inf)

ans =
pi* (1/2)

■ Problem 4

(a)

int (exp (sin (x) ) , x, 0, pi)

Warning: Explicit integral could not be found.

> In C : \MATLABR12\toolbox\symbolic\@sym\int .m at line 58

int (exp (sin (x) ) , x = 0 .. pi)

format long; quadl ( ' exp (sin (x) ) ' , 0, pi)

6.2087580358484585

(b)

quadl ( ' sqrt (x. ~3 + 1)', 0, 1)

1.11144798484585

(C)

MATLAB integrated this one exactly in 4(e) above; let’s integrate it
numerically and compare answers. Unfortunately, the range is infinite,
so to use quadl we have to approximate the interval. Note that for

252

Solutions to the Practice Sets

|*| > 35, the integrand is much smaller than

exp (-35)

6. 3 0511676014698 9e- 016

which is close to the standard floating point accuracy, so:

quadl ('exp(-x.~2) ' , -35, 35)

1.77245385102263

sqrt (pi)

1.77245385090552

The answers agree to 9 digits.

■ Problem 5

(a)

limit (sin (x) /x, x, 0)

ans =

1

(b)

limit ((1 + cos (x) ) / (x + pi), x, -pi)

ans =

0

(C)

limit (x~2*exp ( -x) , x. Inf)

ans

Practice Set B

253

(d)

limit (1/ (x - 1), x, 1, 'left')

ans =

-inf

(e)

limit (sin (l/x) , x, 0, 'right')

ans =

-1 . . 1

This means that every real number in the interval between -1 and +1 is
a “limit point” of sin(l/x) as x tends to zero. You can see why if you plot
sin(l/x) on the interval (0, 1],

ezplot (sin (l/x) , [0 1])

■ Problem 6

(a)

syms k n r x z
symsum(k~2, k, 0, n)

ans =

1/3* (n+1) *3-1/2* (n+1) *2+l/6*n+l/6

254

Solutions to the Practice Sets

simplify (ans)

l/3*n A 3 + l/2*n A '2 + l/6*n

(b)

symsum(r~k, k, 0, n)

ans =

r* (n+1) / (r-1) -1/ (r-1)

pretty (ans)

r-1 r-1

(C)

symsum (x~k/f actorial (k) , k, 0, Inf)

??? Error using ==> fix

Function 'fix' not defined for variables of class ' sym' .

Error in ==> C:\MATLABR12\toolbox\matlab\specfun\factorial.m
On line 14 ==> if (length (n) ~=1) | (fix(n)~= n) | (n < 0)

Here are two ways around this difficulty. The second method will not work
with the Student Version.

symsum (x~k/gamma (k +1), k, 0, Inf)

ans =
exp (x)

symsum (x~k/maple (' factorial' , k) , k, 0, Inf)

ans =
exp (x)

(d)

symsum(l/(z - k)~2, k, -Inf, Inf)

Practice Set B

255

ans =

pi A 2+pi A 2*cot (pi*z) a 2

■ Problem 7

(a)

taylor (exp (x) , 7, 0)

l+x+l/2*x*2+l/6*x*3+l/24*x*4+l/l20*x*5+l/720*x*6

(b)

taylor (sin (x) , 5, 0)

x-l/6*x*3

taylor (sin (x) , 6, 0)

x-l/ 6*x'"'3 + l/l2 0*x'”'5

(C)

taylor (sin (x) , 6, 2)

sin (2) +cos ( 2 ) * (x-2) -l/2*sin(2) * (x-2) *2-l/6*cos (2) * (x-
2) '“'3+1/24* sin (2) * (x-2) *4+l/l20*cos (2) * (x-2) *5

(d)

taylor (tan (x) , 7, 0)

ans =

x+l/3*x*3+2/l5*x*5

(e)

taylor (log (x) , 5, 1)

ans =

x-l- 1/2* (x-l) '”'2 + 1/3* (x-l) *3-1/4* (x-l) *4

(0

taylor (erf (x) , 9, 0)

256

Solutions to the Practice Sets

ans =

2/pi A (1/2) *x-2/3/pi A (1/2) *x A 3+l/5/pi A (1/2) *x A 5-
l/2l/pi A (1/2) *x A 7

■ Problem 8

■ Problem 9

You can’t do animations in an M-book. But from the Command Window, you
can type:

Practice Set B

257

T = 0:0.01:1;
for j = 0:16

fill (4*cos ( j*pi/8) + (1/2) *cos (2*pi*T ) , ...

4*sin ( j *pi/8) + (1/2) *sin (2*pi*T) , 'r');
axis equal; axis ([-5 5 -5 5]);

M( j + 1) = getframe;

end

movie (M)

■ Problem 10

A1 = [34 5; 2 -3 7; 1 -6 1] ; b = [2; -1; 3] ;
format short; x = Al\b

2.6196
-0.2283
-0 . 9891

ans =

2 . 0000
- 1.0000

3 . 0000

A2 = [3 -9 8; 2 -3 7; 1 -6 1] ; b = [2; -1; 3] ;
x = A2\b

Warning: Matrix is close to singular or badly scaled.

Results may be inaccurate. RCOND = 4 . 189521e- 018 .

- 6 . 000
-1.333

1 . 000

258

Solutions to the Practice Sets

The matrix A2 is singular. In fact
det (A2)

(c)

0

A3 = [13 -2 4; -2 3 4 -1; -4 -3 1 2; 2 3 -4 1]; b3
1 ; 1 ];
x = A3\b3

[ 1 ; 1 ;

- 0.5714

0.3333

- 0.2857

- 0.0000

(d)

1.0000

1.0000

1.0000

1.0000

syms ab c dxyuv;

A4 = [a b; c d] ; A4\ [u; v]

[ - (b*v-u*d) / (a*d-c*b) ]

[ (a*v-c*u) / (a*d-c*b) ]

det (A4 )

ans =
a*d-c*b

The determinant of the coefficient matrix is the denominator in the answer.
So the answer is valid only if the coefficient matrix is non singular.

Practice Set B

259

■ Problem 1 1

(a)

rank (Al)

3

rank (A2)

ans =

2

rank (A3)

ans =

4

rank (A4)

ans =

2

MATLAB implicitly assumes ad — be ^0 here.

(b)

Only the second one computed is singular.

(c)

det (Al)
inv (Al)

92

0.4239

0.0543

■ 0.0978

- 0.3696

- 0.0217

0.2391

0.4674

- 0.1196

- 0.1848

det (A2 )

260

Solutions to the Practice Sets

ans =

0

The matrix A2 does not have an inverse.

det (A3)
inv (A3)

0.1837 -0.1531
0.0000 0.1667
0.1633 0.0306
0.2857 -0.0714

-0.2857 -0.3163

-0.0000 0.1667

-0.1429 -0.3367

0 -0.2143

det (A4 )
inv (A4 )

a*d-c*b

[ d/(a*d-c*b),

[ -c/ (a*d-c*b) ,

-b/ (a*d-c*b) ]
a/ (a*d-c*b) ]

■ Problem 12

(a)

[Ul, Rl] = eig(Al)

U1 =

-0.9749
-0.2003
0.0977
Rl =

3.3206

0

0.6036

0.0624 + 0.540H
-0.5522 + 0 . 1877i

0

-1.1603 + 5.1342i

0.6036

0.0624 - 0 . 54 Oli
■0.5522 - 0.18771

0

0

-1.1603

5 . 1342i

Practice Set B

261

A1*U1 - U1*R1

1.0e-014

0.3109

-0.0333

-0.0833

0.2554 - 0 . 3553i
-0.1776 - 0 . 322 Oi
-0.1721 - 0 . 0444i

This is essentially zero. Notice the "e-014".

[U2, R2] = eig (A2)

U2 :

0.9669

0.1240

-0.2231

0.7405

0.4574 - 0 . 2848i
0.2831 + 0 . 2848i

0.2554 + 0 . 3553i
-0.1776 + 0 . 3220i
-0.1721 + 0 . 0444i

0.7405

0.4574 + 0 . 2848i
0.2831 - 0 . 2848i

0.5000 - 6 . 5383i

1.0e-014

-0.3154

-0.2423

-0.1140

-0.3331 - 0 . 4441i
0.0888 + 0 . 3109i
-0.0222 + 0 . 2665i

This is essentially zero as well.

[U3 , R3] = eig (A3)

-0.3331 + 0 . 4441i
0.0888 - 0 . 3109i
-0.0222 - 0 . 2665i

U3 =

-0.2446 - 0 . 4647i -0.2446 + 0.4647i 0.5621 - 0.1062i
0.5621 + 0 . 1062i

0.6254 0.6254 -0.1982 - 0.0654i -

0.1982 + 0 . 0654i

0.0025 + 0 . 3 017i 0.0025 - 0.3017i 0.5833

262

Solutions to the Practice Sets

0.5833

-0.1736 - 0 . 4603i -0.1736
0.2215 - 0 . 4898i
R3 =

4.0755 + 4.15171 0

0

0 4.0755

0

0

0

0

1.0755 - 2 . 7440i

0

0

0 . 4603i 0.2215

0

4 . 1517i 0

-1.0755
0

A3*U3 - U3*R3

1.0e-014 *

-0.2998 - 0 . 4885i
0.0944 - 0 . 3553i

0.1776 + 0 . 3 109i
0.0111 + 0 . 1055i

0.0444 + 0 . 0888i
0.0666 + 0 . 0666i
-0.1110 - 0 . 3109i
0.1776 - 0 . 2970i

•0.2998 + 0 . 4885i
0.1776 - 0 . 3109i
0.0444 - 0 . 0888i
•0.1110 + 0 . 3109i

0.0944 + 0 . 3553i
0.0111 - 0 . 1055i
-0.0666 - 0 . 0666i -
•0.1776 + 0 . 2970i -

Again, with the e-014 term this is essentially zero.

[U4 , R4] = eig (A4)

U4 =

[ 1 ,

1]

[ - (-l/2*d+l/2*a-l/2* (d A 2-2*a*d+a A 2+4*c*b) A (1/2) ) /b, -(-
l/2*d+l/2*a+l/2* (d A 2-2*a*d+a A 2+4*c*b) A (1/2) ) /b]

R4 =

[ l/2*d+l/2*a+l/2* (d^2-2*a*d+a A 2+4*c*b) A (1/2) ,

0]

Practice Set B

263

[ 0, l/2*d+l/2*a-

1/2* (d A 2-2*a*d+a A 2+4*c*b) A (1/2) ]

A4*U4 - U4*R4

[

0 ,

0 ]

[c-d* (-l/2*d+l/2*a-l/2* (d A 2-2*a*d+a A 2+4*c*b) A (1/2) ) /b+ (-
l/2*d+l/2*a-l/2* (d A 2-

2*a*d+a A 2+4*c*b) A (1/2) ) /b* (l/2*d+l/2*a+l/2* (d A 2-
2*a*d+a A 2+4*c*b) A (1/2) ) , c-d* (-l/2*d+l/2*a+l/2* (d A 2-
2*a*d+a A 2+4*c*b) A (1/2) ) /b+ (-l/2*d+l/2*a+l/2* (d A 2-
2*a*d+a A 2+4*c*b) A (1/2) ) /b* (l/2*d+l/2*a-l/2* (d A 2-
2*a*d+a A 2+4*c*b) A (1/2) ) ]

simplify (ans)

ans =

[ 0 , 0 ]

[ 0 , 0 ]

A = [102; -104; -1 -1 5] ;
clear U1 U2 R1 R2
[Ul, Rl] = eig(A)

■0.8165

-0.4082

•0.4082

2 . 0000
0
0

0.5774

0.5774

0.5774

0

3 . 0000
0

0.7071
■0.7071
0 . 0000

0

0

1.0000

B = [5 2 -8; 3 6 -10;
[U2 , R2] = eig(B)

3 3 -7];

264

Solutions to the Practice Sets

0.8165

0.4082

0.4082

2.0000

0

0

- 0.5774

- 0.5774

- 0.5774

0

- 1.0000

0

0.7071

- 0.7071

- 0.0000

0

0

3.0000

We observe that the columns of U1 are negatives of the corresponding
columns of U2 . Finally,

A*B - B*A

0

0

0

■ Problem 13

(a)

If we set X n to be the column matrix with entries x n , y n , z n , and M the
square matrix with entries 1, 1/4, 0; 0, 1/2, 0; 0, 1/4, 1 then X n+ \ = MX n .

