M ATLAB Getting Started with MATLAB The Language of Technical Computing

M ATLAB Getting Started with MATLAB The Language of Technical Computing
MATLAB
®
The Language of Technical Computing
Getting Started with MATLAB®
Version 7
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Getting Started with MATLAB
© COPYRIGHT 1984 - 2005 by The MathWorks, Inc.
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Revision History
December 1996
May 1997
September 1998
September 2000
June 2001
July 2002
August 2002
June 2004
October 2004
March 2005
First printing
Second printing
Third printing
Fourth printing
Online only
Online only
Fifth printing
Sixth printing
Online only
Online only
For MATLAB 5
For MATLAB 5.1
For MATLAB 5.3
Revised for MATLAB 6 (Release 12)
Revised for MATLAB 6.1 (Release 12.1)
Revised for MATLAB 6.5 (Release 13)
Revised for MATLAB 6.5
Revised for MATLAB 7.0 (Release 14)
Revised for MATLAB 7.0.1 (Release 14SP1)
Revised for MATLAB 7.0.4 (Release 14SP2)
Contents
Introduction
1
What Is MATLAB? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2
The MATLAB System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3
MATLAB Documentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-4
MATLAB Online Help . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-4
Starting and Quitting MATLAB . . . . . . . . . . . . . . . . . . . . . . . .
Starting MATLAB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Quitting MATLAB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
MATLAB Desktop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-6
1-6
1-6
1-7
Matrices and Arrays
2
Matrices and Magic Squares . . . . . . . . . . . . . . . . . . . . . . . . . . .
Entering Matrices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
sum, transpose, and diag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Subscripts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The Colon Operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The magic Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-2
2-3
2-4
2-6
2-7
2-8
Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Examples of Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-10
2-10
2-10
2-11
2-11
2-13
Working with Matrices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-14
Generating Matrices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-14
The load Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-15
i
M-Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-15
Concatenation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-16
Deleting Rows and Columns . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-17
More About Matrices and Arrays . . . . . . . . . . . . . . . . . . . . . .
Linear Algebra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Multivariate Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Scalar Expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Logical Subscripting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The find Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-18
2-18
2-21
2-24
2-25
2-26
2-27
Controlling Command Window Input and Output . . . . . . .
The format Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Suppressing Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Entering Long Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Command Line Editing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-28
2-28
2-30
2-30
2-30
Graphics
3
Overview of MATLAB Plotting . . . . . . . . . . . . . . . . . . . . . . . . . 3-2
The Plotting Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2
Graph Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5
Figure Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7
Arranging Graphs Within a Figure . . . . . . . . . . . . . . . . . . . . . 3-13
Selecting Plot Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14
Editing Plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-16
Plot Editing Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-16
Using Functions to Edit Graphs . . . . . . . . . . . . . . . . . . . . . . . . 3-19
Examples — Using MATLAB Plotting Tools . . . . . . . . . . . . . 3-20
Modifying the Graph Data Source . . . . . . . . . . . . . . . . . . . . . . 3-27
Preparing Graphs for Presentation . . . . . . . . . . . . . . . . . . . . 3-29
Modify the Graph to Enhance the Presentation . . . . . . . . . . . 3-30
ii
Contents
Printing the Graph . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-33
Exporting the Graph . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-35
Basic Plotting Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Creating a Plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Multiple Data Sets in One Graph . . . . . . . . . . . . . . . . . . . . . . .
Specifying Line Styles and Colors . . . . . . . . . . . . . . . . . . . . . . .
Plotting Lines and Markers . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Imaginary and Complex Data . . . . . . . . . . . . . . . . . . . . . . . . . .
Adding Plots to an Existing Graph . . . . . . . . . . . . . . . . . . . . . .
Figure Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Multiple Plots in One Figure . . . . . . . . . . . . . . . . . . . . . . . . . . .
Controlling the Axes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Axis Labels and Titles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Saving Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-38
3-38
3-40
3-41
3-41
3-43
3-44
3-46
3-46
3-48
3-49
3-51
Mesh and Surface Plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-52
Visualizing Functions of Two Variables . . . . . . . . . . . . . . . . . . 3-52
Images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-58
Reading and Writing Images . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-59
Printing Graphics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-60
Handle Graphics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Using the Handle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Graphics Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Setting Object Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Specifying the Axes or Figure . . . . . . . . . . . . . . . . . . . . . . . . . .
Finding the Handles of Existing Objects . . . . . . . . . . . . . . . . .
3-62
3-62
3-63
3-65
3-68
3-69
Animations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-71
Erase Mode Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-71
Creating Movies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-73
iii
Programming
4
Flow Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
if, else, and elseif . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
switch and case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
for . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
while . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
continue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
break . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
try - catch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
return . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-2
4-2
4-4
4-5
4-5
4-6
4-7
4-7
4-8
Other Data Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-9
Multidimensional Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-9
Cell Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-11
Characters and Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-13
Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-16
Scripts and Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Scripts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Types of Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Global Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Passing String Arguments to Functions . . . . . . . . . . . . . . . . . .
The eval Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Function Handles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Function Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Vectorization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Preallocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-19
4-20
4-21
4-23
4-25
4-26
4-27
4-28
4-28
4-31
4-31
Creating Graphical User Interfaces
5
What Is GUIDE? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2
iv
Contents
Laying Out a GUI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3
Starting GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3
The Layout Editor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-4
Programming a GUI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-6
Desktop Tools and Development Environment
6
Desktop Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2
Arranging the Desktop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3
Start Button . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-4
Command Window and Command History . . . . . . . . . . . . . . . 6-5
Command Window . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-5
Command History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6
Help Browser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7
Current Directory Browser and Search Path . . . . . . . . . . . 6-10
Current Directory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-10
Search Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-11
Workspace Browser and Array Editor . . . . . . . . . . . . . . . . . . 6-12
Workspace Browser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-12
Array Editor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-13
Editor/Debugger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-14
M-Lint Code Check and Profiler Reports . . . . . . . . . . . . . . . 6-16
M-Lint Code Check Report . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-16
Profiler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-18
Other Development Environment Features . . . . . . . . . . . . . 6-19
v
Index
vi
Contents
1
Introduction
What Is MATLAB? (p. 1-2)
See how MATLAB® can provide solutions for you in technical
computing, what are some of the common applications of
MATLAB, and what types of add-on application-specific
solutions are available in MATLAB toolboxes.
MATLAB Documentation (p. 1-4)
Find out where to look for instruction on how to use each
component of MATLAB, and where to find help when you need
it.
Starting and Quitting MATLAB
(p. 1-6)
Start a new MATLAB session, use the desktop environment,
and terminate the session.
1
Introduction
What Is MATLAB?
MATLAB® is a high-performance language for technical computing. It
integrates computation, visualization, and programming in an easy-to-use
environment where problems and solutions are expressed in familiar
mathematical notation. Typical uses include
• Math and computation
• Algorithm development
• Data acquisition
• Modeling, simulation, and prototyping
• Data analysis, exploration, and visualization
• Scientific and engineering graphics
• Application development, including graphical user interface building
MATLAB is an interactive system whose basic data element is an array that
does not require dimensioning. This allows you to solve many technical
computing problems, especially those with matrix and vector formulations, in
a fraction of the time it would take to write a program in a scalar noninteractive
language such as C or Fortran.
The name MATLAB stands for matrix laboratory. MATLAB was originally
written to provide easy access to matrix software developed by the LINPACK
and EISPACK projects. Today, MATLAB engines incorporate the LAPACK
and BLAS libraries, embedding the state of the art in software for matrix
computation.
MATLAB has evolved over a period of years with input from many users. In
university environments, it is the standard instructional tool for introductory
and advanced courses in mathematics, engineering, and science. In industry,
MATLAB is the tool of choice for high-productivity research, development, and
analysis.
MATLAB features a family of add-on application-specific solutions called
toolboxes. Very important to most users of MATLAB, toolboxes allow you to
learn and apply specialized technology. Toolboxes are comprehensive
collections of MATLAB functions (M-files) that extend the MATLAB
environment to solve particular classes of problems. Areas in which toolboxes
are available include signal processing, control systems, neural networks,
fuzzy logic, wavelets, simulation, and many others.
1-2
What Is MATLAB?
The MATLAB System
The MATLAB system consists of five main parts:
Development Environment. This is the set of tools and facilities that help you use
MATLAB functions and files. Many of these tools are graphical user interfaces.
It includes the MATLAB desktop and Command Window, a command history,
an editor and debugger, and browsers for viewing help, the workspace, files,
and the search path.
The MATLAB Mathematical Function Library. This is a vast collection of computational
algorithms ranging from elementary functions, like sum, sine, cosine, and
complex arithmetic, to more sophisticated functions like matrix inverse, matrix
eigenvalues, Bessel functions, and fast Fourier transforms.
The MATLAB Language. This is a high-level matrix/array language with control
flow statements, functions, data structures, input/output, and object-oriented
programming features. It allows both “programming in the small” to rapidly
create quick and dirty throw-away programs, and “programming in the large”
to create large and complex application programs.
Graphics. MATLAB has extensive facilities for displaying vectors and matrices
as graphs, as well as annotating and printing these graphs. It includes
high-level functions for two-dimensional and three-dimensional data
visualization, image processing, animation, and presentation graphics. It also
includes low-level functions that allow you to fully customize the appearance of
graphics as well as to build complete graphical user interfaces on your
MATLAB applications.
The MATLAB External Interfaces/API. This is a library that allows you to write C and
Fortran programs that interact with MATLAB. It includes facilities for calling
routines from MATLAB (dynamic linking), calling MATLAB as a
computational engine, and for reading and writing MAT-files.
1-3
1
Introduction
MATLAB Documentation
MATLAB provides extensive documentation, in both printed and online
format, to help you learn about and use all of its features. If you are a new user,
start with this Getting Started book. It covers all the primary MATLAB
features at a high level, including many examples.
The MATLAB online help provides task-oriented and reference information
about MATLAB features. MATLAB documentation is also available in printed
form and in PDF format.
MATLAB Online Help
To view the online documentation, select MATLAB Help from the Help menu
in MATLAB. The MATLAB documentation is organized into these main topics:
• Desktop Tools and Development Environment — Startup and shutdown, the
desktop, and other tools that help you use MATLAB
• Mathematics — Mathematical operations and data analysis
• Programming — The MATLAB language and how to develop MATLAB
applications
• Graphics — Tools and techniques for plotting, graph annotation, printing,
and programming with Handle Graphics®
• 3-D Visualization — Visualizing surface and volume data, transparency, and
viewing and lighting techniques
• Creating Graphical User Interfaces — GUI-building tools and how to write
callback functions
• External Interfaces/API — MEX-files, the MATLAB engine, and interfacing
to Java, COM, and the serial port
MATLAB also includes reference documentation for all MATLAB functions:
• Functions - By Category — Lists all MATLAB functions grouped into
categories
• Handle Graphics Property Browser — Provides easy access to descriptions of
graphics object properties
• External Interfaces/API Reference — Covers those functions used by the
MATLAB external interfaces, providing information on syntax in the calling
language, description, arguments, return values, and examples
1-4
MATLAB Documentation
The MATLAB online documentation also includes
• Examples — An index of examples included in the documentation
• Release Notes — New features and known problems in the current release
• Printable Documentation — PDF versions of the documentation suitable for
printing
For more information about using the Help browser, see Chapter 6, “Desktop
Tools and Development Environment.”
1-5
1
Introduction
Starting and Quitting MATLAB
Starting MATLAB
On Windows platforms, start MATLAB by double-clicking the MATLAB
shortcut icon
on your Windows desktop.
On UNIX platforms, start MATLAB by typing matlab at the operating system
prompt.
You can customize MATLAB startup. For example, you can change the
directory in which MATLAB starts or automatically execute MATLAB
statements in a script file named startup.m.
For More Information See “Starting MATLAB” in the Desktop Tools and
Development Environment documentation.
Quitting MATLAB
To end your MATLAB session, select File -> Exit MATLAB in the desktop, or
type quit in the Command Window. You can run a script file named finish.m
each time MATLAB quits that, for example, executes functions to save the
workspace, or displays a quit confirmation dialog box.
For More Information See “Quitting MATLAB” in the Desktop Tools and
Development Environment documentation.
1-6
Starting and Quitting MATLAB
MATLAB Desktop
When you start MATLAB, the MATLAB desktop appears, containing tools
(graphical user interfaces) for managing files, variables, and applications
associated with MATLAB.
The following illustration shows the default desktop. You can customize the
arrangement of tools and documents to suit your needs. For more information
about the desktop tools, see Chapter 6, “Desktop Tools and Development
Environment.”
Enter MATLAB functions at the Command Window prompt.
The Command History maintains a record of
the MATLAB functions you ran.
1-7
1
Introduction
1-8
2
Matrices and Arrays
Matrices and Magic Squares (p. 2-2)
Enter matrices, perform matrix operations, and access
matrix elements.
Expressions (p. 2-10)
Work with variables, numbers, operators, functions, and
expressions.
Working with Matrices (p. 2-14)
Generate matrices, load matrices, create matrices from
M-files and concatenation, and delete matrix rows and
columns.
More About Matrices and Arrays
(p. 2-18)
Use matrices for linear algebra, work with arrays,
multivariate data, scalar expansion, and logical
subscripting, and use the find function.
Controlling Command Window Input
and Output (p. 2-28)
Change output format, suppress output, enter long lines,
and edit at the command line.
2
Matrices and Arrays
Matrices and Magic Squares
In MATLAB, a matrix is a rectangular array of numbers. Special meaning is
sometimes attached to 1-by-1 matrices, which are scalars, and to matrices with
only one row or column, which are vectors. MATLAB has other ways of storing
both numeric and nonnumeric data, but in the beginning, it is usually best to
think of everything as a matrix. The operations in MATLAB are designed to be
as natural as possible. Where other programming languages work with
numbers one at a time, MATLAB allows you to work with entire matrices
quickly and easily. A good example matrix, used throughout this book, appears
in the Renaissance engraving Melencolia I by the German artist and amateur
mathematician Albrecht Dürer.
2-2
Matrices and Magic Squares
This image is filled with mathematical symbolism, and if you look carefully,
you will see a matrix in the upper right corner. This matrix is known as a magic
square and was believed by many in Dürer’s time to have genuinely magical
properties. It does turn out to have some fascinating characteristics worth
exploring.
Entering Matrices
The best way for you to get started with MATLAB is to learn how to handle
matrices. Start MATLAB and follow along with each example.
You can enter matrices into MATLAB in several different ways:
• Enter an explicit list of elements.
• Load matrices from external data files.
• Generate matrices using built-in functions.
• Create matrices with your own functions in M-files.
Start by entering Dürer’s matrix as a list of its elements. You only have to
follow a few basic conventions:
• Separate the elements of a row with blanks or commas.
• Use a semicolon, ; , to indicate the end of each row.
• Surround the entire list of elements with square brackets, [ ].
2-3
2
Matrices and Arrays
To enter Dürer’s matrix, simply type in the Command Window
A = [16 3 2 13; 5 10 11 8; 9 6 7 12; 4 15 14 1]
MATLAB displays the matrix you just entered:
A =
16
5
9
4
3
10
6
15
2
11
7
14
13
8
12
1
This matrix matches the numbers in the engraving. Once you have entered the
matrix, it is automatically remembered in the MATLAB workspace. You can
refer to it simply as A. Now that you have A in the workspace, take a look at
what makes it so interesting. Why is it magic?
sum, transpose, and diag
You are probably already aware that the special properties of a magic square
have to do with the various ways of summing its elements. If you take the sum
along any row or column, or along either of the two main diagonals, you will
always get the same number. Let us verify that using MATLAB. The first
statement to try is
sum(A)
MATLAB replies with
ans =
34
34
34
34
When you do not specify an output variable, MATLAB uses the variable ans,
short for answer, to store the results of a calculation. You have computed a row
vector containing the sums of the columns of A. Sure enough, each of the
columns has the same sum, the magic sum, 34.
How about the row sums? MATLAB has a preference for working with the
columns of a matrix, so the easiest way to get the row sums is to transpose the
matrix, compute the column sums of the transpose, and then transpose the
result. The transpose operation is denoted by an apostrophe or single quote, '.
It flips a matrix about its main diagonal and it turns a row vector into a column
vector.
2-4
Matrices and Magic Squares
So
A'
produces
ans =
16
3
2
13
5
10
11
8
9
6
7
12
4
15
14
1
and
sum(A')'
produces a column vector containing the row sums
ans =
34
34
34
34
The sum of the elements on the main diagonal is obtained with the sum and the
diag functions:
diag(A)
produces
ans =
16
10
7
1
and
sum(diag(A))
produces
ans =
34
2-5
2
Matrices and Arrays
The other diagonal, the so-called antidiagonal, is not so important
mathematically, so MATLAB does not have a ready-made function for it. But a
function originally intended for use in graphics, fliplr, flips a matrix from left
to right:
sum(diag(fliplr(A)))
ans =
34
You have verified that the matrix in Dürer’s engraving is indeed a magic
square and, in the process, have sampled a few MATLAB matrix operations.
The following sections continue to use this matrix to illustrate additional
MATLAB capabilities.
Subscripts
The element in row i and column j of A is denoted by A(i,j). For example,
A(4,2) is the number in the fourth row and second column. For our magic
square, A(4,2) is 15. So to compute the sum of the elements in the fourth
column of A, type
A(1,4) + A(2,4) + A(3,4) + A(4,4)
This produces
ans =
34
but is not the most elegant way of summing a single column.
It is also possible to refer to the elements of a matrix with a single subscript,
A(k). This is the usual way of referencing row and column vectors. But it can
also apply to a fully two-dimensional matrix, in which case the array is
regarded as one long column vector formed from the columns of the original
matrix. So, for our magic square, A(8) is another way of referring to the value
15 stored in A(4,2).
If you try to use the value of an element outside of the matrix, it is an error:
t = A(4,5)
Index exceeds matrix dimensions.
2-6
Matrices and Magic Squares
On the other hand, if you store a value in an element outside of the matrix, the
size increases to accommodate the newcomer:
X = A;
X(4,5) = 17
X =
16
5
9
4
3
10
6
15
2
11
7
14
13
8
12
1
0
0
0
17
The Colon Operator
The colon, :, is one of the most important MATLAB operators. It occurs in
several different forms. The expression
1:10
is a row vector containing the integers from 1 to 10:
1
2
3
4
5
6
7
8
9
10
To obtain nonunit spacing, specify an increment. For example,
100:-7:50
is
100
93
86
79
72
65
58
51
and
0:pi/4:pi
is
0
0.7854
1.5708
2.3562
3.1416
Subscript expressions involving colons refer to portions of a matrix:
A(1:k,j)
is the first k elements of the jth column of A. So
sum(A(1:4,4))
2-7
2
Matrices and Arrays
computes the sum of the fourth column. But there is a better way. The colon by
itself refers to all the elements in a row or column of a matrix and the keyword
end refers to the last row or column. So
sum(A(:,end))
computes the sum of the elements in the last column of A:
ans =
34
Why is the magic sum for a 4-by-4 square equal to 34? If the integers from 1 to
16 are sorted into four groups with equal sums, that sum must be
sum(1:16)/4
which, of course, is
ans =
34
The magic Function
MATLAB actually has a built-in function that creates magic squares of almost
any size. Not surprisingly, this function is named magic:
B = magic(4)
B =
16
5
9
4
2
11
7
14
3
10
6
15
13
8
12
1
This matrix is almost the same as the one in the Dürer engraving and has all
the same “magic” properties; the only difference is that the two middle columns
are exchanged.
To make this B into Dürer’s A, swap the two middle columns:
A = B(:,[1 3 2 4])
2-8
Matrices and Magic Squares
This says, for each of the rows of matrix B, reorder the elements in the order 1,
3, 2, 4. It produces
A =
16
5
9
4
3
10
6
15
2
11
7
14
13
8
12
1
Why would Dürer go to the trouble of rearranging the columns when he could
have used MATLAB ordering? No doubt he wanted to include the date of the
engraving, 1514, at the bottom of his magic square.
2-9
2
Matrices and Arrays
Expressions
Like most other programming languages, MATLAB provides mathematical
expressions, but unlike most programming languages, these expressions
involve entire matrices. The building blocks of expressions are
• “Variables” on page 2-10
• “Numbers” on page 2-10
• “Operators” on page 2-11
• “Functions” on page 2-11
See also “Examples of Expressions” on page 2-13.
Variables
MATLAB does not require any type declarations or dimension statements.
When MATLAB encounters a new variable name, it automatically creates the
variable and allocates the appropriate amount of storage. If the variable
already exists, MATLAB changes its contents and, if necessary, allocates new
storage. For example,
num_students = 25
creates a 1-by-1 matrix named num_students and stores the value 25 in its
single element.
Variable names consist of a letter, followed by any number of letters, digits, or
underscores. MATLAB uses only the first 31 characters of a variable name.
MATLAB is case sensitive; it distinguishes between uppercase and lowercase
letters. A and a are not the same variable. To view the matrix assigned to any
variable, simply enter the variable name.
Numbers
MATLAB uses conventional decimal notation, with an optional decimal point
and leading plus or minus sign, for numbers. Scientific notation uses the letter
e to specify a power-of-ten scale factor. Imaginary numbers use either i or j as
a suffix. Some examples of legal numbers are
3
9.6397238
1i
2-10
-99
1.60210e-20
-3.14159j
0.0001
6.02252e23
3e5i
Expressions
All numbers are stored internally using the long format specified by the IEEE
floating-point standard. Floating-point numbers have a finite precision of
roughly 16 significant decimal digits and a finite range of roughly 10-308 to
10+308.
Operators
Expressions use familiar arithmetic operators and precedence rules.
+
Addition
-
Subtraction
*
Multiplication
/
Division
\
Left division (described in “Matrices and Linear
Algebra” in the MATLAB documentation)
^
Power
'
Complex conjugate transpose
( )
Specify evaluation order
Functions
MATLAB provides a large number of standard elementary mathematical
functions, including abs, sqrt, exp, and sin. Taking the square root or
logarithm of a negative number is not an error; the appropriate complex result
is produced automatically. MATLAB also provides many more advanced
mathematical functions, including Bessel and gamma functions. Most of these
functions accept complex arguments. For a list of the elementary mathematical
functions, type
help elfun
For a list of more advanced mathematical and matrix functions, type
help specfun
help elmat
2-11
2
Matrices and Arrays
Some of the functions, like sqrt and sin, are built in. Built-in functions are
part of the MATLAB core so they are very efficient, but the computational
details are not readily accessible. Other functions, like gamma and sinh, are
implemented in M-files.
There are some differences between built-in functions and other functions. For
example, for built-in functions, you cannot see the code. For other functions,
you can see the code and even modify it if you want.
Several special functions provide values of useful constants.
pi
3.14159265…
i
Imaginary unit,
j
Same as i
eps
Floating-point relative precision,
realmin
Smallest floating-point number, 2
realmax
Largest floating-point number, ( 2 – ε )2
Inf
Infinity
NaN
Not-a-number
–1
ε = 2 –52
– 1022
1023
Infinity is generated by dividing a nonzero value by zero, or by evaluating well
defined mathematical expressions that overflow, i.e., exceed realmax.