(b)

We have X n = MX n _ i = M 2 X n _ 2 = ... = M n X 0 .

(c)

M = [1, 1/4, 0; 0, 1/2, 0; 0, 1/4, 1] ;

[U, R] = eig(M)

U =

1.0000 0 - 0.4082

0 0 0.8165

0 1.0000 - 0.4082

R =

1.0000 0 0

0 1.0000 0

0.5000

Practice Set B

265

(d)

M should be URU _1 . Let’s check:

M - U*R*inv (U)

ans =

0 0 0

0 0 0

0 0 0

It is evident that R^ is the diagonal matrix with entries 1, 1, 0. Since
Mqo = URooU- 1 , we have:

Minf = U*diag ( [1, 1, 0])*inv(U)

Minf =

1.0000 0.5000 0

0 0 0

0 0.5000 1.0000

(e)

syms xO yO zO; XO = [xO ; yO ; zO] ; Minf*X0

[ xO+l/2*yO]

[ 0 ]

[ l/2*y0+z0]

Half of the mixed genotype migrates to the dominant genotype and the
other half of the mixed genotype migrates to the recessive genotype.
These are added to the two original pure types, whose proportions are
preserved.

M~5*X0

ans =

[ x0+31/64*y0]
[ l/32*y0]

[ 31/ 64*y0+z0]

266

Solutions to the Practice Sets

M~10*X0

[ x0+1023/2048*y0]
[ 1/I024*y0]
[ 1023/2048*y0+z0]

If you use the suggested alternate model, then only the first three
columns of the table are relevant, the transition matrix M becomes M =
[1 1/2 0; 0 1/2 1; 0 0 0], and we leave it to you to compute that the
eventual population distribution is [1; 0; 0], independent of the initial
population.

Practice Set C

■ Problem 1

(a)

radiation = inline (vectorize (' 10000/ (4*pi* ( (x - xO) ~2 + (y -
y0)~2 + 1))'), 'x', 'y', 'x0', 'y0')

radiation =

Inline function:

radiation(x,y,x0,y0) = 10000 . / (4 . *pi . * ( (x-xO) . A 2 + (y-
yO) . a 2 + 1) )

x = zeros (1, 5); y = zeros (1, 5);
for j = 1:5
x(j) = 50*rand;
y ( j ) = 50*rand;
end

[X, Y] = meshgrid(0:0.1:50, 0:0.1:50);

contourf (X, Y, radiation (X, Y, x(l), y(l)) + radiation (X, Y,
x(2), y(2)) + radiation(X, Y, x(3), y(3)) + radiation(X, Y,
x (4 ) , y (4) ) + radiation (X, Y, x(5), y(5)) f 20);
colormap ( ' gray' )

Practice Set C

267

It is not so clear from the picture where to hide, although it looks like the
Captain has a pretty good chance of surviving a small number of shots.
But 100 shots may be enough to find him. Intuition says he ought to stay
close to the boundary.

(b)

Below is a series of commands that places Picard at the center of the
arena, fires the death ray 100 times, and then determines the health of
Picard. It uses the function lifeordeath, which computes the fate of
the Captain after a single shot.

function r = lifeordeath (xl, yl, xO, yO)

%This file computes the number of illumatons.

%that arrive at the point (xl, yl) , assuming the death,

%ray strikes 1 meter above the point (xO, yO) .

%If that number exceeds 50, a "1" is returned in the
%variable "r"; otherwise a "0" is returned for "r".
dosage = 10000/ (4*pi* ( (xl - x0)~2 + (yl - y0)~2 + 1));
if dosage > 50

268

Solutions to the Practice Sets

else

r = 0;

Here is the series of commands to test the Captain’s survival
possibilities:

xl = 25; yl = 25; h = 0;
for n = 1:100

xO = 50*rand;
yO = 50*rand;

r = lifeordeath (xl, yl, xO, yO) ;
h = h + r;

end

if h > 0

disp('The Captain is dead!')
else

disp(' Picard lives!')
end

The Captain is dead!

In fact if you run this sequence of commands multiple times, you will see
the Captain die far more often than he lives.

So let’s do a Monte Carlo simulation to see what his odds are:

xl = 25; yl = 25; c = 0;
for k = 1:100
h = 0;

for n = 1:100
xO = 50*rand;
yO = 50*rand;

r = lifeordeath (xl, yl, xO, yO) ;
h = h + r;
end

if h > 0

Practice Set C

269

else

c = c + 1;
end
end

disp(['The chances of Picard surviving are = ', ...

num2str (c/100) ] )

The chances of Picard surviving are = 0.16

We ran this a few times and saw survival chances ranging from 9 to 16%.

xl = 37.5; yl = 25; c = 0;
for k = 1:100
h = 0;

for n = 1:100
xO = 50*rand;
yO = 50*rand;

r = lifeordeath(xl, yl, xO, yO);
h = h + r;
end

if h > 0
else

c = c + 1;
end
end

disp(['The chances of Picard surviving are = ',...
num2str (c/100) ] )

The chances of Picard surviving are = 0.17

This time the numbers were between 10 and 18%. Let’s keep moving him
toward the periphery.

(e)

xl = 50; yl = 25; c = 0;
for k = 1:100

h = 0 ;

270

Solutions to the Practice Sets

for n = 1:100
xO = 50*rand;
yO = 50*rand;

r = lifeordeath (xl, yl, xO, yO)

h = h + r;
if h > 0
else

c = c + 1;

end

disp(['The chances
num2str (c/100) ] )

of Picard surviving are = ' , . . .

The chances of Picard surviving are = 0.44

The numbers now hover between 36 and 47% upon multiple runnings of
this scenario; so finally, suppose he cowers in the corner.

xl = 50; yl = 50; c
for k = 1:100
h = 0;

for n = 1:100
xO = 50*rand;

= 0;

yO = 50*rand;

r = lifeordeath (xl, yl, xO, yO)

h = h + r;

if h > 0
c = c;
else

c = c + 1;

end

disp(['The chances
num2str (c/100) ] )

of Picard surviving are = ' , . . .

The chances of Picard surviving are = 0.64

We saw numbers between 56 and 64%.

Practice Set C

271

They say a brave man dies but a single time, but a coward dies a
thousand deaths. But the person who said that probably never
encountered a Cardassian. Long live Picard!

■ Problem 2

(a)

Consider the status of the account on the last day of each month. At the
end of the first month, the account has M+ M x J = M(1 + J) dollars.
Then at the end of the second month the account contains
[M( 1 + J)](1 + J) = M(1 + J) 2 dollars. Similarly, at the end of n months,
the account will hold M(1 + J) n dollars. Therefore, our formula is

T = M(1 + J) n .

(b)

Now we take M = 0 and S dollars deposited monthly. At the end of the
first month the account has S+ S x J = S(1 + J) dollars. S dollars are
added to that sum the next day, and then at the end of the second month
the account contains [S(l + J) + S](l + J) = S[(l + J) 2 + (1 + J)]
dollars. Similarly, at the end of n months, the account will hold

S[(l + J) n H h (1 + =/)]

dollars. We recognize the geometric series — with the constant term “1”
missing, so the amount T in the account after n months will equal

T = S[((1 + J) n+1 - 1)/((1 + J) - 1) - 1] = S[((1 + J) n+1 - 1)/J- 1],

(c)

By combining the two models it is clear that in an account with an initial
balance M and monthly deposits S, the amount of money T after n
months is given by

T = M(1 + J) n + S [((1 + J) n+1 - 1 )/J - 1] .

(d)

We are asked to solve the equation
(1 + J) n = 2

with the values J = 0.05/12 and J = 0.1/12.

272

Solutions to the Practice Sets

months = solve('(l + (0 . 05) /12) ~n = 2')
years = months/12

months =

166.70165674865177999568182581405
years =

13.891804729054314999640152151171

months = solve ('(1 + (0.1)/12)~n = 2')
years = months/12

months =

83.523755900375880189555262964714
years =

6 . 9603129916979900157962719137262

If you double the interest rate, you roughly halve the time to achieve the
goal.

solve ( ' 1000000 = S* ( ( ( (1 + (0 . 08/12) )~ (35*12 + 1) -
l)/(0. 08/12) ) - 1) ')

433.05508895308253849797798306477

You need to deposit $433.06 every month.

solve ( ' 1000000 = 300* ( ( ( (1 + (0.08/12) ) ~ (n + 1) -
l)/(0. 08/12)) - 1)')

472.38046393034711345013989217261

ans/12

39.365038660862259454178324347717

Practice Set C

273

You have to work nearly 5 more years.

(g)

First, taking the whole bundle at once, after 20 years the $65,000 left after
taxes generates

format bank; optionl = 65000* (1 + (. 05/12) ) “ (12*20)

optionl =

176321.62

The stash grows to about $176,322. The second option yields

S = .8* (100000/240)

s =

333.33

option2 = S*((l/(. 05/12 ))*(((1 + (. 05/12) ) ~241) - 1) - 1)

option2 =

137582.10

You only accumulate $137,582 this way. Taking the lump sum up front
is clearly the better strategy.

<h)

rates = [.13, .15, -.03, .05, .10, .13, .15, -.03, .05];

clear T
for k = 0:4
T = 50000;
for j =1:5

T = T* (1 + rates (k + j));
disp ( [k + 1, T] )

1.00

2.00

72794 . 74
72794 . 74

274 Solutions to the Practice Sets

3.00

4.00

5.00

72794.74

72794.74

72794.74

The results are all the same; you wind up with $72,795 regardless of
where you enter in the cycle, because the product rii</<5(l + rates! /))
is independent of the order in which you place the factors. If you put the
$50,000 in a bank account paying 8%, you get

50000* (1.08) ~5

73466.40

that is, $73,466 — better than the market. The market’s volatility hurts
you compared to the bank’s stability. But of course that assumes you can
find a bank that will pay 8%. Now let’s see what happens with no stash,
but an annual investment instead. The analysis is more subtle here. Set
S = 10, 000 (which now represents a yearly deposit). At the end of one
year, the account contains S(1 + ri); then at the end of the second year
(S(l + ri) + S)(l + r 2 ), where we have written rj for rates! j). So at the
end of 5 years, the amount in the account will be the product of S and the
number

Iljfcitt + r j) + n y > 2 (l + r j) + n 7 > 3 (l + rj) + n 7 > 4 (l + Tj ) + (1 + rj).

If you enter at a different year in the business cycle the terms get cycled
appropriately. So now we can compute

format short
for k = 0:4
T = ones (1, 5) ;
for j =1:5
TT = 1;
for m = j : 5

TT = TT* ( 1 + rates (k + m) ) ;
end

T ( j ) = TT;

ans

end

Practice Set C

275

disp([k + 1, sum(T)])

1.0000

2.0000

3.0000

4.0000

5.0000

6.1196

6.4000

6.8358

6.1885

6.0192

Multiplying each of these by $10,000 gives the portfolio amounts for the
five scenarios. Not surprisingly, all are less than what one obtains by
investing the original $50,000 all at once. But in this model it matters
where you enter the business cycle. It’s clearly best to start your
investment program when a recession is in force and end in a boom.
Incidentally, the bank model yields in this case

(1/.08)*(((1.08)~6) - l) - l

ans =

6.3359

which is better than the results of some of the previous investment models
and worse than others.

■ Problem 3

(a)

First we define an expression that computes whether Tony gets a hit or
not during a single at bat, based on a random number chosen between 0
and 1. If the random number is less than or equal to 0.339, Tony is
credited with a hit, whereas if the number exceeds 0.339, he is retired by
the opposition.