Not-a-number is generated by trying to evaluate expressions like 0/0 or
Inf-Inf that do not have well defined mathematical values.
The function names are not reserved. It is possible to overwrite any of them
with a new variable, such as
eps = 1.e-6
and then use that value in subsequent calculations. The original function can
be restored with
clear eps
2-12
Expressions
Examples of Expressions
You have already seen several examples of MATLAB expressions. Here are a
few more examples, and the resulting values:
rho = (1+sqrt(5))/2
rho =
1.6180
a = abs(3+4i)
a =
5
z = sqrt(besselk(4/3,rho-i))
z =
0.3730+ 0.3214i
huge = exp(log(realmax))
huge =
1.7977e+308
toobig = pi*huge
toobig =
Inf
2-13
2
Matrices and Arrays
Working with Matrices
This section introduces you to other ways of creating matrices:
• “Generating Matrices” on page 2-14
• “The load Function” on page 2-15
• “M-Files” on page 2-15
• “Concatenation” on page 2-16
• “Deleting Rows and Columns” on page 2-17
Generating Matrices
MATLAB provides four functions that generate basic matrices.
zeros
All zeros
ones
All ones
rand
Uniformly distributed random elements
randn
Normally distributed random elements
Here are some examples:
Z = zeros(2,4)
Z =
0
0
0
0
0
0
F = 5*ones(3,3)
F =
5
5
5
5
5
5
5
5
5
0
0
N = fix(10*rand(1,10))
N =
9
2
6
4
2-14
8
7
4
0
8
4
Working with Matrices
R = randn(4,4)
R =
0.6353
0.0860
-0.6014
-2.0046
0.5512
-0.4931
-1.0998
0.4620
-0.3210
1.2366
-0.6313
-2.3252
-1.2316
1.0556
-0.1132
0.3792
The load Function
The load function reads binary files containing matrices generated by earlier
MATLAB sessions, or reads text files containing numeric data. The text file
should be organized as a rectangular table of numbers, separated by blanks,
with one row per line, and an equal number of elements in each row. For
example, outside of MATLAB, create a text file containing these four lines:
16.0
5.0
9.0
4.0
3.0
10.0
6.0
15.0
2.0
11.0
7.0
14.0
13.0
8.0
12.0
1.0
Store the file under the name magik.dat. Then the statement
load magik.dat
reads the file and creates a variable, magik, containing our example matrix.
An easy way to read data into MATLAB in many text or binary formats is to
use Import Wizard.
M-Files
You can create your own matrices using M-files, which are text files containing
MATLAB code. Use the MATLAB Editor or another text editor to create a file
containing the same statements you would type at the MATLAB command
line. Save the file under a name that ends in .m.
For example, create a file containing these five lines:
A = [ ...
16.0
3.0
5.0
10.0
9.0
6.0
4.0
15.0
2.0
11.0
7.0
14.0
13.0
8.0
12.0
1.0 ];
2-15
2
Matrices and Arrays
Store the file under the name magik.m. Then the statement
magik
reads the file and creates a variable, A, containing our example matrix.
Concatenation
Concatenation is the process of joining small matrices to make bigger ones. In
fact, you made your first matrix by concatenating its individual elements. The
pair of square brackets, [], is the concatenation operator. For an example, start
with the 4-by-4 magic square, A, and form
B = [A
A+32; A+48
A+16]
The result is an 8-by-8 matrix, obtained by joining the four submatrices:
B =
16
5
9
4
64
53
57
52
3
10
6
15
51
58
54
63
2
11
7
14
50
59
55
62
13
8
12
1
61
56
60
49
48
37
41
36
32
21
25
20
35
42
38
47
19
26
22
31
34
43
39
46
18
27
23
30
45
40
44
33
29
24
28
17
This matrix is halfway to being another magic square. Its elements are a
rearrangement of the integers 1:64. Its column sums are the correct value for
an 8-by-8 magic square:
sum(B)
ans =
260
260
260
260
260
260
260
260
But its row sums, sum(B')', are not all the same. Further manipulation is
necessary to make this a valid 8-by-8 magic square.
2-16
Working with Matrices
Deleting Rows and Columns
You can delete rows and columns from a matrix using just a pair of square
brackets. Start with
X = A;
Then, to delete the second column of X, use
X(:,2) = []
This changes X to
X =
16
5
9
4
2
11
7
14
13
8
12
1
If you delete a single element from a matrix, the result is not a matrix anymore.
So, expressions like
X(1,2) = []
result in an error. However, using a single subscript deletes a single element,
or sequence of elements, and reshapes the remaining elements into a row
vector. So
X(2:2:10) = []
results in
X =
16
9
2
7
13
12
1
2-17
2
Matrices and Arrays
More About Matrices and Arrays
This section shows you more about working with matrices and arrays, focusing
on
• “Linear Algebra” on page 2-18
• “Arrays” on page 2-21
• “Multivariate Data” on page 2-24
• “Scalar Expansion” on page 2-25
• “Logical Subscripting” on page 2-26
• “The find Function” on page 2-27
Linear Algebra
Informally, the terms matrix and array are often used interchangeably. More
precisely, a matrix is a two-dimensional numeric array that represents a linear
transformation. The mathematical operations defined on matrices are the
subject of linear algebra.
Dürer’s magic square
A = [16
5
9
4
3
10
6
15
2
11
7
14
13
8
12
1 ]
provides several examples that give a taste of MATLAB matrix operations. You
have already seen the matrix transpose, A'. Adding a matrix to its transpose
produces a symmetric matrix:
A + A'
ans =
32
8
11
17
8
20
17
23
11
17
14
26
17
23
26
2
The multiplication symbol, *, denotes the matrix multiplication involving inner
products between rows and columns. Multiplying the transpose of a matrix by
the original matrix also produces a symmetric matrix:
2-18
More About Matrices and Arrays
A'*A
ans =
378
212
206
360
212
370
368
206
206
368
370
212
360
206
212
378
The determinant of this particular matrix happens to be zero, indicating that
the matrix is singular:
d = det(A)
d =
0
The reduced row echelon form of A is not the identity:
R = rref(A)
R =
1
0
0
0
0
1
0
0
0
0
1
0
1
-3
3
0
Since the matrix is singular, it does not have an inverse. If you try to compute
the inverse with
X = inv(A)
you will get a warning message:
Warning: Matrix is close to singular or badly scaled.
Results may be inaccurate. RCOND = 9.796086e-018.
Roundoff error has prevented the matrix inversion algorithm from detecting
exact singularity. But the value of rcond, which stands for reciprocal condition
estimate, is on the order of eps, the floating-point relative precision, so the
computed inverse is unlikely to be of much use.
2-19
2
Matrices and Arrays
The eigenvalues of the magic square are interesting:
e = eig(A)
e =
34.0000
8.0000
0.0000
-8.0000
One of the eigenvalues is zero, which is another consequence of singularity.
The largest eigenvalue is 34, the magic sum. That is because the vector of all
ones is an eigenvector:
v = ones(4,1)
v =
1
1
1
1
A*v
ans =
34
34
34
34
When a magic square is scaled by its magic sum,
P = A/34
the result is a doubly stochastic matrix whose row and column sums are all 1:
P =
0.4706
0.1471
0.2647
0.1176
2-20
0.0882
0.2941
0.1765
0.4412
0.0588
0.3235
0.2059
0.4118
0.3824
0.2353
0.3529
0.0294
More About Matrices and Arrays
Such matrices represent the transition probabilities in a Markov process.
Repeated powers of the matrix represent repeated steps of the process. For our
example, the fifth power
P^5
is
0.2507
0.2497
0.2500
0.2496
0.2495
0.2501
0.2498
0.2506
0.2494
0.2502
0.2499
0.2505
0.2504
0.2500
0.2503
0.2493
This shows that as k approaches infinity, all the elements in the k th power,
k
p , approach 1 ⁄ 4 .
Finally, the coefficients in the characteristic polynomial
poly(A)
are
1
-34
-64
2176
0
This indicates that the characteristic polynomial
det(A – λI )
is
4
3
2
λ – 34λ – 64λ + 2176λ
The constant term is zero, because the matrix is singular, and the coefficient of
the cubic term is -34, because the matrix is magic!
Arrays
When they are taken away from the world of linear algebra, matrices become
two-dimensional numeric arrays. Arithmetic operations on arrays are done
element by element. This means that addition and subtraction are the same for
arrays and matrices, but that multiplicative operations are different. MATLAB
uses a dot, or decimal point, as part of the notation for multiplicative array
operations.
2-21
2
Matrices and Arrays
The list of operators includes
+
Addition
-
Subtraction
.*
Element-by-element multiplication
./
Element-by-element division
.\
Element-by-element left division
.^
Element-by-element power
.'
Unconjugated array transpose
If the Dürer magic square is multiplied by itself with array multiplication
A.*A
the result is an array containing the squares of the integers from 1 to 16, in an
unusual order:
ans =
256
25
81
16
9
100
36
225
4
121
49
196
169
64
144
1
Building Tables
Array operations are useful for building tables. Suppose n is the column vector
n = (0:9)';
Then
pows = [n
2-22
n.^2
2.^n]
More About Matrices and Arrays
builds a table of squares and powers of 2:
pows =
0
1
2
3
4
5
6
7
8
9
0
1
4
9
16
25
36
49
64
81
1
2
4
8
16
32
64
128
256
512
The elementary math functions operate on arrays element by element. So
format short g
x = (1:0.1:2)';
logs = [x log10(x)]
builds a table of logarithms.
logs =
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
0
0.04139
0.07918
0.11394
0.14613
0.17609
0.20412
0.23045
0.25527
0.27875
0.30103
2-23
2
Matrices and Arrays
Multivariate Data
MATLAB uses column-oriented analysis for multivariate statistical data. Each
column in a data set represents a variable and each row an observation. The
(i,j)th element is the ith observation of the jth variable.
As an example, consider a data set with three variables:
• Heart rate
• Weight
• Hours of exercise per week
For five observations, the resulting array might look like
D = [ 72
81
69
82
75
134
201
156
148
170
3.2
3.5
7.1
2.4
1.2 ]
The first row contains the heart rate, weight, and exercise hours for patient 1,
the second row contains the data for patient 2, and so on. Now you can apply
many MATLAB data analysis functions to this data set. For example, to obtain
the mean and standard deviation of each column, use
mu = mean(D), sigma = std(D)
mu =
75.8
161.8
3.48
sigma =
5.6303
25.499
2.2107
For a list of the data analysis functions available in MATLAB, type
help datafun
If you have access to the Statistics Toolbox, type
help stats
2-24
More About Matrices and Arrays
Scalar Expansion
Matrices and scalars can be combined in several different ways. For example,
a scalar is subtracted from a matrix by subtracting it from each element. The
average value of the elements in our magic square is 8.5, so
B = A - 8.5
forms a matrix whose column sums are zero:
B =
7.5
-3.5
0.5
-4.5
-5.5
1.5
-2.5
6.5
-6.5
2.5
-1.5
5.5
4.5
-0.5
3.5
-7.5
sum(B)
ans =
0
0
0
0
With scalar expansion, MATLAB assigns a specified scalar to all indices in a
range. For example,
B(1:2,2:3) = 0
zeroes out a portion of B:
B =
7.5
-3.5
0.5
-4.5
0
0
-2.5
6.5
0
0
-1.5
5.5
4.5
-0.5
3.5
-7.5
2-25
2
Matrices and Arrays
Logical Subscripting
The logical vectors created from logical and relational operations can be used
to reference subarrays. Suppose X is an ordinary matrix and L is a matrix of the
same size that is the result of some logical operation. Then X(L) specifies the
elements of X where the elements of L are nonzero.