Here is an M-file, called atbat .m, which computes the outcome of a single
at bat:

%This file simulates a single at bat.

%The variable r contains a "1" if Tony gets a hit,

%that is, if rand <= 0.339; and it contains a "0"

%if Tony fails to hit safely, that is, if rand > 0.339.
s = rand;

276

Solutions to the Practice Sets

if s <= 0.339
r = 1;

r = 0;

end

We can simulate a year in Tony’s career by evaluating the script M-file
atbat 500 times. The following program does exactly that. Then it
computes his average by adding up the number of hits and dividing by
the number of at bats, that is, 500. We build in a variable that allows for
a varying number of at bats in a year, although we shall only use 500.

function y = yearbattingaverage (n)

%This function file computes Tony's batting average for
%a single year, by simulating n at bats, adding up the
%number of hits, and then dividing by n.

X = zeros (1, n) ;
for i = l:n
atbat;

X(i) = r;

end

y = sum(X)/n;
yearbattingaverage (500)

0.3200

Now let’s write a function M-file that simulates a 20-year career. As with
the number of at bats in a year, we’ll allow for a varying length career.

function y = career (n,k)

%This function file computes the batting average for each
%year in a k-year career, asuming n at bats in each year.

%Then it lists the maximum, minimum, and lifetime average.

Y = zeros (1, k) ;
for j = l:k

Y(j) = yearbattingaverage (n) ;

end

Practice Set C

277

disp(['Best avg: num2str (max (Y) ) ] )

disp( ['Worst average: ', num2str (min (Y) ) ] )
disp ( ['Lifetime avg: ', num2str (sum(Y) /k) ] )

career(500, 20)

Best avg: 0.376
Worst average: 0.316
Lifetime avg: 0.3439

(C)

Now we run the simulation four more times:

career (500 , 20)

Best avg: 0.366
Worst average : 0.31
Lifetime avg: 0.3393

career(500, 20)

Best avg: 0.38
Worst average: 0.288
Lifetime avg: 0.3381

career(500, 20)

Best avg: 0.378
Worst average: 0.312
Lifetime avg: 0.3428

career(500, 20)

Best avg: 0.364
Worst average: 0.284
Lifetime avg: 0.3311

(d)

The average for the five different 20-year careers is:

278

Solutions to the Practice Sets

(.3439 + .3393 + .3381 + .3428 + .3311)/5

0.33904000000000

How about that!

If we ran the simulation 100 times and took the average it would likely
be extremely close to .339 — even closer than the previous number.

■ Problem 4

Our solution and its output are below. First we set n to 500 to save typing in
the following lines and make it easier to change this value later. Then we set
up a row vector j and a zero matrix A of the appropriate sizes and begin a
loop that successively defines each row of the matrix. Notice that on the line
defining A ( i , j ) , i is a scalar and j is a vector. Finally, we extract the
maximum value from the list of eigenvalues of A.

n = 500;
j = lsn;

A = zeros (n) ;
for i = l:n

A (i , j ) = l./(i + j - 1);
end

max (eig (A) )

■ Problem 5

Again we display below our solution and its output. First we define a vector
t of values between 0 and 27T, in order to later represent circles
parametrically as x = r cos t,y = r sin t. Then we clear any previous figure
that might exist and prepare to create the figure in several steps. Let’s say
the red circle will have radius 1; then the first black ring should have inner
radius 2 and outer radius 3, and thus the tenth black ring should have inner
radius 20 and outer radius 21. We start drawing from the outside in because

Practice Set C

279

the idea is to fill the largest circle in black, then fill the next largest circle in
white leaving only a ring of black, then fill the next largest circle in black
leaving a ring of white, etc. The i f statement tests true when r is odd and
false when it is even. We stop the alternation of black and white at a radius
of 2 to make the last circle red instead of black; then we adjust the axes to
make the circles appear round.

t = linspace(0, 2*pi, 100);
cla reset; hold on
for r = 21: -1:2
if mod(r, 2)

fill (r*cos (t) , r*sin (t) , 'k')
else

f ill (r*cos (t) , r*sin(t), 'w')
end
end

fill (cos (t) , sin (t) , 'r')
axis equal; hold off

■ Problem 6

Here are the contents of our solution M-file:

function m = mylcm (varargin)
nums = [varargin{ : }] ;

if -isnumeric (nums) |any(nums ~= round (real (nums) ) ) | ...
any (nums <= 0)

280

Solutions to the Practice Sets

error ( 'Arguments must be positive integers.')
end

for k = 2 : length (nums) ;

nuns (k) = lcm(nums(k), nums (k - 1) ) ;

m = nums (end) ;

Here are some examples:

mylcm( [4 5 6] )

60

mylcm(6, 7, 12, 15)

420

mylcm(4.5, 6)

??? Error using ==> mylcm
Arguments must be positive integers.

mylcm ('a', 'b', 'c')

??? Error using ==> mylcm
Arguments must be positive integers.

■ Problem 7

Here is our solution M-file:

function letcount (f ile)
if isunix

[stat, str] = unix(['cat ' file]);
else

[stat, str] = dos(['type ' file]);
end

Practice Set C

281

letters = 'abcdefghijklmnopqrstuvwxyz';
caps = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
for n = 1:26

count (n) = sumfstr == letters (n) ) + sumlstr == caps (n) ) ;
end

bar (count)

ylabel 'Number of occurrences'

title ( ['Letter frequencies in ' file])

set (gca, 'XLim', [0 27], 'XTick', 1:26, 'XTickLabel' , ...

letters' )

Here is the result of running this M-file on itself:

letcount ( ' letcount .m' )

■ Problem 8

We let w, x, y, and z, denote the number of residences canvassed in the four
cities Gotham, Metropolis, Oz, and River City, respectively. Then the linear
inequalities specified by the given data are as follows:

Nonnegative data: w > 0, x > 0, y > 0, z > 0;

Pamphlets: w+x+y+z< 50,000;

Travel cost: 0.5ic + 0.5x + y + 2z < 40,000;

Time available: 2w + 3x + y + 4z < 18,000;

Preferences: w < x, x + y < z;

282

Solutions to the Practice Sets

Contributions: w + 0.25x + 0.5y + 3 z > 10,000.

The quantity to be maximized is:

Voter support: 0.6u; + 0.6x + 0.5y + 0.3z.

(a)

This enables us to set up and solve the linear programming problem in
MATLAB as follows:

f = [-0.6 -0.6 -0.5 -0.3] ;

A = [1111; 0.5 0.5 1 2; 2 3 1 4; 1 -1 0 0; 0 1 1 -1; -1 -
0.25 -0.5 -3; -100 0; 0 -1 0 0; 0 0 -1 0; 0 0 0 -1];
b = [50000; 40000; 18000; 0; 0; -10000; 0; 0; 0; 0] ;

simlp ( f , A, b)

Optimization terminated successfully.

1.0e+003 *

1.2683

1.2683

1.3171

2.5854

Jane should canvass 1268 residences in each of Gothan and Metropolis,
1317 residences in Oz, and 2585 residences in River City.

(b)

If the allotment for time doubles then

b = [50000; 40000; 36000; 0; 0; -10000; 0; 0; 0; 0] ;
simlp (f. A, b)

Optimization terminated successfully.

1.0e+003 *

4 . 0000

4 . 0000
0 . 0000

4 . 0000

Practice Set C

283

Jane should canvass 4000 residences in each of Gotham, Metropolis, and
River City and ignore Oz.

(c)

Finally, if in addition she needs to raise $20,000 in contributions, then
b = [50000; 40000; 36000; 0; 0; -20000; 0; 0; 0; 0] ;
simlp(f. A, b)

Optimization terminated successfully,
ans =

1.0e+003 *

2.5366

2.5366

2.6341

5.1707

Jane needs to canvass 2537 residences in each of Gotham and Metropolis,
2634 residences in Oz, and 5171 in River City.

■ Problem 9

We let w, x, y, and z, denote the number of hours that Nerv spends with the
quarterback, the running backs, the receivers, and the linemen, respectively.
Then the linear inequalities specified by the given data are as follows:
Nonnegative data: w > 0, x > 0, y > 0, z > 0;

Time available: w + x + y + z < 50;

Point production: 0.5w + 0.3* + 0.4y + 0.1 z > 20;

Criticisms: w + 2x + 3y + 0.5z < 75;

Prima Donna status: x = y, w > x + y, x > z.

The quantity to be maximized is:

Personal satisfaction: 0.2w + OAx + 0.3y + 0.6z.

(a)

This enables us to set up and solve the linear programming problem in
MATLAB as follows:

f = [-0.2 -0.4 -0.3 -0.6] ;

A = [1111; -0.5 -0.3 -0.4 -0.1; 123 0.5; 0-110; ...

284

Solutions to the Practice Sets

01-1 0 ; - 1110 ; 0-10 1 ; -1 0 0 0 ; 0 -1 0 0 ; ...
0 0 -1 0 ; 0 0 0 - 1 ];

b = [50; -20; 75; 0; 0; 0; 0; 0; 0; 0; 0];

simlp(f. A, b)

Optimization terminated successfully.

25.9259

9.2593

9.2593

5.5556

Nerv should spend 7.5 hours each with the running backs and receivers;
6.9 hours with the linemen; and the majority of his time, 28.1 hours, with
the quarterback.

(b)

If the team only needs 15 points to win, then
b = [50; -15; 75; 0; 0; 0; 0; 0; 0; 0; 0] ;
simlp(f. A, b)

Optimization terminated successfully,
ans =

20.0000

10.0000

10.0000

10.0000

Nerv can spread his time more evenly, 10 hours each with the r unnin g
backs, receivers, and linemen; but still the biggest chunk of his time, 20
hours, should be spent with the quarterback.

(c)

Finally if in addition the number of criticisms is reduced to 70, then
b = [50; -15; 70; 0; 0; 0; 0; 0; 0; 0; 0];

simlp(f. A, b)

Practice Set C

285

Optimization terminated successfully.

18.6667

9.3333

9.3333

9.3333

Nerv must spend 18 1 hours with the quarterback and 9| hours with
each of the other three groups. Note that the total is less than 50, leaving
Nerv some free time to look for a job for next year.

■ Problem 10

syms VO R 10 VT x
f = x - VO + R*I0*exp (x/VT)

f =

x-V0+R*I0*exp (x/VT)

(a)

VD = fzero (char (subs (f, [VO, R, 10, VT] , [1.5, 1000, 10~(-5),

.0025])), [0, 1.5])

VD =

0.0125

That’s the voltage; the current is therefore

I = (1.5 - VD) /1000

I =

0 . 0015

(b)

g = subs (f , [V0, R] , [1.5, 1000])

g =

x-3/2+1000*I0*exp (x/VT)

286

Solutions to the Practice Sets

fzero (char (subs (g, [10, VT] , [ (1/2) *10" (-5) , .0025])),

[0, 1.5])

0.0142

Not surprisingly, the voltage goes up slightly.

<c)

fzero (char (subs (g, [10, VT] , [10"(-5), .0025/2])), [0, 1.5])
??? Error using ==> fzero

Function values at interval endpoints must be finite and
real .

The problem is that the values of the exponential are too big at the
right-hand endpoint of the test interval. We have to specify an interval
big enough to catch the solution, but small enough to prevent the
exponential from blowing up too drastically at the right endpoint. This
will be the case even more dramatically in part (e) below.

fzero (char (subs (g, [10, VT] , [10"(-5), .0025/2])), [0, 0.5])

ans =

0 . 0063

This time the voltage goes down.

(d)

Next we halve both:

fzero (char (subs (g, [10, VT] , [ (1/2) *10" (-5) , .0025/2])), [0,

0.5] )

ans =

0 . 0071

The voltage is less than in part (b) but more than in part (c).

(e)

syms u

h = subs (g, [10, VT] , [10" (-5)

0 . 0025*u] )

Practice Set C

287

h =

x-3/2+l/l00*u*exp (400*x/u)

X = zeros (6) ;

I = zeros (6) ;
for j =0:5

X ( j + 1) = f zero (char (subs (h, u, 10“(-j))), ...