This kind of subscripting can be done in one step by specifying the logical
operation as the subscripting expression. Suppose you have the following set of
data:
x = [2.1 1.7 1.6 1.5 NaN 1.9 1.8 1.5 5.1 1.8 1.4 2.2 1.6 1.8];
The NaN is a marker for a missing observation, such as a failure to respond to
an item on a questionnaire. To remove the missing data with logical indexing,
use isfinite(x), which is true for all finite numerical values and false for NaN
and Inf:
x = x(isfinite(x))
x =
2.1 1.7 1.6 1.5 1.9 1.8 1.5 5.1 1.8 1.4 2.2 1.6 1.8
Now there is one observation, 5.1, which seems to be very different from the
others. It is an outlier. The following statement removes outliers, in this case
those elements more than three standard deviations from the mean:
x = x(abs(x-mean(x)) <= 3*std(x))
x =
2.1 1.7 1.6 1.5 1.9 1.8 1.5 1.8 1.4 2.2 1.6 1.8
For another example, highlight the location of the prime numbers in Dürer’s
magic square by using logical indexing and scalar expansion to set the
nonprimes to 0. (See “The magic Function” on page 2-8.)
A(~isprime(A)) = 0
A =
0
5
0
0
2-26
3
0
0
0
2
11
7
0
13
0
0
0
More About Matrices and Arrays
The find Function
The find function determines the indices of array elements that meet a given
logical condition. In its simplest form, find returns a column vector of indices.
Transpose that vector to obtain a row vector of indices. For example, start
again with Dürer’s magic square. (See “The magic Function” on page 2-8.)
k = find(isprime(A))'
picks out the locations, using one-dimensional indexing, of the primes in the
magic square:
k =
2
5
9
10
11
13
Display those primes, as a row vector in the order determined by k, with
A(k)
ans =
5
3
2
11
7
13
When you use k as a left-hand-side index in an assignment statement, the
matrix structure is preserved:
A(k) = NaN
A =
16
NaN
9
4
NaN
10
6
15
NaN
NaN
NaN
14
NaN
8
12
1
2-27
2
Matrices and Arrays
Controlling Command Window Input and Output
So far, you have been using the MATLAB command line, typing functions and
expressions, and seeing the results printed in the Command Window. This
section describes
• “The format Function” on page 2-28
• “Suppressing Output” on page 2-30
• “Entering Long Statements” on page 2-30
• “Command Line Editing” on page 2-30
The format Function
The format function controls the numeric format of the values displayed by
MATLAB. The function affects only how numbers are displayed, not how
MATLAB computes or saves them. Here are the different formats, together
with the resulting output produced from a vector x with components of
different magnitudes.
Note To ensure proper spacing, use a fixed-width font, such as Courier.
x = [4/3 1.2345e-6]
format short
1.3333
0.0000
format short e
1.3333e+000
1.2345e-006
format short g
1.3333
2-28
1.2345e-006
Controlling Command Window Input and Output
format long
1.33333333333333
0.00000123450000
format long e
1.333333333333333e+000
1.234500000000000e-006
format long g
1.33333333333333
1.2345e-006
format bank
1.33
0.00
format rat
4/3
1/810045
format hex
3ff5555555555555
3eb4b6231abfd271
If the largest element of a matrix is larger than 103 or smaller than 10-3,
MATLAB applies a common scale factor for the short and long formats.
In addition to the format functions shown above
format compact
suppresses many of the blank lines that appear in the output. This lets you
view more information on a screen or window. If you want more control over
the output format, use the sprintf and fprintf functions.
2-29
2
Matrices and Arrays
Suppressing Output
If you simply type a statement and press Return or Enter, MATLAB
automatically displays the results on screen. However, if you end the line with
a semicolon, MATLAB performs the computation but does not display any
output. This is particularly useful when you generate large matrices. For
example,
A = magic(100);
Entering Long Statements
If a statement does not fit on one line, use an ellipsis (three periods), ...,
followed by Return or Enter to indicate that the statement continues on the
next line. For example,
s = 1 -1/2 + 1/3 -1/4 + 1/5 - 1/6 + 1/7 ...
- 1/8 + 1/9 - 1/10 + 1/11 - 1/12;
Blank spaces around the =, +, and - signs are optional, but they improve
readability.
Command Line Editing
Various arrow and control keys on your keyboard allow you to recall, edit, and
reuse statements you have typed earlier. For example, suppose you mistakenly
enter
rho = (1 + sqt(5))/2
You have misspelled sqrt. MATLAB responds with
Undefined function or variable 'sqt'.
Instead of retyping the entire line, simply press the ↑ key. The statement you
typed is redisplayed. Use the ← key to move the cursor over and insert the
missing r. Repeated use of the ↑ key recalls earlier lines. Typing a few
characters and then the ↑ key finds a previous line that begins with those
characters. You can also copy previously executed statements from the
Command History. For more information, see “Command History” on page 6-6.
Following is the list of arrow and control keys you can use in the Command
Window. If the preference you select for Command line key bindings is
Emacs (MATLAB standard), you can also use the Ctrl+key combinations
2-30
Controlling Command Window Input and Output
shown. See also general keyboard shortcuts for desktop tools in the MATLAB
Desktop Tools and Development Environment documentation.
Key
Control Key for Emacs
(MATLAB standard)
Preference
Operation
Ctrl+P
Recall previous line. Works only at command line.
Ctrl+N
Recall next line. Works only at command line if
you previously used the up arrow or Ctrl+P.
Ctrl+B
Move back one character.
Ctrl+F
Move forward one character.
Ctrl+
Move right one word.
Ctrl+
Move left one word.
Home
Ctrl+A
Move to beginning of command line.
End
Ctrl+E
Move to end of command line.
Ctrl+Home
Move to top of Command Window.
Ctrl+End
Move to end of Command Window.
Esc
Ctrl+U
Clear command line.
Delete
Ctrl+D
Delete character at cursor in command line.
Backspace
Ctrl+H
Delete character before cursor in command line.
Ctrl+K
Cut contents (kill) to end of command line.
Shift+Home
Highlight to beginning of command line.
Shift+End
Highlight to end of last line. Can start at any line
in the Command Window.
2-31
2
Matrices and Arrays
2-32
3
Graphics
Overview of MATLAB Plotting (p. 3-2)
Create plots, include multiple data sets, specify property
values, and save figures.
Editing Plots (p. 3-16)
Edit plots interactively and using functions, and use the
property editor.
Mesh and Surface Plots (p. 3-52)
Visualize functions of two variables.
Images (p. 3-58)
Work with images.
Printing Graphics (p. 3-60)
Print and export figures.
Handle Graphics (p. 3-62)
Work with graphics objects and set object properties.
Animations (p. 3-71)
Create moving graphics.
3
Graphics
Overview of MATLAB Plotting
MATLAB provides a wide variety of techniques to display data graphically.
Interactive tools enable you to manipulate graphs to achieve results that reveal
the most information about your data. You can also annotate and print graphs
for presentations, or export graphs to standard graphics formats for
presentation in web browsers or other media.
For More Information “Graphics” and “3-D Visualization” in the MATLAB
documentation provide in-depth coverage of MATLAB graphics and
visualization tools. Access these topics from the Help browser.
The Plotting Process
The process of visualizing data typically involves a series of operations. This
section provides a “big picture” view of the plotting process and contains links
to sections that have examples and specific details about performing each
operation.
Creating a Graph
The type of graph you choose to create depends on the nature of your data and
what you want to reveal about the data. MATLAB predefines many graph
types, such as line, bar, histogram, and pie graphs. There are also 3-D graphs,
such as surfaces, slice planes, and streamlines.
There are two basic ways to create graphs in MATLAB:
• Use plotting tools to create graphs interactively.
See “Examples — Using MATLAB Plotting Tools” on page 3-20.
• Use the command interface to enter commands in the Command Window or
create plotting programs.
See “Basic Plotting Functions” on page 3-38.
You might find it useful to combine both approaches. For example, you might
issue a plotting command to create a graph and then modify the graph using
one of the interactive tools.
3-2
Overview of MATLAB Plotting
Exploring Data
Once you create a graph, you can extract specific information about the data,
such as the numeric value of a peak in a plot, the average value of a series of
data, or you can perform data fitting.
For More Information See “Data Exploration Tools” and “The Basic Fitting
Interface” in the MATLAB documentation.
Editing the Graph Components
Graphs are composed of objects, which have properties you can change. These
properties affect the way the various graph components look and behave.
For example, the axes used to define the coordinate system of the graph has
properties that define the limits of each axis, the scale, color, etc. The line used
to create a line graph has properties such as color, type of marker used at each
data point (if any), line style, etc.
Note that the data used to create a line graph are properties of the line. You
can, therefore, change the data without actually creating a new graph.
See “Editing Plots” on page 3-16.
Annotating Graphs
Annotations are the text, arrows, callouts, and other labels added to graphs to
help viewers see what is important about the data. You typically add
annotations to graphs when you want to show them to other people or when you
want to save them for later reference.
For More Information See “Annotating Graphs” in the MATLAB
documentation or select Annotating Graphs from the figure Help menu.
3-3
3
Graphics
Printing and Exporting Graphs
You can print your graph on any printer connected to your computer. The print
previewer enables you to view how your graph will look when printed. It
enables you to add headers, footers, a date, and so on. The page setup dialog
lets you control the size, layout, and other characteristics of the graph (select
Page Setup from the figure File menu).
Exporting a graph means creating a copy of it in a standard graphics file
format, such as TIF, JPEG, or EPS. You can then import the file into a word
processor, include it in an HTML document, or edit it in a drawing package
select Export Setup from the figure File menu).
For More Information See the print command reference page and
“Printing and Exporting” in the MATLAB documentation or select Printing
and Exporting from the figure Help menu.
Saving Graphs to Reload into MATLAB
There are two ways to save graphs that enable you to save the work you have
invested in their preparation:
• Save the graph as a FIG-file (select Save from the figure File menu).
• Generate MATLAB code that can recreate the graph (select Generate
M-File from the figure File menu).
FIG-Files. FIG-files are a binary format that saves a figure in its current state.
This means that all graphics objects and property settings are stored in the file
when you create it. You can reload the file into a different MATLAB session,
even if you are running MATLAB on a different type of computer. When you
load a FIG-file, MATLAB creates a new figure in the same state as the one you
saved.
Note that the states of any figure tools (i.e., any items on the toolbars) are not
saved in a FIG-file; only the contents of the graph are saved.
3-4
Overview of MATLAB Plotting
Generated Code. You can use the MATLAB M-code generator to create code that
recreates the graph. Unlike a FIG-file, the generated code does not contain any
data. You must pass the data to the generated function when you run the code.
Note that studying the generating code for a graph is a good way to learn how
to program with MATLAB.
For More Information See the print command reference page and “Saving
Your Work” in the MATLAB documentation.
Graph Components
MATLAB displays graphs in a special window known as a figure. To create a
graph, you need to define a coordinate system. Therefore every graph is placed
within axes, which are contained by the figure.
The actual visual representation of the data is achieved with graphics objects
like lines and surfaces. These objects are drawn within the coordinate system
defined by the axes, which MATLAB automatically creates specifically to
accommodate the range of the data. The actual data is stored as properties of
the graphics objects.
See “Handle Graphics” on page 3-62 for more information about graphics object
properties.
The following picture shows the basic components of a typical graph.
3-5
3
Graphics
Figure window displays
graphs.
y = 1.5cos(x) + 4e−0.01xcos(x) + e0.07xsin(3x)
20
15
10
Axes define a coordinate
system for the graph.
Line plot represents
data.
Y Axis
5
0
−5
−10
−15
−20
−10
3-6
−5
0
5
10
15
X Axis
20
25
30
35
40
Overview of MATLAB Plotting
Figure Tools
The figure is equipped with sets of tools that operate on graphs. The figure
Tools menu provides access to many graph tools.
For More Information See “MATLAB Plotting Tools” in the MATLAB
documentation or select Plotting Tools from the figure Help menu.
3-7
3
Graphics
Access to Tools
You can access the figure toolbars and the plotting tools from the View menu,
as shown in the following picture.
Enable the figure toolbars and the plotting tools from the View menu
Figure Toolbars
Figure toolbars provide easy access to many graph modification features. There
are three toolbars. When you place the cursor over a particular tool, a text box
pops up with the tool name. The following picture shows the three toolbars
displayed with the cursor over the Data Cursor tool.
For More Information See “Figure Toolbars” in the MATLAB
documentation.
3-8
Overview of MATLAB Plotting
Plotting Tools
Plotting tools are attached to figures and create an environment for creating
graphs. These tools enable you to do the following:
• Select from a wide variety of graph types
• Set the properties of graphics objects
• Annotate graphs with text, arrows, etc.
• Create and arrange subplots in the figure
• Drag and drop data into graphs
Display the plotting tools from the View menu or by clicking in the figure
toolbar, as shown in the following picture.
Enable plotting tools from the View menu or toolbar.
There are three components to the plotting tools:
• Figure Palette — Specify and arrange subplots, access workspace variables
for plotting or editing, and add annotations.
• Plot Browser — Select objects in the graphics hierarchy, control visibility,
and add data to axes.
• Property Editor — Change key properties of the selected object. Click
Inspector for access to all object properties.
3-9
3
Graphics
The following picture shows a figure with the plotting tools enabled.
3-10
Overview of MATLAB Plotting
Plotting Tools and MATLAB Commands
You can enable the plotting tools on any graph, even if you created it using
MATLAB commands. For example, suppose you create the following graph:
t = 0:pi/20:2*pi;
y = exp(sin(t));
plotyy(t,y,t,y,'plot','stem')
xlabel('X Axis')
ylabel('Plot Y Axis')
title('Two Y Axes')
This graph contains two y-axes, one for each plot type (lineseries and stem
graphs). The plotting tools make it easy to select any of the objects that the
graph contains and set their properties.
3-11
3
Graphics
For example, adding a label for the y-axis that corresponds to the stem plot is
easily accomplished by selecting that axes in the Plot Browser and setting the
YLabel property in the Property Editor.
3-12
Overview of MATLAB Plotting
Arranging Graphs Within a Figure
You can place a number of axes within a figure by selecting the layout you want
from the Figure Palette. For example, the following picture shows how to
specify four 2-D axes in the figure.
Click to add one axes to bottom of
current layout.
Click and drag right to specify axes layout.
Select the axes you want to target for plotting. You can also use the subplot
command to create multiple axes.
3-13
3
Graphics
Selecting Plot Types
You can use the Plot Catalog to select from a variety of techniques for plotting
data. To access the Plot Catalog,
1 Select the variables you want to plot in the Figure Palette.
2 Right-click to display the context menu.
3 Select More Plots to display the catalog.
3-14
Overview of MATLAB Plotting
MATLAB displays the Plot Catalog with the selected variables ready to plot,
once you select a plot type.
Specify variables to plot.
See a description of each plot type.
Select a category of graphs and then choose a specific type.
3-15
3
Graphics
Editing Plots
MATLAB automatically formats graphs by setting the scale of the axes, adding
tick marks along axes, and using colors and line styles to distinguish the data
plotted in the graph. However, if you are creating graphics for presentation,
you can change the default formatting or add descriptive labels, titles, legends
and other annotations to help explain your data.
Plot Editing Mode
Plot editing mode enables you to perform point-and-click editing of the graphics
objects in your graph.
Enabling Plot Edit Mode
To enable plot edit mode, click the arrowhead in the figure toolbar:
Enable plot edit mode.
You can also select Edit Plot from the figure Tools menu.
Setting Object Properties
Once you have enabled plot edit mode, you can select objects by clicking on
them in the graph. Selection handles appear and indicate that the object is
selected. Select multiple objects using Shift+click.
Right-click with the pointer over the selected object to display the object’s
context menu:
3-16
Editing Plots
The context menu provides quick access to the most commonly used operations
and properties.
Using the Property Editor
In plot edit mode, double-clicking on an object in a graph starts the Property
Editor with that object’s major properties displayed. The Property Editor
provides access to the most used object properties. It is updated to display the
properties of whatever object you select.
Click to display Property Inspector.
3-17
3
Graphics
Accessing All Properties — Property Inspector
The Property Inspector is a tool that enables you to access all object properties.
If you do not find the property you want to set in the Property Editor, click the
Inspector button to display the Property Inspector. You can also use the
inspect command to start the Property Inspector.
The following picture shows the Property Inspector displaying the properties of
a graph’s axes. It lists each property and provides a text field or other
appropriate device (such as a color picker) from which you can set the value of
the property.
As you select different objects, the Property Inspector is updated to display the
properties of the current object.
3-18
Editing Plots
Using Functions to Edit Graphs
If you prefer to work from the MATLAB command line, or if you are creating
an M-file, you can use MATLAB commands to edit the graphs you create. You
can use the set and get commands to change the properties of the objects in a
graph. For more information about using graphics commands, see “Handle
Graphics” on page 3-62.
3-19
3
Graphics
Examples — Using MATLAB Plotting Tools
Suppose you want to graph the function y = x3 over the x domain -1 to 1. The
first step is to generate the data to plot.
It is simple to evaluate a function because MATLAB can distribute arithmetic
operations over all elements of a multivalued variable.
For example, the following statement creates a variable x that contains values
ranging from -1 to 1 in increments of 0.1 (you could also use the linspace
function to generate data for x). The second statement raises each value in x to
the third power and stores these values in y:
x = -1:.1:1; % Define the range of x
y = x.^3;
% Raise each element in x to the third power
Now that you have generated some data, you can plot it using the MATLAB
plotting tools. To start the plotting tools, type
plottools
MATLAB displays a figure with plotting tools attached.
3-20
Examples — Using MATLAB Plotting Tools
Variables in workspace
Figure plotting area
3-21
3
Graphics
Plotting Two Variables
A simple line graph is a suitable way to display x as the independent variable
and y as the dependent variable. To do this, select both variables (click to
select, then Shift-click to select again), then right-click to display the context
menu.
Select plot(x,y) from the menu. MATLAB creates the line graph in the figure
area. The black squares indicate that the line is selected and you can edit its
properties with the Property Editor.
3-22
Examples — Using MATLAB Plotting Tools
Changing the Appearance
Next change the line properties so that the graph displays only the data point.
Use the Property Editor to set following properties:
• Line to no line
• Marker to o
• Marker size to 4.0
• Marker fill color to red
Set Line to no line.
Set Marker to o .
Set Marker size to 4.0.
Set Marker fill
color to red.
3-23
3
Graphics
Adding More Data to the Graph
You can add more data to the graph by defining more variables or by specifying
an expression that MATLAB uses to generate data for the plot. This second
approach makes it easy to explore variations of the data already plotted.
To add data to the graph, select the axes in the Plot Browser and click the Add
Data button. When you are using the plotting tools, MATLAB always adds
data to the existing graph, instead of replacing the graph, as it would if you
issued repeated plotting commands. That is, the plotting tools are in a hold on
state.
1. Select axes.
3. Enter expression
2. Click Add Data.
The picture above shows how to configure the Add Data to Axes dialog to create
a line plot of y = x4, which is added to the existing plot of y = x3.
3-24
Examples — Using MATLAB Plotting Tools
Changing the Type of Graph
The plotting tools enable you to easily view your data with a variety of plot
types. The following picture shows the same data as above converted to stem
plots. To change the plot type,
1 Select the plotted data in the Plot Browser.
2 Select Stem in the Plot Type menu.
3-25
3
Graphics
Select both sets of data.
Select Stem as the Plot
Type.
3-26
Examples — Using MATLAB Plotting Tools
Modifying the Graph Data Source
You can link graph data to variables in your workspace. When you change the
values contained in the variables, you can then update the graph to use the new
data without having to create a new graph. (See also the refresh function.)
Suppose you have the following data:
x = linspace(-pi,pi,25); % 25 points between -π and π
y = sin(x);
Using the plotting tools, create a graph of y = sin(x):
plottools
MATLAB sets the data source to the
selected variables.
3-27
3
Graphics
New Values for the Data Source
The data that defines the graph is linked to the x and y variables in the base
workspace. If you assign new values to these variables and click the Refresh
Data button, MATLAB updates the graph to use the new data:
x = linspace(-2*pi,2*pi,25); % 25 points between -2pi and 2pi
y = sin(x); % Recalculate y based on the new x values
Click Refresh Data to
update the plot.
3-28
Preparing Graphs for Presentation
Preparing Graphs for Presentation
Suppose you plot the following data and want to create a graph that presents
certain information about the data:
x = -10:.005:40;
y = [1.5*cos(x)+4*exp(-.01*x).*cos(x)+exp(.07*x).*sin(3*x)];
plot(x,y)
This picture shows the graph created by the code above.
Now suppose you want to save copies of the graph by
• Printing the graph on a local printer so you have a copy for your notebook
• Exporting the graph to an Encapsulated PostScript (EPS) file to incorporate
into a word processor document
3-29
3
Graphics
Modify the Graph to Enhance the Presentation
To obtain a better view, zoom in on the graph using horizontal zoom.
After enabling zoom mode from the figure toolbar, right-click to display the
context menu. Select Horizontal Zoom (2-D Plots Only) from the Zoom
Options.
Left-click to zoom in on a region of the graph and use the panning tool to
position the points of interest where you want them on the graph.
Label some key points using data tips.
3-30
Preparing Graphs for Presentation
Finally, add text annotations, axis labels, and a title. You can add the title and
axis labels using the following commands:
title ('y = 1.5cos(x) + 4e^{-0.01x}cos(x) + e^{0.07x}sin(3x)')
xlabel('X Axis')
ylabel('Y Axis')
Note that the text string passed to the title command uses TEX syntax to
produce the exponents. See the text String property for more information on
using TEX syntax to produce mathematical symbols.
The graph is now ready to print and export.
3-31
3
Graphics
3-32
Preparing Graphs for Presentation
Printing the Graph
Before printing the graph, use the print previewer to see how the graph will be
laid out on the page. Display the graph in the print previewer by selecting
Print Preview from the figure File menu.
• Click the Header button to add some descriptive text to the top of the page.
• Next click the Page Setup button. The Page Setup dialog box enables you
to set a number of properties that control how the page is printed.
Note that MATLAB recalculates the values of the axes tick marks because
the printed graph is larger than the one displayed on the computer screen.
To force MATLAB to use the same tick marks and limits, select Keep screen
limits and ticks from the Axes and Figure tab in the Page Setup dialog.
3-33
3
Graphics
• Click OK to accept the setting and dismiss the dialog.
• Click Print on the print previewer to send the graph to your default printer.
The Page Setup dialog provides many other options for controlling how printed
graphs look. Click Help for more information.
3-34
Preparing Graphs for Presentation
Exporting the Graph
Exporting a graph is the process of creating a standard graphics file format of
the graph (such as EPS or TIFF), which you can then import into other
applications like word processors, drawing packages, etc.
This example exports the graph as an EPS file with the following
requirements:
• The size of the picture when imported into the word processor document
should be four inches wide and three inches high.