[0, lO-(-j-l)]);

I(j + 1) = 10~(-j-5);

loglog (I, X)

The loglog plot has a slope of approximately 1, reflecting a linear
dependence.

■ Problem 1 1

(a)

dsolve('Dx = x - x~2')

ans =

1/ (1+exp (-t) *C1)

syms xO; sol = dsolve('Dx = x - x~2', 'x(O) = x0')

288

Solutions to the Practice Sets

sol =

1/ (1-exp (-t) * (-1+xO) /xO)

Note that this includes the zero solution; indeed

bettersol = simplify (sol)

bettersol =

-xO/ ( -xO-exp ( -t ) +exp ( -t) *x0)

subs (bettersol, xO, 0)

ans =

0

(b)

T = 0:0. 1:5;
hold on

solcurves = inline (vectorize (bettersol) , 't', 'xO');
for initval = 0:0.25:2.0
plot (T, solcurves (T, initval))
end

axis tight

title 'Solutions of Dx = x - x~2, with x(0) = 0, 0.25,..., 2'
xlabel ' t'
ylabel 'x'
hold off

Practice Set C

289

The graphical evidence suggests that: The solution that starts at zero stays
there; all the others tend toward the constant solution 1.

clear all; close all; hold on

f = inline ( ' [x (1) - x(l)~2 - 0 . 5*x (1) *x (2) ; x(2) - x(2)~2 -
0 . 5*x (1) *x (2 ) ] ' , 't', 'x');

for a = 0:1/12:13/12
for b = 0:1/12:13/12

[t, xa] = ode45 (f , [0 3], [a,b] ) ;

plot (xa ( : , 1), xa ( : , 2))
echo off
end
end

axis ( [0 13/12 0 13/12] )

The endpoints on the curves are the start points. So clearly any curve
that starts out inside the first quadrant, that is, one that corresponds to
a situation in which both populations are present at the outset, tends
toward a unique point — which from the graph appears to be about
(2/3, 2/3). In fact if x = y = 2/3, then the right sides of both equations in
(4) vanish, so the derivatives are zero and the values of x{t ) and y(t)
remain constant — they don’t depend on t. If only one species is present
at the outset, that is, you start out on one of the axes, then the solution

(c)

(d)

290 Solutions to the Practice Sets

tends toward either (1,0) or (0,1) depending on whether x or y is the
species present. That is precisely the behavior we saw in part (b).

close all; hold on

f = inline ( ' [x (1) - x(l)~2 - 2*x (1) *x (2) ; x(2) - x(2)~2 -

2*x (1) *x (2) ] ' , 't', 'x');
for a = 0:1/12:13/12
for b = 0:1/12:13/12

[t, xa] = ode45 (f , [0 3], [a,b] ) ;

plot (xa ( : , 1) , xa ( : , 2))
echo off
end
end

axis ( [0 13/12 0 13/12])

This time most of the curves seem to be tending toward one of the points
(1,0) or (0,1) — in particular, any solution curve that starts on one of the
axes (corresponding to no initial poulation for the other species) does so. It
seems that whichever species has a greater population at the outset will
eventually take over all the population — the other will die out. But there
is a delicate balance in the middle — it appears that if the two populations
are about equal at the outset, then they tend to the unique population
distribution at which, if you start there, nothing happens. That value
looks like (1/3, 1/3). In fact this is the value that renders both sides of (5)
zero and its role is analogous to that played by (2/3, 2/3) in part (d).

(e)

Practice Set C

291

It makes sense to refer to the model (4) as “peaceful coexistence”, since
whatever initial populations you have — provided both are present —
you wind up with equal populations eventually. “Doomsday” is an
appropriate name for model (5), since if you start out with unequal
populations, then the smaller group becomes extinct. The lower
coefficient 0.5 means relatively small interaction between the species,
allowing for coexistence. The larger coefficient 2 means stronger
interaction and competition, precluding the survival of both.

■ Problem 12

Here is a SIMULINK model for redoing the pendulum application from
Chapter 9:

With the initial conditions x(0) = 0, x(0) = 10, the XY Graph block shows the
following phase portrait:

292

Solutions to the Practice Sets

Meanwhile, the Scope block gives the following graph of x as a function of t:

■ Problem 13

Here is a SIMULINK model for studying the equation of motion of a baseball:

Practice Set C

293

The way this works is fairly straightforward. The Integrator block in the
upper left integrates the acceleration (a vector quantity) to get the velocity
(also a vector — we have chosen the option, from the Format menu, of
indicating vector quantities with thicker arrows). This block requires the
initial value of the velocity as an initial condition; we define it in the “initial
velocity” Constant block. Output from the first Integrator goes into the
second Integrator, which integrates the velocity to get the position (also a
vector). The initial condition for the position, [0, 4], is stored in the
parameters of this second Integrator. The position vector is fed into a Demux
block, which splits off the horizontal and vertical components of the position.
These are fed into the XY Graph block, and also the vertical component is fed
into a scope block so that we can see the height of the ball as a function of
time. The hardest part is the computation of the acceleration:

x = [0, -g] -c||x||x.

This is computed by adding the two terms on the right with the Sum block
near the lower left. The value of [0, —g] is stored in the “gravity” Constant
block. The second term on the right is computed in the Product block labeled
“Compute acceleration due to drag”, which multiplies the velocity (a vector)
by -c times the speed (a scalar). We compute the speed by taking the dot

294

Solutions to the Practice Sets

product of the velocity with itself and then taking the square root; then we
multiply by — c in the Gain block in the middle bottom of the model. The
Scope block in the lower right plots the ball’s speed as a function of time.

(a)

With c set to 0 (no air resistance) and the initial velocity set to [80, 80], the
ball follows a familiar parabolic trajectory, as seen in the following picture:

Note that the ball travels about 400 feet before hitting the ground, and so
the trajectory is just about what is required for a home run in most
ballparks. We can read off the flight time and final speed from the other
two scopes:

Practice Set C

295

Thus the ball stays in the air about 5 seconds and is traveling about 115
ft/sec when it hits the ground.

Now let’s see what happens when we factor in air resistance, again with
the initial velocity set to [80, 80]. First we take c = 0.0017. The trajectory
now looks like this:

Note the enormous difference air resistance makes; the ball only travels
about 270 feet. We can also investigate the flight time and speed with the
other two scopes:

296

Solutions to the Practice Sets

So the ball is about 80 feet high at its peak, and hits the ground in about
4g seconds. Its final speed can be read off from the picture:

So the final speed is only about 80 ft/sec, which is much gentler on the
hands of the outfielder than in the no-air-resistance case.

Let’s now redo exactly the same calculation with c = 0.0014
(corresponding to playing in Denver). The ball’s trajectory is now:

Practice Set C

297

I

400

500

The ball goes about 285 feet, or about 15 feet further than when playing
at sea level. This particular ball is probably an easy play, but with some
hard-hit balls, those extra 15 feet could mean the difference between an
out and a home run. If we look at the height scope for the Denver
calculation, we see:

So there is a very small increase in the flight time. Similarly, if we look at
the speed scope for the Denver calculation, we see:

298

Solutions to the Practice Sets

and so the final speed is a bit faster, about 83 ft/sec.

(c)

One would expect that batting averages would be higher in Denver, as
indeed is the case according to Major League Baseball statistics.

Glossary

We present here the most commonly used MATLAB objects in six categories:
operators, built-in constants, built-in functions, commands, graphics com-
mands, and MATLAB programming constructs. Though MATLAB does
not distinguish between commands and functions, it is convenient to think
of a MATLAB function as we normally think of mathematical functions. A
MATLAB function is something that can be evaluated or plotted; a com-
mand is something that manipulates data or expressions or that initiates a
process.

We list each operator, function, and command together with a short
description of its effect, followed by one or more examples. Many MATLAB
commands can appear in a number of different forms, because you can apply
them to different kinds of objects. In our examples, we have illustrated the
most commonly used forms of the commands. Many commands also have nu-
merous optional arguments; in this glossary, we have only included some very
common options. You can find a full description of all forms of a command,
and get a more complete accounting of all the optional arguments available
for it, by reading the help text — which you can access either by typing help
<commandname> or by invoking the Help Browser, shown in Figure G-l.

This glossary does not contain a comprehensive list of MATLAB commands.
We have selected the commands that we feel are most important. You can find
a comprehensive list in the Help Browser. The Help Browser is accessible
from the Command Window by typing helpdesk or helpwin, or from the
Launch Pad window in your Desktop under MATLAB : Help. Exactly what
commands are covered in your documentation depends on your installation, in
particular which toolboxes and what parts of the overall documentation files
you installed.

*® = ’ See Online Help in Chapter 2 for a detailed description of the Help Browser.

299

300

Glossary

Figure G-l : The Help Browser, Opened to "Graphics".

MATLAB Operators

\ Left matrix division. X = A\B is the solution of the equation A*X = B. Type help
slash for more information.

A = [1 0; 2 1] ; B = [3; 5] ;

A\B

/ Ordinary scalar division, or right matrix division. For matrices, A/B is essentially
equivalent to A*inv (B) . Type help slash for more information.

* Scalar or matrix multiplication. See the online help for mtimes.

. Not a true MATLAB operator. Used in conjunction with arithmetic operators to
force element-by-element operations on arrays. Also used to access fields of a struc-
ture array.

a = [1 2 3] ; b = [4-68];
a. *b

syms x y; solve (x + y - 2, x-y); ans.x

. * Element-by-element multiplication of arrays. See the previous entry and the
online help for times.

~ Scalar or matrix powers. See the online help for mpower.

. ~ Element-by-element powers. See the online help for power.

Glossary

301

: Range operator, used for defining vectors and matrices. Type help colon for more

information.

' Complex conjugate transpose of a matrix. See ctranspose. Also delimits the
beginning and end of a string.

; Suppresses output of a MATLAB command, and can be used to separate commands
on a command line. Also used to separate the rows of a matrix or column vector.

X = 0:0.1:30;

[1; 2; 3]

, Separates elements of a row of a matrix, or arguments to a command. Can also be
used to separate commands on a command line.

. ' Transpose of a matrix. See transpose.

. . . Line continuation operator. Cannot be used inside quoted strings. Type help
punct for more information.

1+3+5+7+9+11 ...

+ 13 + 15 + 17

['This is a way to create very long strings ...

'that span more than one line. Note the square brackets.']

! Run command from operating system.

!C: \Programs\program.bat

% Comment. MATLAB will ignore the rest of the same line.

@ Creates a function handle.
fminbnd(@cos, 0, 2*pi)

Built-in Constants

eps Roughly the size of the computer’s floating point round-off error; on most
computers it is around 2 x 10 -16 .

exp (1) e = 2.71828 .... Note that e has no special meaning.

i i = V— 1. This assignment can be overridden, for example, if you want to use i as
an index in a for loop. In that case j can be used for the imaginary unit.

Inf oo. Also inf (in lower-case letters).

NaN Not a number. Used for indeterminate expressions such as 0/0.
pi n = 3.14159

302

Glossary

Built-in Functions

abs |x|.
acos arccos x .
asin arcsinx.

atan arctanx. Use atan2 instead if you want the angular coordinate 0 of the point
(x.y).

bessel Bessel functions; besselj (n, x) and bessely (n, x) are linearly inde-
pendent solutions of Bessel’s equation of order n.

con j Gives the complex conjugate of a complex number.

conj (1 - 5*i)
cos cosx.
cosh coshx.
cot cotx.

erf The error function erffx ) = (2/^/jv) f* e~ t2 dt.
exp e x .

expm Matrix exponential.

gamma The gamma function r(x) = / 0 °° e~ t t x ~ 1 dt (when Re x > 0). The property
T(k + 1) = k\, for nonnegative integers k, is sometimes useful.

imag imag(z), the imaginary part of a complex number,
log The natural logarithm lnx = log e x.
real real(z), the real part of a complex number,
sec secx.
sech sechx.

sign Returns —1, 0, or 1, depending on whether the argument is negative, zero, or
positive.

sin sinx.
sinh sinhx.
sqrt *Jx.
tan tanx.
tanh tanhx.

Glossary

303

MATLAB Commands

addpath Adds the specified directory to MATLAB’s file search path,
addpath C:\my_mfiles

ans A variable holding the value of the most recent unassigned output,
cd Makes the specified directory the current (working) directory,
cd C:\mydocs\mfiles

char Converts a symbolic expression to a string. Useful for defining inline functions,
syms x y

f = inline (char (sin (x) *sin (y) ) )

clear Clears values and definitions for variables and functions. If you specify one
or more variables, then only those variables are cleared,
clear
clear £ g

collect Collects coefficients of powers of the specified symbolic variable in a given
symbolic expression,
syms x y

collect (x~2 - 2*x~2 + 3*x + x*y, x)
compose Composition of functions.

syms x y; f = exp (x) ; g = sin(y); h = composetf, g)

ctranspose Conjugate transpose of a matrix. Usually invoked with the ' operator.
Equivalent to transpose for real matrices.

A = [1 3 i]

A'

D Not a true MATLAB command. Used in dsolve to denote differentiation. See di f f .

dsolve('x*Dy + y = sin (x) ' , 'x')

delete Deletes a file.

delete <filename>
det The determinant of a matrix,
det ( [1 3; 4 5])

diag Gives a square matrix with a prescribed diagonal vector, or picks out the
diagonal in a square matrix.

V = [2345]; diag (V)

X = [2 3; 4 5] ; diag(X)

304

Glossary

diary Writes a transcript of a MATLAB session to a file,
diary <filename>
diary off

dif f Symbolic differentiation operator (also difference operator),
syms x; diff(x~3)
dif f ( ' x*y~2 ' , 'y')

dir Lists the files in the current working directory. Similar to Is.
disp Displays output without first giving its name,
x = 5.6; disp(x)
syms x; disp(x~2)

disp ('This will print without quotes.')

double Gives a double-precision value for either a numeric or symbolic quantity.
Applied to a string, double returns a vector of ASCII codes for the characters in
the string.

z = sym('pi'); double (z)
double ( ' think' )

dsolve Symbolic ODE solver. By default, the independent variable is t, but a diff-
erent variable can be specified as the last argument,
dsolve ('D2y - x*y = O', 'x')

dsolve('Dy + y~2 = O', 'y(O) = 1', 'x')

[x, y] = dsolve ('Dx = 2x + y' , 'Dy = - x')

echo Turns on or off the echoing of commands inside script M-files.
edit Opens the specified M-file in the Editor/Debugger,
edit mymfile

eig Computes eigenvalues and eigenvectors of a square matrix,
eig ( [2 , 3; 4, 5])

[e, v] = eig( [1, 0, 0; 1, 1, 1; 1, 2, 4])
end Last entry of a vector. Also a programming command,
v (end)
v (3 : end)

eval Used for evaluating strings as MATLAB expressions. Useful in M-files.
eval ( ' cos (x) ' )

expand Expands an algebraic expression,
syms x y; expand ( (x - y)~2)

Glossary

305

eye The identity matrix of the specified size,
eye (5)

factor Factors a polynomial,
syms x y; factor (x~4 - y~4)

f eval Evaluates a function specified by a string. Useful in function M-files.
feval ( ' exp' , 1)

find Finds the indices of nonzero elements of a vector or matrix.

X = [205]; find (X)

fminbnd Finds the smallest (approximate) value of a function over an interval,
fminbnd ( ' x~4 - x~2 +1', 0, 1)

f = inline ( ' x~3 - 7*x~2 - 5*x +2', 'x'); fminbnd(f, 4, 6)
format Specifies the output format for numerical variables,
format long

f zero Tries to find a zero of the specified function near a given starting point or on
a specified interval,
f zero (inline (' cos (x) - x' ) , 1)
fzero(@cos, [-pi 0])

guide Opens the GUI Design Environment,
guide mygui

help Asks for documentation for a MATLAB command. See also lookfor.
help factor

inline Constructs a MATLAB inline function from a string expression,
f = inline ( ' x~5 - x' ) ; f(3)

sol = dsolve('Dy = x~2 + y' , 'y(O) = 2', 'x');
fsol = inline (vectorize (sol) , 'x')

int Integration operator for both definite and indefinite integrals,
int ( ' 1/ (1 + x~2 ) ' , 'x')
syms x; int(exp(-x), x, 0, Inf)

inv Inverse of a square matrix,
inv ( [1 2; 3 5] )

j acobian Computes the Jacobian matrix, or for a scalar function, the symbolic gra-
dient.

syms x y; f = x'"2*y"3; jacobian(f)

306

Glossary

length Returns the number of elements in a vector or string,
length ( ' abode ' )

limit Finds a two-sided limit, if it exists. Use 'right' or 'left' for one-sided
limits.

syms x; limit (sin (x) /x, x, 0)
syms x; limit (1/x, x. Inf, 'left')

1 inspace Generates a vector of linearly spaced points.

Iinspace(0, 2*pi, 30)
load Loads Workspace variables from a disk file,
load filename

lookfor Searches for a specified string in the first line of all M-files found in the
MATLAB path,
lookfor ode

Is Lists files in the current working directory. Similar to dir.

maple String access to the Maple kernel; generally is used in the form
maple ( ' function' , ' arg' ) . Not available in the Student Version,
maple (' help' , 'csgn')

max Computes the arithmetic maximum of the entries of a vector.

X = [3 5 1 -6 23 -56 100]; max (X)
mean Computes the arithmetic average of the entries of a vector.

X = [3 5 1 -6 23 -56 100]; mean(X)
syms xyz; X= [xyz]; mean(X)

median Computes the arithmetic median of the entries of a vector.

X = [3 5 1 -6 23 -56 100]; median(X)
min Computes the arithmetic minimum of the entries of a vector.

X = [3 5 1 -6 23 -56 100]; min (X)

more Turns on (or off) page-by-page scrolling of MATLAB output. Use the SPACE BAR
to advance to the next page, the RETURN key to advance line-by-line, and Q to abort
the output,
more on
more off

notebook Opens an M-book (Windows only),
notebook probleml.doc
notebook -setup

Glossary

307

num2str Converts a number to a string. Useful in programming,
constant = ['a' num2str(l)]

ode45 Numerical ODE solver for first-order equations. See MATLAB’s online help
for ode45 for a list of other MATLAB ODE solvers,
f = inline('t~2 + y' , 't', 'y')

[x, y] = ode45 (f , [0 10] , 1) ;

plot(x, y)

ones Creates a matrix of ones,
ones (3)
ones (3, 1)

open Opens a file. The way this is done depends on the filename extension,
open myfigure.fig

path Without an argument, displays the search path. With an argument, sets the
search path. Type help path for details.

pretty Displays a symbolic expression in a more readable format,
syms x y; expr = x/ (x - 3)/(x + 2/y)
pretty (expr)

prod Computes the product of the entries of a vector.

X = [3 5 1 -6 23 -56 100]; prod(X)

pwd Shows the name of the current (working) directory.

quadl Numerical integration command. In MATLAB 5.3 or earlier, use quad8 in-
stead.

format long; quadl (' sin (exp (x) )' , 0, 1)
g = inline (' sin (exp (x) )') ; quad8 (g, 0, 1)

quit Terminates a MATLAB session.

rand Random number generator; gives a random number between 0 and 1.

rank Gives the rank of a matrix.

A = [2 3 5; 4 6 8] ; rank (A)

roots Finds the roots of a polynomial whose coefficients are given by the elements
of the vector argument of roots,
roots ( [1 2 2] )

round Rounds a number to the nearest integer.

save Saves Workspace variables to a specified file. See also diary and load.

save filename

308

Glossary

sim Runs a SIMULINK model.
sim(' model' )

simple Attempts to simplify an expression using multiple methods.

syms x y; [expression, how] = simple (sin (x) *cos (y) + cos (x) *sin (y) )
simplify Attempts to simplify an expression symbolically.

syms x; simplify (1/ (1 + x) ~2 - 1/(1 - x) ~2)
s imu link Opens the SIMULINK library.

size Returns the number of rows and the number of columns in a matrix.

A = [1 3 2; 4 1 5]

[r, c] = size (A)

solve Solves an equation or set of equations. If the right-hand side of the equation
is omitted, ‘0’ is assumed.
solve('2*x~2 - 3*x + 6')

[x, y] = solvel'x + 3*y =4', ' -x - 5*y = 3', 'x', 'y')
sound Plays a vector through the computer speakers.

sound (sin (0 : 0 . l*pi : 1000*pi) )
s treat Concatenates two or more strings.

strcat('This 'is ', 'a ', 'long ', 'string.')
str2num Converts a string to a number. Useful in programming,
constant = 'a7'
index = str2num (constant (2 ) )

subs Substitutes for parts of an expression,
subs ( ' x~3 - 4*x + 1', 'x', 2)
subs ( ' sin (x) ~2 + cos (x) ' , 'sin(x)', 'z')

sum Sums a vector, or sums the columns of a matrix,
k = 1:10; sum (k)

sym Creates a symbolic variable or number,
sym pi

x = sym('x')
constant = sym ('1/2')

syms Shortcut for creating symbolic variables. The command syms x is
equivalent to x = sym ( ' x ' ) .
syms x y z

Glossary

309

symsum Performs a symbolic summation of a vector, possibly with infinitely many
entries.

syms x k n; symsum (x~k, k, 0, n)
syms n; symsum (n~ ( -2 ) , n, 1, Inf)

taylor Gives a Taylor polynomial approximation of a specified order (the default is
5) at a specified point (default is 0).
syms x; taylor (cos (x) , 8, 0)
taylor (exp (1/x) , 10, Inf)

transpose Transpose of a matrix (compare ctranspose). Converts a column vector
to a row vector, and vice versa. Usually invoked with the . ' operator.

A = [13 4]

A. '

type Displays the contents of a specified file,
type myfile.m

vectorize Vectorizes a symbolic expression. Useful in defining inline functions,
f = inline (vectorize ( 'x~2 - 1/x'))

vpa Evaluates an expression to the specified degree of accuracy using variable
precision arithmetic,
vpa ( ' 1/3 ' , 20)

whos Lists current information on all the variables in the Workspace.

zeros Creates a matrix of zeros,
zeros (10)
zeros (3, 1)

Graphics Commands

area Produces a shaded graph of the area between the x axis and a curve.

X = 0:0.1 : 4*pi ; Y = sin(X); area(X, Y)

axes Creates an empty figure window.

axis Sets axis scaling and appearance.

axis ( [xmin xmax ymin ymax] ) — sets ranges for the axes,
axis tight — sets the axis limits to the full range of the data,
axis equal — makes the horizontal and vertical scales equal,
axis square — makes the axis box square,
axis off — hides the axes and tick marks.

310

Glossary

bar Draws a bar graph.

bar ( [2 , 7, 1.5, 6])
cla Clear axes.

close Closes the current figure window; close all closes all figure windows.

colormap Sets the colormap features of the current figure; type help graph3d to
see examples of colormaps.

X = 0:0.1: 4*pi ; Y = sin(X); colormap cool
comet Displays an animated parametric plot.

t = 0:0.1: 4*pi ; comet (t . *cos (t) , t.*sin(t))

contour Plots the level curves of a function of two variables; usually used with
meshgrid.

[X, Y] = meshgrid(-3:0.1:3, -3:0. 1:3);
contour (X, Y, X.~2 - Y.~2)

contour f Filled contour plot. Often used with colormap.