• All the text in the figure should have a size of 8 points.
Specifying the Size of the Graph
To set the size, use the Export Setup dialog (select Export Setup from the
figure File menu).
Set the size of the graph in the exported file.
3-35
3
Graphics
Specifying the Font Size
To set the font size of all the text in the graph, select Fonts in the Export Setup
dialog’s Properties selector. Then select Use fixed font size and enter 8 in the
text box.
3-36
Preparing Graphs for Presentation
Selecting the File Format
Once you finish setting options for the exported graph, click the Export button.
MATLAB displays a Save As dialog that enables you to specify a name for the
file as well as select the type of file format you want to use.
For this example, select EPS, as shown in the following picture.
Select EPS from the
drop-down menu.
You can import the saved file into any application that supports EPS files. The
Save as type drop-down menu lists other options for file types.
You can also use the print command to print figures on your local printer or to
export graphs to standard file types.
For More Information See the print command reference page and
“Printing and Exporting” in the MATLAB documentation or select Printing
and Exporting from the figure Help menu.
3-37
3
Graphics
Basic Plotting Functions
This section describes important graphics functions and provides examples of
some typical applications. The plotting tools, described in previous sections,
make use of MATLAB plotting functions and use these functions to generate
code for graphs:
• “Creating a Plot” on page 3-38
• “Multiple Data Sets in One Graph” on page 3-40
• “Specifying Line Styles and Colors” on page 3-41
• “Plotting Lines and Markers” on page 3-41
• “Imaginary and Complex Data” on page 3-43
• “Adding Plots to an Existing Graph” on page 3-44
• “Figure Windows” on page 3-46
• “Multiple Plots in One Figure” on page 3-46
• “Controlling the Axes” on page 3-48
• “Axis Labels and Titles” on page 3-49
• “Saving Figures” on page 3-51
Creating a Plot
The plot function has different forms, depending on the input arguments. If y
is a vector, plot(y) produces a piecewise linear graph of the elements of y
versus the index of the elements of y. If you specify two vectors as arguments,
plot(x,y) produces a graph of y versus x.
For example, these statements use the colon operator to create a vector of x
values ranging from 0 to 2π, compute the sine of these values, and plot the
result:
x = 0:pi/100:2*pi;
y = sin(x);
plot(x,y)
3-38
Basic Plotting Functions
Now label the axes and add a title. The characters \pi create the symbol π. See
text strings for more symbols:
xlabel('x = 0:2\pi')
ylabel('Sine of x')
title('Plot of the Sine Function','FontSize',12)
Plot of the Sine Function
1
0.8
0.6
0.4
Sine of x
0.2
0
−0.2
−0.4
−0.6
−0.8
−1
0
1
2
3
4
5
6
7
x = 0:2π
3-39
3
Graphics
Multiple Data Sets in One Graph
Multiple x-y pair arguments create multiple graphs with a single call to plot.
MATLAB automatically cycles through a predefined (but user settable) list of
colors to allow discrimination among sets of data. See the axes ColorOrder and
LineStyleOrder properties.
For example, these statements plot three related functions of x, with each
curve in a separate distinguishing color:
x = 0:pi/100:2*pi;
y = sin(x);
y2 = sin(x-.25);
y3 = sin(x-.5);
plot(x,y,x,y2,x,y3)
The legend command provides an easy way to identify the individual plots:
legend('sin(x)','sin(x-.25)','sin(x-.5)')
1
sin(x)
sin(x−.25)
sin(x−.5)
0.8
0.6
0.4
0.2
0
−0.2
−0.4
−0.6
−0.8
−1
3-40
0
1
2
3
4
5
6
7
Basic Plotting Functions
For More Information See “Defining the Color of Lines for Plotting” in
“Axes Properties” in the MATLAB documentation.
Specifying Line Styles and Colors
It is possible to specify color, line styles, and markers (such as plus signs or
circles) when you plot your data using the plot command:
plot(x,y,'color_style_marker')
color_style_marker is a string containing from one to four characters
(enclosed in single quotation marks) constructed from a color, a line style, and
a marker type:
• Color strings are 'c', 'm', 'y', 'r', 'g', 'b', 'w', and 'k'. These correspond
to cyan, magenta, yellow, red, green, blue, white, and black.
• Line style strings are '-' for solid, '--' for dashed, ':' for dotted, '-.' for
dash-dot. Omit the line style for no line.
• The marker types are '+', 'o', '*', and 'x', and the filled marker types are
's' for square, 'd' for diamond, '^' for up triangle, 'v' for down triangle,
'>' for right triangle, '<' for left triangle, 'p' for pentagram, 'h' for
hexagram, and none for no marker.
You can also edit color, line style, and markers interactively. See “Editing
Plots” on page 3-16 for more information.
Plotting Lines and Markers
If you specify a marker type but not a line style, MATLAB draws only the
marker. For example,
plot(x,y,'ks')
plots black squares at each data point, but does not connect the markers with
a line.
3-41
3
Graphics
The statement
plot(x,y,'r:+')
plots a red dotted line and places plus sign markers at each data point.
Placing Markers at Every Tenth Data Point
You might want to use fewer data points to plot the markers than you use to
plot the lines. This example plots the data twice using a different number of
points for the dotted line and marker plots:
x1 = 0:pi/100:2*pi;
x2 = 0:pi/10:2*pi;
plot(x1,sin(x1),'r:',x2,sin(x2),'r+')
1
0.8
0.6
0.4
0.2
0
−0.2
−0.4
−0.6
−0.8
−1
0
3-42
1
2
3
4
5
6
7
Basic Plotting Functions
Imaginary and Complex Data
When the arguments to plot are complex, the imaginary part is ignored except
when you pass plot a single complex argument. For this special case, the
command is a shortcut for a graph of the real part versus the imaginary part.
Therefore,
plot(Z)
where Z is a complex vector or matrix, is equivalent to
plot(real(Z),imag(Z))
For example,
t = 0:pi/10:2*pi;
plot(exp(i*t),'-o')
axis equal
draws a 20-sided polygon with little circles at the vertices. The command
axis equal makes the individual tick-mark increments on the x- and y-axes
the same length, which makes this plot more circular in appearance.
1
0.8
0.6
0.4
0.2
0
−0.2
−0.4
−0.6
−0.8
−1
−1
−0.5
0
0.5
1
3-43
3
Graphics
Adding Plots to an Existing Graph
The hold command enables you to add plots to an existing graph. When you
type
hold on
MATLAB does not replace the existing graph when you issue another plotting
command; it adds the new data to the current graph, rescaling the axes if
necessary.
For example, these statements first create a contour plot of the peaks function,
then superimpose a pseudocolor plot of the same function:
[x,y,z] = peaks;
pcolor(x,y,z)
shading interp
hold on
contour(x,y,z,20,'k')
hold off
The hold on command causes the pcolor plot to be combined with the contour
plot in one figure.
3-44
Basic Plotting Functions
For More Information See “Creating Specialized Plots” in the MATLAB
documentation for information on a variety of graph types.
3-45
3
Graphics
Figure Windows
Graphing functions automatically open a new figure window if there are no
figure windows already on the screen. If a figure window exists, MATLAB uses
that window for graphics output. If there are multiple figure windows open,
MATLAB targets the one that is designated the “current figure” (the last figure
used or clicked in).
To make an existing figure window the current figure, you can click the mouse
while the pointer is in that window or you can type
figure(n)
where n is the number in the figure title bar. The results of subsequent
graphics commands are displayed in this window.
To open a new figure window and make it the current figure, type
figure
Clearing the Figure for a New Plot
When a figure already exists, most plotting commands clear the axes and use
this figure to create the new plot. However, these commands do not reset figure
properties, such as the background color or the colormap. If you have set any
figure properties in the previous plot, you might want to use the clf command
with the reset option,
clf reset
before creating your new plot to restore the figure’s properties to their defaults.
For More Information See “Figure Properties” and “ Graphics Window —
the Figure” in the MATLAB documentation for information on figures.
Multiple Plots in One Figure
The subplot command enables you to display multiple plots in the same
window or print them on the same piece of paper. Typing
subplot(m,n,p)
3-46
Basic Plotting Functions
partitions the figure window into an m-by-n matrix of small subplots and selects
the pth subplot for the current plot. The plots are numbered along first the top
row of the figure window, then the second row, and so on. For example, these
statements plot data in four different subregions of the figure window:
t = 0:pi/10:2*pi;
[X,Y,Z] = cylinder(4*cos(t));
subplot(2,2,1); mesh(X)
subplot(2,2,2); mesh(Y)
subplot(2,2,3); mesh(Z)
subplot(2,2,4); mesh(X,Y,Z)
5
5
0
0
−5
40
−5
40
30
20
0
30
20
20
10
20
0
0
1
1
0.5
0.5
0
40
10
0
0
5
30
20
20
0
10
0
5
0
0
−5
−5
3-47
3
Graphics
Controlling the Axes
The axis command provides a number of options for setting the scaling,
orientation, and aspect ratio of graphs. You can also set these options
interactively. See “Editing Plots” on page 3-16 for more information.
Setting Axis Limits
By default, MATLAB finds the maxima and minima of the data and chooses the
axis limits to span this range. The axis command enables you to specify your
own limits:
axis([xmin xmax ymin ymax])
or for three-dimensional graphs,
axis([xmin xmax ymin ymax zmin zmax])
Use the command
axis auto
to reenable MATLAB automatic limit selection.
Setting Axis Aspect Ratio
axis also enables you to specify a number of predefined modes. For example,
axis square
makes the x-axis and y-axis the same length:
axis equal
makes the individual tick mark increments on the x-axes and y-axes the same
length. This means
plot(exp(i*[0:pi/10:2*pi]))
followed by either axis square or axis equal turns the oval into a proper
circle:
axis auto normal
returns the axis scaling to its default automatic mode.
3-48
Basic Plotting Functions
Setting Axis Visibility
You can use the axis command to make the axis visible or invisible.
axis on
makes the axes visible. This is the default.
axis off
makes the axes invisible.
Setting Grid Lines
The grid command toggles grid lines on and off. The statement
grid on
turns the grid lines on, and
grid off
turns them back off again.
For More Information See the axis and axes reference pages and “Axes
Properties” in the MATLAB documentation.
Axis Labels and Titles
The xlabel, ylabel, and zlabel commands add x-, y-, and z-axis labels. The
title command adds a title at the top of the figure and the text function
inserts text anywhere in the figure.
3-49
3
Graphics
You can produce mathematical symbols using LaTeX notation in the text
string, as the following example illustrates:
t = -pi:pi/100:pi;
y = sin(t);
plot(t,y)
axis([-pi pi -1 1])
xlabel('-\pi \leq {\itt} \leq \pi')
ylabel('sin(t)')
title('Graph of the sine function')
text(1,-1/3,'{\itNote the odd symmetry.}')
Graph of the sine function
1
0.8
0.6
0.4
sin(t)
0.2
0
−0.2
Note the odd symmetry.
−0.4
−0.6
−0.8
−1
−3
−2
−1
0
−π ≤ t ≤ π
1
2
3
You can also set these options interactively. See “Editing Plots” on page 3-16
for more information.
Note that the location of the text string is defined in axes units (i.e., the same
units as the data). See the annotation function for a way to place text in
normalized figure units.
3-50
Basic Plotting Functions
Saving Figures
Save a figure by selecting Save from the File menu to display a file save dialog.
MATLAB saves the data it needs to recreate the figure and its contents (i.e.,
the entire graph) in a file with a .fig extension.