[X , Y] = meshgrid (-2:0. 1:2, -2:0. 1:2); contourf (X, Y, X . ~2 - Y.~3);
colormap autumn

ezcontour Easy plot command for contour or level curves.
ezcontour('x A 2 - y~2')
syms x y; ezcontour (x - y~2)

ezmesh Easy plot command for mesh view of surfaces,
ezmesh ('x~2 + y~2')
syms x y; ezmesh (x*y)

ezplot Easy plot command for symbolic expressions,
ezplot ( ' exp ( -x~2 ) ' , [-5, 5])

syms x; ezplot (sin (x) )

ezplot3 Easy plot command for 3D parametric curves.
ezplot3 ( ' cos (t) ' , 'sin(t)', 't')

syms t; ezplot3 (1 - cos(t), t - sin(t), t, [0 4*pi] )
ezsurf Easy plot command for standard shaded view of surfaces,
ezsurf ( ' (x~2 + y~2 ) *exp ( - (x~2 + y~2))')
syms x y; ezsurf (sin (x*y) , [-pi pi -pi pi])

figure Creates a new figure window,
fill Creates a filled polygon. See also patch.

fill ( [0 110], [0011], 'b'); axis equal tight

Glossary

311

f indob j Finds graphics objects with specified property values.

f indob j ( ' Type' , 'Line')
gca Gets current axes,
gcf Gets current figure,
get Gets properties of a figure,
get (gcf)

getframe Command to get the frames of a movie or animation.

T = 0:0.1: 2*pi ;
for j = 1:12

plot(5*cos(j*pi/6) + cos(T), 5*sin ( j*pi/6) + sin(T));
axis ( [ - 6 6 -6 6]);

M ( j ) = getframe;

movie (M)

ginput Gathers coordinates from a figure using the mouse (press the RETURN key
to finish).

[X, Y] = ginput
grid Puts a grid on a figure,
gtext Places a text label using the mouse.

gtext (' Region of instability')
hist Draws a histogram,
for j = 1:100

Y(j) = rand;

end

hist (Y)

hold Holds the current graph. Superimposes any new graphics generated by
MATLAB on top of the current figure,
hold on
hold off

legend Creates a legend for a figure,
t = 0:0.1: 2*pi ;
plot ( t # cos(t), t, sin (t) )
legend (' cos (t) ' , 'sin(t)')

loglog Creates a log-log plot.

x = 0.0001:0.1:12; loglog(x, x.~5)

312

Glossary

mesh Draws a mesh surface.

[X, Y] = meshgrid(-2: .1:2, -2:. 1:2);
mesh(X, Y, sin (pi*X) . *cos (pi*Y) )

meshgrid Creates a vector array that can be used as input to a graphics command,
for example, contour, quiver, or surf.

[X, Y] = meshgrid (0:0. 1:1, 0:0. 1:2)
contour (X, Y, X.~2 + Y.~2)

movie Plays back a movie. See the entry for getf rame.

patch Creates a filled polygon or colored surface patch. See also fill.

t = (0 : 1: 5) *2*pi/5; patch (cos (t) , sin(t), 'r'); axis equal
pie Draws a pie plot of the data in the input vector.

Z = [34 5 32 6] ; pie(Z)
plot Plots vectors of data.

X = [0:0. 1:2] ;
plot (X, X.~3)

plot 3 Plots curves in 3D space,
t = [0:0.1:30] ;

plot3 (t, t.*cos(t), t.*sin(t))
polar Polar coordinate plot command.

theta = 0:0.1: 2*pi ; rho = theta; polar(theta, rho)
print Sends the contents of the current figure window to the printer or to a file,
print

print -deps picture.eps
quiver Plots a (numerical) vector field in the plane.

[x, y] = meshgrid ( -4 : 0 . 5 : 4 , -4:0. 5:4);
quiver (x, y, x.*(y - 2), y.*x); axis tight

semilogy Creates a semilog plot, with logarithmic scale along the vertical axis.

x = 0:0.1:12; semilogy (x, exp(x))
set Set properties of a figure.

set(gcf, 'Color', [0, 0.8, 0.8])
subplot Breaks the figure window into a grid of smaller plots.
subplot(2, 2, 1), ezplot ( ' x~2 ' )
subplot (2, 2, 2), ezplot ( 'x~3 ' )

Glossary

313

subplot(2, 2, 3), ezplot ( 'x~4 ' )
subplot(2, 2, 4), ezplot ('x~5' )

surf Draws a solid surface.

[X, Y] = meshgrid(-2: .1:2, -2:. 1:2);
surf (X, Y, sin (pi*X) . *cos (pi*Y) )

text Annotates a figure, by placing text at specified coordinates,
text (x, y, 'string')

title Assigns a title to the current figure window,
title 'Nice Picture'

xlabel Assigns a label to the horizontal coordinate axis,
xlabel ('Year' )

ylabel Assigns a label to the vertical coordinate axis,
ylabel ( ' Population' )

view Specifies a point from which to view a 3D graph.

ezsurf('(x~2 + y~2 ) *exp ( - (x~2 + y~2))'); view([0 0 1])
syms x y; ezmesh (x*y) ; view([l 0 0])

zoom Rescales a figure by a specified factor; zoom by itself enables use of the mouse
for zooming in or out.
zoom
zoom (4)

MATLAB Programming

any True if any element of an array is nonzero.

if any(imag(x) ~= 0); error (' Inputs must be real.'); end
all True if all the elements of an array are nonzero,
break Breaks out of a for or while loop,
case Used to delimit cases after a switch statement,
computer Outputs the type of computer on which MATLAB is running,
dbclear Clears breakpoints from a file,
dbclear all

dbcont Returns to an M-file after stopping at a breakpoint,
dbquit Terminates an M-file after stopping at a breakpoint.

314

Glossary

dbstep Executes an M-file line-by-line after stopping at a breakpoint,
dbstop Insert a breakpoint in a file.

dbstop in <filename> at <linenumber>

dos Runs a command from the operating system, saving the result in a variable.
Similar to unix.

end Terminates an if, for, while, or switch statement,
else Alternative in a conditional statement. See if.

elseif Nested alternative in a conditional statement. See the online help for if.
error Displays an error message and aborts execution of an M-file.
find Reports indices of nonzero elements of an array,
n = find (isspace (mystring) ) ;
if ~isempty(n)

firstword = mystring (1 :n (1) - 1) ;
restofstring = mystring (n (1) +1 : end) ;

end

for Repeats a block of commands a specified number of times. Must be terminated
by end.

close; axes; hold on
t = -1:0.05:1;
for k = 0:10

plot(t, t.~k)

end

function Used on the first line of an M-file to make it a function M-file.
function y = myfunction (x)

i f Allows conditional execution of MATLAB statements. Must be terminated by end.
if (x >= 0)
sqrt (x)
else

error (' Invalid input.')
end

input Prompts for user input.

answer = input (' Please enter [x, y] coordinates: ')
isa Checks whether an object is of a given class (double, sym, etc.).
isa(x, 'sym')

ischar True if an array is a character string.

Glossary

315

is empty True if an array is empty.

is finite Checks whether elements of an array are finite.

isfinited./ [-1 0 1] )
ishold True if hold on is in effect,
isinf Checks whether elements of an array are infinite,
isletter Checks whether elements of a string are letters of the alphabet,
str = 'remove my spaces'; str (isletter (str) )

isnan Checks whether elements of an array are “not-a-number” (which results from
indeterminate forms such as 0/0).
isnan ( [-1 0 1] /0)

isnumeric True if an object is of a numeric class,
ispc True if MATLAB is running on a Windows computer,
isreal True if an array consists only of real numbers,
isspace Checks whether elements of a string are spaces, tabs, etc.
isunix True if MATLAB is running on a UNIX computer.

keyboard Returns control from an M-file to the keyboard. Useful for debugging
M-files.

mex Compiles a MEX program.

nargin Returns the number of input arguments passed to a function M-file.

if (nargin < 2); error ('Wrong number of arguments'); end
nargout Returns the number of output arguments requested from a function M-file.
otherwise Used to delimit an alternative case after a switch statement,
pause Suspends execution of an M-file until the user presses a key.

return Terminates execution of an M-file early or returns to an M-file after a
keyboard command,
if abs(err) < tol; return; end

switch Alternative to if that allows branching to more than two cases. Must be
terminated by end.
switch num

disp (' Yes . ' )

316

Glossary

disp ( 'No. ' )
otherwise

disp ( 'Maybe . ' )

end

unix Runs a command from the operating system, saving the result in a variable.
Similar to dos.

varargin Used in a function M-file to handle a variable number of inputs.

varargout Used in a function M-file to allow a variable number of outputs.

warning Displays a warning message.

warning (' Taking the square root of negative number.')

while Repeats a block of commands until a condition fails to be met. Must be termi-
nated by end.
mysum = 0 ;
x = 1;

while x > eps

mysum = mysum + x;
x = x/2;

mysum

Index

The index uses the same conventions for fonts that are used throughout the
book. MATLAB commands, such as dsolve, are printed in a boldface type-
writer font. Menu options, such as File, are printed in boldface. Names of
keys, such as ENTER, are printed in small caps. Everything else is printed in
a standard font.

! , 118-119, 301

\, 60, 79, 88, 172, 257-258, 300

%, 38, 57, 301

70, 300

&, 104-105

| , 104-105, 279

' , 22-23, 301

-, 105-106

' * ' , 80

~=, 106

' + ',80

' - ' , 80

aborting calculations, 5

' - - ' , 80

abs, 102, 302

' : ' , 80

accuracy, floating point, 110

*, 14, 300

acos, 302

,,51, 301

activate, 131

. , 19, 22, 26, 300

Activate Figure, 131

.',301

Add Folder..., 35

.*,22, 59, 300

Add with Subfolders..., 35

. . . , 39, 53, 99, 301

addition, 9

. /, 22, 59

addpath, 34, 303

.'',22, 59, 300

air resistance, 212-213, 294-296

/, 300

Align Objects, 128

21, 59, 107, 114, 171, 195, 279, 289-290,

all, 106, 313

301

amperes, 210

; , 23-24, 29, 50, 301

animation, 77, 256, 311

=, 16

ans, 9, 13, 40, 58, 114, 219, 303

==, 103-104, 109, 115

any, 106, 279, 313

?, 5, 15

Application Options, 130, 133

@, 55, 116, 194, 301

area, 174-175, 309

317

318

Index

argument, 40, 52, 57-58, 112-114
arithmetic, 8

arithmetic operations, 220, 226
arithmetic symbols, 222, 226
arithmetic, complex, 58-59
arithmetic, exact, 10-11
arithmetic, floating point, 11
arithmetic, variable precision, 11
array, 13, 23, 51
Array Editor, 33
asin, 63, 302

assignment, 16-17, 103, 219
asterisk, 8
atan, 24, 48, 302
atan2, 58, 116, 302
Auto Correct..., 99-100
axes, 26, 81
axes, 43, 45, 309
Axes Properties..., 81, 125
axes, scale, 80

axis, 27, 49, 69-70, 77-78, 212, 242, 309
axlimdlg, 132

background, 3
backslash, 60, 87, 257-258
Band-Limited White Noise, 126
bar, 161, 164, 281, 310
baseball, 213, 292
batting average, 206, 213, 298
Beethoven, 85
bell curve, 152, 154
bessel, 302
besselj, 24, 76, 302
bessely, 302
bifurcation, 163

Block Parameters, 124-125, 192
blocks, 121

Boltzmann’s constant, 209
border, 82

boundary condition, 185
braces, 113, 220, 222
brackets, 113, 220-222
brackets, curly, 222
branch line, 124
branching, 101, 103, 106, 111
break, 111, 313
breakpoint, 117, 227, 230

Breakpoints, 227

Bring MATLAB to Front, 97

bug fixes, 214
bug reports, 214
built-in constant, 24
built-in function, 24, 226
bulls-eye, 207
Bytes, 13

C, xiii, xvi, 118-119
calculus, 61

callback function, 131-132, 134-135

Captain Picard, 204

Cardassian, 204, 271

case, 109, 222, 313

casino, 149, 153-154

cd, 34, 118, 303

cell array, 113, 222

Cell Markers, 96

cell, corrupted, 100

Central Limit Theorem, 152-153

chaotic behavior, 165, 167

char, 53, 226, 285-287, 303

Checkbox, 130, 134

chessboard, 83

Children, 82

circuit, electrical, 209

cla, 279, 310

class, 13, 51

clear, 13, 17, 33, 38, 46, 57, 303
clear values, 17, 226
clear variables, 225
close, 38, 43, 45^6, 98, 310
Close, 28

closed Leontief model, 168
collect, 303

colon, xvii, 21, 59, 107, 171, 301
color, 80
Color, 82

color, background, 82
color, RGB, 82, 230
colormap, 137, 266, 310
column, 59

column vector, 22-23, 40, 52
comet, 77, 310
comma, 51, 301
command history, 15, 36

Index

319

Command History window, 3, 31, 220
Command Window, 3, 8, 14, 31-33, 35-36,
39, 43, 45, 91, 93, 96, 98-99, 103, 299
comment, 38, 42, 46
common mistakes, 226
competing species, 211
complex arithmetic, 58-59
complex number, 19, 24, 58-59, 102, 105,
114, 224, 246
compose, 303
computer, 118, 313
computer won’t respond, 218, 226
concatenation, 53
conductivity, 189
con j , 302
Constant, 213, 293
constant, built-in, 24
constraint, 173-175, 177-179
Continuous library, 121, 200
continuous model, 211
contour, 69-70, 73, 86, 176, 204, 246-247,
310

contour plot, 69
contourf, 138-139, 266, 310
CONTROL key, 5
Copy, 32

cos, 24, 302
cosh, 302

cot, 302

Create Shortcut, 32
Create subsystem, 200
ctranspose, 22—23, 303
CRTL+C, 5, 43, 47, 226
CRTL+ENTER, 94, 96, 100
current, 209
Current Directory, 34
Current Directory browser, 3, 31-32,

35-36, 99
CurrentAxes, 82
curve, 86
curve, level, 69
Cut, 126

cylindrical coordinates, 74
D, 61, 303

damped harmonic oscillator, 121
dashed line, 80, 243

data class, 13, 20, 51-52, 54
dbclear, 117, 227, 313
dbcont, 117, 227, 313
dblquad, 63
dbquit, 117, 313
dbstep, 117, 314
dbstop, 117, 227, 314
debug, 46, 227
debugger, 36, 45, 227
debugging, 117
default variables, 65
Define Auto Init Cell, 97
Define Calc Zone, 97
Define Input Cell, 96
definite integral, 62
definitions, conflicting, 219
delete, 42, 46, 118, 303
Delete, 34

delimiters, 220-221, 226
delimiters, mismatched, 220
demand, 171-172
demo, 5

Demux, 200, 293
Denver, Colorado, 213, 296-298
derivative, 61, 86
Derivative, 125
descendents, 83

Desktop, xv, 3-4, 6-8, 14, 31-33, 99, 121,
214, 220, 299
Desktop Layout, 31
det, 88, 258-260, 303
determinant, 88
diag, 265, 303
dialog box, 132
diary, 42^13, 46, 304
diary file, xv, 31, 42, 45-46, 98
dif f, 61, 186, 190, 249, 304
difference equation, 210
differential equation, 121, 125, 191, 210
differential/difference equation, 197
differentiation, 61
diffusion equation, 184
digit, 17, 48
dingbats, xvii
diode, 209
dir, 35, 118, 304
Dirichlet boundary condition, 185

320

Index

discrete model, 211
Discrete Pulse Generator, 166
disp, 51, 114^115, 145, 147, 149, 268-270,
277, 304
display pane, 15
division, 9, 220
Dock, 6, 32
documentation, 16, 214
documentation, online, 15
doomsday, 212, 291
DOS, 118
dos, 119, 280, 314
dotted line, 80
double, 18, 53, 246, 304
double array, 13
double precision, 10, 13
down-arrow key, 15, 36
dsolve, 66, 211, 287, 304
Duplicate, 129

e, 301

echo, 37-38, 41-43, 45-46, 99, 304
edit, 36, 101, 304

Edit, 34, 80-81, 84, 96, 126, 200, 217

Edit Axes..., 84
Edit ButtondownFcn, 135
Edit Callback, 132
Edit Menubar, 130

Edit Text, 129, 131, 133-134
editing, 36
editing input, 15

Editor/Debugger, 36, 45-46, 99, 102, 117,
130, 132, 227, 230, 232
editpath, 35

eig, 60, 88, 148, 169, 261-264, 278, 304
eigenpair, 60

eigenvalue, 60, 88, 90, 148, 168-169, 198,
206, 278

eigenvector, 60, 88, 90, 148, 168-169, 198
element, 23, 59

element-by-element, 22-23, 59
else, 102-103, 106, 268, 279-280, 314
elseif, 102, 105, 109, 314
emacs, 36

end, 41, 45, 102-103, 106, 109, 111, 186,
190, 304, 314

enter, 4, 13, 15, 36, 39, 47, 53, 116, 134

eps, 301

EPS (Encapsulated PostScript), 43
equal sign, 16-17, 50
equilibrium, 168
erf, 24, 255, 302
error, 17

error, 113-114, 116, 280, 314
error function, 302
error message, 14, 224
error message, plotting, 223
error messages, multiple, 223
error, syntax, 14
errors, common, 15
eval, 116, 133, 304
Evaluate Calc Zone, 97
Evaluate Cell, 97
Evaluate Loop, 97
Evaluate M-book, 97
evaluation, 116

evaluation, suppressing, 54-55

exact arithmetic, 10—11

execute, 37

Exit MATLAB, 7

exp, 11, 22, 24, 302

exp (1) , 301

expand, 11, 304

expm, 22, 302

exponential decay, 156, 159
exponential function, 22
exponential function, matrix, 22
exponential growth, 156, 159, 211
Export..., 215
expression, 54

External Interfaces/API, 119

external program, 118

eye, 59, 172, 192, 305

ezcontour, 70, 310

ezcontourf, 137—138

ezmesh, 74-75, 310

ezplot, 26, 30, 49, 67-68, 70, 112, 310

ezplot3, 72, 310

ezsurf , 75, 226, 256,310

factor, 4, 11, 52, 237, 305
factorial, 41
factorial, 254
Feigenbaum parameter, 165

Index

321

feval, 116, 305
field, vector, 71
figure, 77

figure, 78, 98, 310
Figure Properties..., 80

figure window, 6

File, xvii, 7, 28, 33, 35-36, 43, 84, 93-94,
120-121, 124, 128, 130, 215
file extension, 34, 37, 124, 127, 130, 307
fill, 83-84, 257, 279,310
find, 305, 314
Find, 92

f indob j, 83, 133, 311
finite difference scheme, 185, 188, 194
finite difference scheme, stability of,
188-189

finite element method, 191
floating point arithmetic, 10-11
floating point number, 51, 53
fminbnd, 305
follow-the-leader, 196
FontName, 82
football coach, 208
fopen, 120

for, 41, 45, 84, 101, 103, 109, 111,
278-279, 281, 314
for loop, 41, 45
forced oscillations, 125
format, 10, 36-38, 146-147, 177, 235, 305
Format, 199, 293
Format Object..., 97
FORTRAN, xiii, xvi, 118-119
fprintf, 120
fraction, 10
frequency, 85
friction, 181
front end, 91
function, 54
function, 39, 112, 314
function handle, 55, 116, 194, 301
function M-file, 31, 36, 39-40, 55, 57, 101,
112

function names, capitalized, 223
function, built-in, 24
function, user-defined, 25
Fundamental Theorem of Linear
Programming, 177

f zero, 17, 19-20, 49, 142, 210, 223, 240,
244, 285-287, 305

Gain, 123-124, 193, 200-201
Gain, matrix, 193
Galileo, 212
gamma, 24, 87, 254, 302
gamma function, 87, 302
gca, 81-82, 281, 311
gcbf, 133
gcbo, 133
gcf, 78, 82, 311
genotype, 89-90, 265
geometric series, 64, 146, 271
get, 82, 133, 311
getframe, 77, 202, 257, 311
ginput, 115-116, 311
gradient, 305
graph, 49
graph2d, 67
graph3d, 67, 137
Graphical User Interface, xvi
graphics, 26, 43, 67
graphics window, 7
graphics, customizing, 82
graphics, editing, 216
graphics, exporting, 215
graphics, M-book, 97
gravity, 212
Greek letters, 79
grid, 311
Group Cells, 96
gtext, 311

GUI, xvi, 127-128, 130-135
GUI, 127

GUIDE, 127-128, 130-133, 135
guide, 305
Gwynn, Tony, 206

half-life, 204

handle, 82, 133

HandleVisibility, 133

heat equation, 136, 184, 187

helix, 72

help, 4

help, 10, 15-16, 43, 57, 223, 305
Help, 5, 299

322

Index

Help Browser, 4-5, 15-16, 32, 78-79,
119, 299

Help Navigator, 15
help text, 52, 57
helpdesk, 4-5, 15, 78, 299
helpwin, 4, 16, 299
Hide Cell Markers, 96
hist, 151-155, 311
histogram, 151, 207
history, command, 15, 36
hold, 30, 44-46, 98, 116, 311
homogeneous equation, 121
HTML, 42, 215

i, 19, 24, 58, 301
if, 101-106, 109, 111, 113, 268,
279-280, 314
illumatons, 204
illumination, 136
imag, 302
image, 83

imaginary number, 59

implicit plot, 70

Import Data..., 120

Import Wizard, 120

improper integral, 62

increment, 21

increment, fractional, 21

increment, negative, 21

indefinite integral, 62

indeterminate form, 106

inf, 24, 63, 251-252, 254, 301

infinite loop, 110

inhomogeneous equation, 125

initial condition, 125, 185, 195, 200

initial value, 211, 293

inline, 19, 25-26, 39, 51, 53, 55, 305

inline function, 51, 55, 117

input, 8, 40, 57, 112-113

input, 47, 115, 132, 314

input, editing, 15

input-output model, 168-169

inputdlg, 132

Insert, 84

installation, 2

int, 52, 55, 62-63, 250, 305

integral, 86

integral, definite, 62
integral, improper, 62
integral, indefinite, 62
integral, multiple, 63
Integrator, 125, 192, 213, 293
intensity, 137, 139-140, 142-144
interest, 145, 149, 205
interface, 91
Internet, 214
Internet Explorer, 216
interrupting calculations, 5, 226
intersection point, 49
inv, 88, 259-260, 265, 305
isa, 314

ischar, 112, 314
isempty, 115, 315
isequal, 115
isfinite, 315
ishold, 116, 315
isinf, 315
isletter, 315
isnan, 315

isnumeric, 112, 114, 279, 315
ispc, 118, 280, 315
isreal, 315
isspace, 315
isunix, 118, 280, 315

j, 301

jacobian, 305
Java, 119
JPEG, 215

keyboard, 47, 99, 115, 117, 227, 315
Kirchhoff s Current Law, 209
Kirchhoff s Voltage Law, 209

label, 27

Launch Pad, 3, 31-33, 121, 299
Layout, 128, 130
Layout Editor, 128-135
lcm, 207, 280

least common multiple, 207
left, 306

left-arrow key, 15, 36
left-matrix-divide, 87
legend, 78, 98, 311

Index

323

lemniscate, 70, 78
length, 56, 107, 280, 306
Leontief, Wassily, 168
level curve, 69, 86
light fixtures, 137
limit, 63

limit, 63, 252, 306
limit point, 253
limit, one-sided, 63
line, 83
Line, 82
line style, 80
line, dashed, 80
line, dotted, 80
line, solid, 80
linear algebra, 20, 88
linear economic models, 136
linear exchange model, 168
linear programming, 136, 173, 207-208,
282-284
LineStyle, 83
linprog, 177

linspace, 187, 195, 279, 306
Linux, 1
Listbox, 130
In, 24

load, 34, 119, 306

local variable, 40, 57

location of MATLAB, 2

log, 11, 24, 48, 58, 63, 302

logical array, 104-105

logical expression, 104, 106

logistic growth, 159, 161

loglog, 210, 287, 311

lookfor, 15, 35, 306

loop, 41, 45, 84, 101, 109-111, 119, 278

loops, avoiding, 107-108, 155, 206

lowercase, 17, 223, 227

Is, 35, 306

M-book, 2, 41, 46, 91, 93, 99, 136, 215
m-book . dot, 92, 94

M-file, xv, 31, 36, 41, 43, 46, 55-57, 98-99,
101, 112, 115, 117, 127, 132, 135, 214,
219