To save a figure using a standard graphics format, such as TIFF, for use with
other applications, select Export Setup from the File menu. You can also save
from the command line — use the saveas command, including any options to
save the figure in a different format.
See “Exporting the Graph” on page 3-35 for an example.
Saving Workspace Data
You can save the variables in your workspace using the Save Workspace As
item in the figure File menu. You can reload saved data using the Import Data
item in the figure File menu. MATLAB supports a variety of data file formats,
including MATLAB data files, which have a .mat extension.
Generating M-Code to Recreate a Figure
You can generate MATLAB code that recreates a figure and the graph it
contains by selecting the Generate M-File item from the figure File menu.
This option is particularly useful if you have developed a graph using plotting
tools and want to create a similar graph using the same or different data.
Saving Figures That Are Compatible with Previous Version of MATLAB
Create backward-compatible FIG-files by following these two steps:
• Ensure that any plotting functions used to create the contents of the figure
are called with the 'v6' argument, where applicable.
• Use the '-v6' option with the hgsave command.
For More Information See “Plot Objects and Backward Compatibility” in
the MATLAB documentation more information.
3-51
3
Graphics
Mesh and Surface Plots
MATLAB defines a surface by the z-coordinates of points above a grid in the x-y
plane, using straight lines to connect adjacent points. The mesh and surf
plotting functions display surfaces in three dimensions. mesh produces
wireframe surfaces that color only the lines connecting the defining points.
surf displays both the connecting lines and the faces of the surface in color.
The figure colormap and figure properties determine how MATLAB colors the
surface.
Visualizing Functions of Two Variables
To display a function of two variables, z = f (x,y),
• Generate X and Y matrices consisting of repeated rows and columns,
respectively, over the domain of the function.
• Use X and Y to evaluate and graph the function.
The meshgrid function transforms the domain specified by a single vector or
two vectors x and y into matrices X and Y for use in evaluating functions of two
variables. The rows of X are copies of the vector x and the columns of Y are
copies of the vector y.
Example — Graphing the sinc Function
This example evaluates and graphs the two-dimensional sinc function, sin(r)/r,
between the x and y directions. R is the distance from the origin, which is at the
center of the matrix. Adding eps (a MATLAB command that returns a small
floating-point number) avoids the indeterminate 0/0 at the origin:
[X,Y] = meshgrid(-8:.5:8);
R = sqrt(X.^2 + Y.^2) + eps;
Z = sin(R)./R;
mesh(X,Y,Z,'EdgeColor','black')
3-52
Mesh and Surface Plots
1
0.5
0
−0.5
10
5
10
5
0
0
−5
−5
−10
−10
By default, MATLAB colors the mesh using the current colormap. However,
this example uses a single-colored mesh by specifying the EdgeColor surface
property. See the surface reference page for a list of all surface properties.
You can create a mesh with see-through faces by disabling hidden line removal:
hidden off
See the hidden reference page for more information on this option.
Example — Colored Surface Plots
A surface plot is similar to a mesh plot except that MATLAB colors the
rectangular faces of the surface. The color of each faces is determined by the
values of Z and the colormap (a colormap is an ordered list of colors). These
statements graph the sinc function as a surface plot, specify a colormap, and
add a color bar to show the mapping of data to color:
surf(X,Y,Z)
colormap hsv
colorbar
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3
Graphics
1
0.8
1
0.6
0.5
0.4
0
0.2
−0.5
10
5
10
0
5
0
0
−5
−5
−10
−0.2
−10
See the colormap reference page for information on colormaps.
For More Information See “Creating 3-D Graphs” in the MATLAB
documentation for more information on surface plots.
Transparent Surfaces
You can make the faces of a surface transparent to a varying degree.
Transparency (referred to as the alpha value) can be specified for the whole
object or can be based on an alphamap, which behaves in a way analogous to
colormaps. For example,
surf(X,Y,Z)
colormap hsv
alpha(.4)
3-54
Mesh and Surface Plots
produces a surface with a face alpha value of 0.4. Alpha values range from 0
(completely transparent) to 1 (not transparent).
For More Information See “Transparency” in the MATLAB documentation
for more information on using this feature.
Surface Plots with Lighting
Lighting is the technique of illuminating an object with a directional light
source. In certain cases, this technique can make subtle differences in surface
shape easier to see. Lighting can also be used to add realism to
three-dimensional graphs.
This example uses the same surface as the previous examples, but colors it red
and removes the mesh lines. A light object is then added to the left of the
“camera” (the camera is the location in space from where you are viewing the
surface):
surf(X,Y,Z,'FaceColor','red','EdgeColor','none')
camlight left; lighting phong
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3
Graphics
Manipulating the Surface
The figure toolbar and the camera toolbar provide ways to explore 3-D graphics
interactively. Display the camera toolbar by selecting Camera Toolbar from
the figure View menu.
The following picture shows both toolbars with the Rotate 3D tool selected.
These tools enables you to move the camera around the surface object, zoom,
add lighting, and perform other viewing operations without issuing commands.
3-56
Mesh and Surface Plots
The following picture shows the surface viewed by orbiting the camera toward
the bottom using Rotate 3D. A scene light has been added to illuminate the
underside of the surface, which is not lit by the light added in the previous
section.
For More Information See “Lighting as a Visualization Tool” and “View
Control with the Camera Toolbar” in the MATLAB documentation for
information on these techniques.
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Graphics
Images
Two-dimensional arrays can be displayed as images, where the array elements
determine brightness or color of the images. For example, the statements
load durer
whos
Name
X
caption
map
Size
Bytes
Class
648x509
2x28
128x3
2638656
112
3072
double array
char array
double array
load the file durer.mat, adding three variables to the workspace. The matrix X
is a 648-by-509 matrix and map is a 128-by-3 matrix that is the colormap for this
image.
MAT-files, such as durer.mat, are binary files that can be created on one
platform and later read by MATLAB on a different platform.
The elements of X are integers between 1 and 128, which serve as indices into
the colormap, map. Then
image(X)
colormap(map)
axis image
reproduces Dürer’s etching shown at the beginning of this book. A
high-resolution scan of the magic square in the upper right corner is available
in another file. Type
load detail
and then use the up arrow key on your keyboard to reexecute the image,
colormap, and axis commands. The statement
colormap(hot)
adds some twentieth century colorization to the sixteenth century etching. The
function hot generates a colormap containing shades of reds, oranges, and
yellows. Typically a given image matrix has a specific colormap associated with
it. See the colormap reference page for a list of other predefined colormaps.
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Images
Reading and Writing Images
You can read standard image files (TIFF, JPEG, BMP, etc.) into MATLAB
using the imread function. The type of data returned by imread depends on the
type of image you are reading.
You can write MATLAB data to a variety of standard image formats using the
imwrite function. See the reference pages for these functions for more
information and examples.
For More Information See “Displaying Bit-Mapped Images” in the
MATLAB documentation for information on the image processing capabilities
of MATLAB.
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Graphics
Printing Graphics
You can print a MATLAB figure directly on a printer connected to your
computer or you can export the figure to one of the standard graphics file
formats supported by MATLAB. There are two ways to print and export
figures:
• Use the Print or Export Setup options under the File menu
• Use the print command to print or export the figure
See “Preparing Graphs for Presentation” on page 3-29 for an example.
Printing from the Menu
There are four menu options under the File menu that pertain to printing:
• The Page Setup option displays a dialog box that enables you to adjust
characteristics of the figure on the printed page.
• The Print Setup option displays a dialog box that sets printing defaults, but
does not actually print the figure.
• The Print Preview option enables you to view the figure the way it will look
on the printed page.
• The Print option displays a dialog box that lets you select standard printing
options and print the figure.
Generally, use Print Preview to determine whether the printed output is what
you want. If not, use the Page Setup dialog box to change the output settings.
Select the Page Setup dialog box Help button to display information on how to
set up the page.
Exporting Figure to Graphics Files
The Export Setup option in the File menu enables you to set a variety of figure
characteristics, such as size and font type, as well as apply predefined
templates to achieve standard-looking graphics files. After setup, you can
export the figure to a number of standard graphics file formats.
3-60
Printing Graphics
Using the Print Command
The print command provides more flexibility in the type of output sent to the
printer and allows you to control printing from M-files. The result can be sent
directly to your default printer or stored in a specified file. A wide variety of
output formats, including TIFF, JPEG, and PostScript, is available.
For example, this statement saves the contents of the current figure window as
color Encapsulated Level 2 PostScript in the file called magicsquare.eps. It
also includes a TIFF preview, which enables most word processors to display
the picture:
print -depsc2 -tiff magicsquare.eps
To save the same figure as a TIFF file with a resolution of 200 dpi, use the
command
print -dtiff -r200 magicsquare.tiff
If you type print on the command line,
print
MATLAB prints the current figure on your default printer.
For More Information See the print reference page and “Printing and
Exporting” in the MATLAB documentation for more information on printing.
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3
Graphics
Handle Graphics
Handle Graphics refers to a system of graphics objects that MATLAB uses to
implement graphing and visualization functions. Each object created has a
fixed set of properties. You can use these properties to control the behavior and
appearance of your graph.
When you call a plotting function, MATLAB creates the graph using various
graphics objects, such as a figure window, axes, lines, text, and so on. MATLAB
enables you to query the value of each property and set the values of most
properties.
For example, the following statement creates a figure with a white background
color and without displaying the figure toolbar:
figure('Color','white','Toolbar','none')
Using the Handle
Whenever MATLAB creates a graphics object, it assigns an identifier (called a
handle) to the object. You can use this handle to access the object’s properties
with the set and get functions. For example, the following statements create
a graph and return a handle to a lineseries object in h:
x = 1:10;
y = x.^3;
h = plot(x,y);
You can use the handle h to set the properties of the lineseries object. For
example, you can set its Color property:
set(h,'Color','red')
You can also specify properties when you call the plotting function:
h = plot(x,y,'Color','red');
When you query the lineseries properties,
get(h,'LineWidth')
MATLAB returns the answer:
ans =
0.5000
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Handle Graphics
Use the handle to see what properties a particular object contains:
get(h)
Graphics Objects
Graphics objects are the basic elements used to display graphs and user
interface components. These objects are organized into a hierarchy, as shown
by the following diagram.
Root
Figure
UI Objects
Axes
Core Objects
Plot Objects
Hidden
Annotation Axes
Group Objects
Annotation Objects
Key Graphics Objects
When you call a function to create a graph, MATLAB creates a hierarchy of
graphics objects. For example, calling the plot function creates the following
graphics objects:
• Lineseries plot objects — Represent the data passed to the plot function
• Axes — Provide a frame of reference and scaling for the plotted lineseries
• Text — Label the axes tick marks and are used for titles and annotations
• Figures — Are the windows that contain axes toolbars, menus, etc.
Different types of graphs use different objects to represent data: however, all
data objects are contained in axes and all objects (except root) are contained in
figures.
The root is an abstract object that primarily stores information about your
computer or MATLAB state. You cannot create an instance of the root object.
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Graphics
For More Information See “Handle Graphics Objects” in the MATLAB
documentation for information on graphics objects.
User interface objects are used to create graphical user interfaces (GUIs).
These objects include components like push buttons, editable text boxes, and
list boxes.
For More Information See “Creating Graphical User Interfaces” in the
MATLAB documentation for more information on user interface objects.
Creating Objects
Plotting functions (like plot and surf) call the appropriate low-level function
to draw their respective graph. For information about an object’s properties,
see the Handle Graphics Property Browser in the MATLAB online
documentation.
Commands for Working with Objects
This table lists commands commonly used when working with objects.