M-file editor, 216

M-file, function, 31, 36, 39-40, 55, 57

M-file, script, 31, 36-38, 41, 45, 56-57

M-files, problems with, 218

Macintosh, 1

Macro, 93

Maple, 61-63, 306

maple, 254, 306

Math library, 124, 212

mating, 89

MATLAB, 119, 299

MATLAB Desktop, 3

MATLAB Java Interface, 119

MATLAB Manuals, 78

matrix, 23, 52, 59-60

matrix multiplication, 22

max, 224, 277-278, 306

mean, 186, 190, 306

median, 306

memory, managing, 155, 226
menu bar, 5, 33, 95, 130
Menu Editor, 130
menu items, xvii
mesh, 73, 224, 312

meshgrid, 69-70, 73, 139, 176, 246-247,
312

mex, 119, 315
MEXfile, 119

Microsoft Word, xiii, xvi, 2, 92-93, 215
millionaire, 205
min, 277, 306
minus sign, 100

mismatched delimiters, 220, 226

mistake, 218

mldivide, 60

mod, 279

Model, 121

model window, 121

Monte Carlo simulation, 136, 149, 206

monthly payment, 145-148

more, 15, 99, 306

mortgage, 136, 145, 149

Move Down, 35

Move Up, 35

movie, 87, 202-203, 311

movie, 77, 257, 312

movieview, 77

multiple curves, 30

multiple error messages, 218

324

Index

multiple integral, 63
multiplication, 9
Mux, 200

NaN, 106, 301
nargin, 112, 315
nargout, 112, 114, 315
Navigator, 217
Netscape Navigator, 216
New, 94, 121, 128
New Courier, 94
New M-book, 94
Newton, 212
nodesktop, 32
nonsingular, 88
notebook, 306
Notebook, 94, 96-98
Notebook Options, 97
notepad, 36

num2str, 53, 115, 269-270, 277, 307
numerical differentiation, 125
numerical integration, 62, 125
numerical ODE solver, 194, 211
numerical output, 11

ode45, 55, 181-183, 191, 203,
289-290, 307
Ohm’s Law, 209
ohms, 210
OK, 33

ones, 59, 76, 107, 193, 307
online help, 4, 15, 52, 136
open, 307

open Leontief model, 169
Open..., xvii, 93
openvar, 33
order of integration, 63
otherwise, 109, 315
Out of memory, 155
output, 8, 40, 57, 112, 114
Output options, 203
output, numeric, 11
output, suppressing, 24
output, symbolic, 11
output, unexpected, 218
output, wrong, 218
overloaded, 52

PaperSize, 82

parameter, 39, 67

parametric plot, 67

parametric surface, 73

parentheses, 104, 220, 221

parsing, 112

partial derivative, 61

partial differential equation, 184, 194

Paste, 32

patch, 230, 312

path, 34

path, 35, 307

Path Browser, 33, 35

pathtool, 35

pause, 42-43, 46^7, 99, 115, 315
PC, 1

PC File Viewer, 217
PDE Toolbox, 191
pdepe, 194
PDF, 78, 121
peaceful coexistence, 291
pendulum, 136, 180, 182, 212
percent sign, 38, 57
period, 22

periodic behavior, 164, 167
phase portrait, 181
pi, 11-12, 24, 301
pie, 172, 312
pitch, of sound, 85
platform, 1

plot, 28-30, 43, 45, 49, 53, 67, 72, 77, 80,
84, 181, 219, 230, 243, 312
plot style, 80
plot, 2-dimensional, 67
plot, 3-dimensional, 72
plot, contour, 69
plot, log-log, 311
plot, parametric, 67
plot, semilog, 312
plot3, 72, 80, 312
plotting several functions, 30
polar, 312

polar coordinates, 57, 74, 114
polynomial, 48
population dynamics, 136
population growth, 210
Popup Menu, 130-131, 133

Index

325

PostScript, 43
precedence, 220, 226
Preferences, 217
pretty, 45, 254, 307
pretty print, 45
previously saved M-file, 224
prima donnas, 208
prime numbers, 52
principal, 145
print, 43, 46, 312
Print..., 43

Printable Documentation, 78
printing, suppressing, 24
problems, 218
prod, 64, 107, 307
Product, 293

product, element-by-element, 59
Professional Version, 2, 225

Programs, 216

Properties, 32

Property Editor, 84

Property Inspector, 128-130, 132-134

punct, 301

Purge Output Cells, 96

Push Button, 129, 132
pwd, 35, 118, 307

quad, 62
quad8, 62, 307
quadl, 62, 251, 307
quit, 307
quit MATLAB, 7
quiver, 71, 312
quote marks, 221

Radio Button, 130, 134
Ramp, 200

rand, 150, 266, 268-270, 275, 307
range, 27
rank, 88

rank, 88, 259, 307
real, 302
rectangle, 83

relational operator, 104-106, 109
release, 2
relop, 104-105
Remove, 35

Rename, 33
Report Generator, 215
rescale, 313
reshape, 107
resistor, 209

return, 47, 107-108, 227, 315
RETURN, 4, 13, 15, 53, 306
RGB, 82, 230
right, 306

right-arrow key, 15, 36
roots, 307
Rotate 3D, 80
round, 307
round-off error, 301
row, 59

row vector, 22-23, 40, 52

saddle point, 71

saddle surface, 73

save, 34, 119, 307

Save as..., 124

Save As..., 130, 215

save states to Workspace, 193

scalar, 13, 23, 52

scale factor, 71

scale, axes, 80

Scope, 124-125, 166, 193, 200, 292
Scope window, rescaling axes, 125
script M-file, 31, 36-38, 40-41, 45,
56-57, 202
sec, 302
sech, 302
Security..., 93
Select All, 96

semicolon, 23-24, 29, 50-51, 77, 301

semi logy, 312

sensitivity coefficient, 197

sensitivity parameter, 200-201

sequence of evaluation, 225

set, 81-84, 133-134, 281, 312

Set Path..., 35

shift, 197

Shortcut, 33

Show Workspace, 33

sign, 103, 302

Signal dimensions, 199

Signals and Systems library, 200

326

Index

sim, 277, 308

simlp, 177-178, 180, 282-284
simple, 11, 49, 238, 308
Simple, 31

simplify, 11, 49, 238, 308
simulation, 136
Simulation, 125, 193, 203
Simulation Parameters..., 193, 203
SIMULINK, xiii, xvi, 32, 121, 136, 166,

177, 191-192, 199, 213
simulink, 121, 308
SIMULINK Library Browser, 121
SIMULINK library window, 121

sin, 4, 13, 24, 61, 302
sinh, 302

Sinks library, 124
size, 76, 107, 308
slash, 59
smiley face, 100
Solaris, 118

solve, 17-19, 49, 53, 58, 66, 236, 239, 245,
308

Solver, 193, 203
solving equations, 17
sound, 85
sound, 85, 308
Sources library, 126, 200
space bar, 15

space, in argument of command, 221
specgraph, 67
spelling error, 218, 223
sphere, 74

spherical coordinates, 74
spiral, 199
sqrt, 9, 24, 59, 302
square, 69-70
sscanf, 119
stability, 188, 197, 199
StarOffice, 217
Start, 92, 125, 216
starting MATLAB, 2
Static Text, 128
stock market, 205
str2num, 53, 119, 308
strcat, 308
strcmp, 109

string, 12, 17, 51-53, 55, 80, 220-221,

226, 301

structure array, 202, 300

Student Edition, xvii, 2

Student Version, xvii, 2, 225-226, 254, 306

style, line, 80

subfunction, 112, 194-195

submatrix, 59

submenu items, xvii

subplot, 76, 172, 312

subs, 56, 146, 149, 245, 285-287, 308

substitution, 56

subsystem, 199

subtraction, 9

Sum, 124, 166, 200, 213

sum, 64, 169, 276-277, 281, 308

sum, symbolic, 64

summation sign, 79

suppressing output, 50

surf, 73, 188, 193, 313

surface, 83

switch, 101, 108-109, 222, 315
sym, 12, 51, 53, 60, 238, 308
sym object, 13
symbol, 79
Symbol font, 82

symbolic expression, 11, 13, 26, 51—53, 55
Symbolic Math Toolbox, xvii, 2, 10-11, 48,
61, 86, 111, 204
symbolic output, 11
symbolic variable, 13
syms, 10, 12-13, 26, 51, 53, 308
symsum, 64, 111, 253-254, 309
syntax error, 218, 220, 222

tag, 132

tan, 24, 302

tanh, 302

Task Bar, 92

taylor, 65, 255, 309

Taylor polynomial, 65, 87

technical coefficients, 171

template, 92

T^X, 79

text, 78-79, 313

Text Properties..., 80
textedit, 36
TIFF, 216

time-lapse photo, 204

title, 27, 30, 43, 70, 78, 98, 172, 281, 313

Index

327

To Workspace, 202
Toggle Button, 129, 134

Toggle Graph Output for Cell, 98

tool bar, 5, 33, 36

toolboxes, 2, 16, 32, 35, 136, 177, 299
Tools, 80-81, 84, 93, 99-100, 128, 130,
131, 133

traffic flow, stability of, 197
traffic flow, vehicular, 136, 196
Transport Delay, 200
transpose, 22-23
transpose, 23, 309
Trigonometric Function, 212
type, 55, 101, 112, 118, 309
Type, 83

uicontextmenu, 127
uicontrol, 127
uiimport, 120
uimenu, 127
Undefine Cells, 96
underscore, 17, 70
Undo, 100
Ungroup Cells, 96
UNIX, 1, 118, 315
unix, 118-120, 280, 316
up-arrow key, 15, 36
uppercase, 17, 223, 227
user-defined function, 25
Using MATLAB, 119

valid name, 17

varargin, 112-114, 222, 279, 316
varargout, 112, 114, 222, 316
variable, 9-10, 17

variable precision arithmetic, 11-12

variable, symbolic, 10

variables, clearing, 219

variables, default, 65

vector, 18, 21, 25, 28, 59

vector field, 71

vector, column, 23, 52

vector, element of, 59

vector, row, 23, 52

vectorize, 25, 53, 62, 309

Verhulst, Pierre, 161

version, 1

vertical range, 27

vi, 36

view, 80, 313

View, 5-6, 14-15, 31-32

viewpoint, 80

Visual Basic, 92

voltage, 209

volts, 210

vpa, 12, 239, 309

warning, 114, 316
Web, 214

Web Server, MATLAB, 216
web site, for this book, 215
web site, The Math Works, 214
which, 219
while, 109-111, 316
whos, 13, 33, 51-52, 104, 309
Wide nonscalar lines, 199
Windows, 1, 118, 315
Windows 2000, 1
Windows 95, 1
Windows 98, 1
Windows Explorer, 93
Windows NT, 1
Word 2000, 92, 97
Word 97, 92
word processor, 92

working directory, 2, 33-36, 39, 43, 45
Workspace, 34, 52, 56-57, 119, 193, 226
workspace, 14, 33
Workspace, 33

Workspace browser, 3, 14, 31-34, 52, 99

Workspace I/O, 193

wrap, 45

wrong output, 218
XData, 82

xlabel, 27, 78, 98, 187, 189-190, 194, 313
XTick, 81, 281
XTickLabel, 81, 281
XY Graph, 213, 291, 293

YData, 82

ylabel, 27, 78, 98, 187, 189-190, 194, 281,
289, 313

zeros, 59, 157, 186, 190, 192, 276, 278, 309
zlabel, 78, 187, 189-190, 194
zoom, 313