3-64
Function
Purpose
allchild
Find all children of specified objects
ancestor
Find ancestor of graphics object
copyobj
Copy graphics object
delete
Delete an object
findall
Find all graphics objects (including hidden handles)
findobj
Find the handles of objects having specified property
values
gca
Return the handle of the current axes
Handle Graphics
Function
Purpose
gcf
Return the handle of the current figure
gco
Return the handle of the current object
get
Query the values of an object’s properties
ishandle
True if value is valid object handle
set
Set the values of an object’s properties
Setting Object Properties
All object properties have default values. However, you might find it useful to
change the settings of some properties to customize your graph. There are two
ways to set object properties:
• Specify values for properties when you create the object.
• Set the property value on an object that already exists.
Setting Properties from Plotting Commands
You can specify object property value pairs as arguments to many plotting
functions, such as plot, mesh, and surf.
For example, plotting commands that create lineseries or surfaceplot objects
enable you to specify property name/property value pairs as arguments. The
command
surf(x,y,z,'FaceColor','interp',...
'FaceLighting','gouraud')
plots the data in the variables x, y, and z using a surfaceplot object with
interpolated face color and employing the Gouraud face light technique. You
can set any of the object’s properties this way.
Setting Properties of Existing Objects
To modify the property values of existing objects, you can use the set command
or the Property Editor. This section describes how to use the set command. See
“Using the Property Editor” on page 3-17 for more information.
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Graphics
Most plotting functions return the handles of the objects that they create so you
can modify the objects using the set command. For example, these statements
plot a five-by-five matrix (creating five lineseries, one per column) and then set
the Marker property to a square and the MarkerFaceColor property to green:
h = plot(magic(5));
set(h,'Marker','s','MarkerFaceColor','g')
In this case, h is a vector containing five handles, one for each of the five
lineseries in the graph. The set statement sets the Marker and
MarkerFaceColor properties of all lineseries to the same values.
Setting Multiple Property Values
If you want to set the properties of each lineseries to a different value, you can
use cell arrays to store all the data and pass it to the set command. For
example, create a plot and save the lineseries handles:
h = plot(magic(5));
Suppose you want to add different markers to each lineseries and color the
marker’s face color the same color as the lineseries. You need to define two cell
arrays — one containing the property names and the other containing the
desired values of the properties.
The prop_name cell array contains two elements:
prop_name(1) = {'Marker'};
prop_name(2) = {'MarkerFaceColor'};
The prop_values cell array contains 10 values: five values for the Marker
property and five values for the MarkerFaceColor property. Notice that
prop_values is a two-dimensional cell array. The first dimension indicates
which handle in h the values apply to and the second dimension indicates
which property the value is assigned to:
prop_values(1,1)
prop_values(1,2)
prop_values(2,1)
prop_values(2,2)
prop_values(3,1)
prop_values(3,2)
prop_values(4,1)
prop_values(4,2)
3-66
=
=
=
=
=
=
=
=
{'s'};
{get(h(1),'Color')};
{'d'};
{get(h(2),'Color')};
{'o'};
{get(h(3),'Color')};
{'p'};
{get(h(4),'Color')};
Handle Graphics
prop_values(5,1) = {'h'};
prop_values(5,2) = {get(h(5),'Color')};
The MarkerFaceColor is always assigned the value of the corresponding line’s
color (obtained by getting the lineseries Color property with the get
command).
After defining the cell arrays, call set to specify the new property values:
set(h,prop_name,prop_values)
25
20
15
10
5
0
1
1.5
2
2.5
3
3.5
4
4.5
5
3-67
3
Graphics
Specifying the Axes or Figure
MATLAB always creates an axes or figure if one does not exist when you issue
a plotting command. However, when you are creating a graphics M-file, it is
good practice to explicitly create and specify the parent axes and figure,
particularly if others will use your program. Specifying the parent prevents the
following problems:
• Your M-file overwrites the graph in the current figure. Note that a figure
becomes the current figure whenever a user clicks on it.
• The current figure might be in an unexpected state and not behave as your
program expects.
The following examples shows a simple M-file that plots a function and the
mean of the function over the specified range:
function myfunc(x)
% x = -10:.005:40; Here's a value you can use for x
y = [1.5*cos(x) + 6*exp(-.1*x) + exp(.07*x).*sin(3*x)];
ym = mean(y);
hfig = figure('Name','Function and Mean',...
'Pointer','fullcrosshair');
hax = axes('Parent',hfig);
plot(hax,x,y)
hold on
plot(hax,[min(x) max(x)],[ym ym],'Color','red')
hold off
ylab = get(hax,'YTick');
set(hax,'YTick',sort([ylab ym]))
title ('y = 1.5cos(x) + 6e^{-0.1x} + e^{0.07x}sin(3x)')
xlabel('X Axis'); ylabel('Y Axis')
3-68
Handle Graphics
y = 1.5cos(x) + 6e−0.1x + e0.07xsin(3x)
20
15
10
5
Y Axis
3.1596
0
−5
−10
−15
−20
−10
−5
0
5
10
15
X Axis
20
25
30
35
40
Finding the Handles of Existing Objects
The findobj function enables you to obtain the handles of graphics objects by
searching for objects with particular property values. With findobj you can
specify the values of any combination of properties, which makes it easy to pick
one object out of many. findobj also recognizes regular expressions (regexp).
For example, you might want to find the blue line with square marker having
blue face color.You can also specify which figures or axes to search, if there are
more than one. The following sections provide examples illustrating how to use
findobj.
Finding All Objects of a Certain Type
Because all objects have a Type property that identifies the type of object, you
can find the handles of all occurrences of a particular type of object. For
example,
h = findobj('Type','patch');
finds the handles of all patch objects.
3-69
3
Graphics
Finding Objects with a Particular Property
You can specify multiple properties to narrow the search. For example,
h = findobj('Type','line','Color','r','LineStyle',':');
finds the handles of all red dotted lines.
Limiting the Scope of the Search
You can specify the starting point in the object hierarchy by passing the handle
of the starting figure or axes as the first argument. For example,
h = findobj(gca,'Type','text','String','\pi/2');
finds the string π/2 only within the current axes.
Using findobj as an Argument
Because findobj returns the handles it finds, you can use it in place of the
handle argument. For example,
set(findobj('Type','line','Color','red'),'LineStyle',':')
finds all red lines and sets their line style to dotted.
3-70
Animations
Animations
MATLAB provides three ways of generating moving, animated graphics:
• “Erase Mode Method” on page 3-71 — Continually erase and then redraw the
objects on the screen, making incremental changes with each redraw.
• “Creating Movies” on page 3-73 — Save a number of different pictures and
then play them back as a movie.
• Using AVI files. See avifile for more information and examples.
Erase Mode Method
Using the EraseMode property is appropriate for long sequences of simple plots
where the change from frame to frame is minimal. Here is an example showing
simulated Brownian motion. Specify a number of points, such as
n = 20
and a temperature or velocity, such as
s = .02
The best values for these two parameters depend upon the speed of your
particular computer. Generate n random points with (x,y) coordinates between
-1/2 and +1/2:
x = rand(n,1)-0.5;
y = rand(n,1)-0.5;
Plot the points in a square with sides at -1 and +1. Save the handle for the
vector of points and set its EraseMode to xor. This tells the MATLAB graphics
system not to redraw the entire plot when the coordinates of one point are
changed, but to restore the background color in the vicinity of the point using
an exclusive or operation:
h = plot(x,y,'.');
axis([-1 1 -1 1])
axis square
grid off
set(h,'EraseMode','xor','MarkerSize',18)
3-71
3
Graphics
Now begin the animation. Here is an infinite while loop, which you can
eventually exit by typing Ctrl+c. Each time through the loop, add a small
amount of normally distributed random noise to the coordinates of the points.
Then, instead of creating an entirely new plot, simply change the XData and
YData properties of the original plot:
while 1
drawnow
x = x + s*randn(n,1);
y = y + s*randn(n,1);
set(h,'XData',x,'YData',y)
end
See how long it takes for one of the points to get outside the square and how
long before all the points are outside the square.
1
0.8
0.6
0.4
0.2
0
−0.2
−0.4
−0.6
−0.8
−1
−1
3-72
−0.5
0
0.5
1
Animations
Creating Movies
If you increase the number of points in the Brownian motion example to n =
300 and s = .02, the motion is no longer very fluid; it takes too much time to
draw each time step. It becomes more effective to save a predetermined number
of frames as bitmaps and to play them back as a movie.
First, decide on the number of frames:
nframes = 50;
Next, set up the first plot as before, except using the default EraseMode
(normal):
x = rand(n,1)-0.5;
y = rand(n,1)-0.5;
h = plot(x,y,'.');
set(h,'MarkerSize',18);
axis([-1 1 -1 1])
axis square
grid off
Generate the movie and use getframe to capture each frame:
for k = 1:nframes
x = x + s*randn(n,1);
y = y + s*randn(n,1);
set(h,'XData',x,'YData',y)
M(k) = getframe;
end
Finally, play the movie 5 times:
movie(M,5)
3-73
3
Graphics
3-74
4
Programming
Flow Control (p. 4-2)
Use flow control constructs including if, switch and
case, for, while, continue, and break.
Other Data Structures (p. 4-9)
Work with multidimensional arrays, cell arrays,
character and text data, and structures.
Scripts and Functions (p. 4-19)
Write scripts and functions, use global variables, pass
string arguments to functions, use eval to evaluate text
expressions, vectorize code, preallocate arrays, reference
functions using handles, and use functions that operate
on functions.
4
Programming
Flow Control
MATLAB has several flow control constructs:
• “if, else, and elseif” on page 4-2
• “switch and case” on page 4-4
• “for” on page 4-5
• “while” on page 4-5
• “continue” on page 4-6
• “break” on page 4-7
• “try - catch” on page 4-7
• “return” on page 4-8
if, else, and elseif
The if statement evaluates a logical expression and executes a group of
statements when the expression is true. The optional elseif and else
keywords provide for the execution of alternate groups of statements. An end
keyword, which matches the if, terminates the last group of statements. The
groups of statements are delineated by the four keywords — no braces or
brackets are involved.
The MATLAB algorithm for generating a magic square of order n involves
three different cases: when n is odd, when n is even but not divisible by 4, or
when n is divisible by 4. This is described by
if rem(n,2) ~= 0
M = odd_magic(n)
elseif rem(n,4) ~= 0
M = single_even_magic(n)
else
M = double_even_magic(n)
end
In this example, the three cases are mutually exclusive, but if they weren’t, the
first true condition would be executed.
4-2
Flow Control
It is important to understand how relational operators and if statements work
with matrices. When you want to check for equality between two variables, you
might use
if A == B, ...
This is valid MATLAB code, and does what you expect when A and B are
scalars. But when A and B are matrices, A == B does not test if they are equal,
it tests where they are equal; the result is another matrix of 0’s and 1’s showing
element-by-element equality. (In fact, if A and B are not the same size, then A
== B is an error.)
A = magic(4);
A == B
ans =
0
1
1
1
1
1
1
1
B = A;
1
1
1
1
B(1,1) = 0;
1
1
1
1
The proper way to check for equality between two variables is to use the
isequal function,
if isequal(A, B), ...
isequal returns a scalar logical value of 1 (representing true) or 0 (false),
instead of a matrix, as the expression to be evaluated by the if function. Using
the A and B matrices from above, you get
isequal(A, B)
ans =
0
Here is another example to emphasize this point. If A and B are scalars, the
following program will never reach the “unexpected situation”. But for most
pairs of matrices, including our magic squares with interchanged columns,
none of the matrix conditions A > B, A < B, or A == B is true for all elements
and so the else clause is executed:
4-3
4
Programming
if A > B
'greater'
elseif A < B
'less'
elseif A == B
'equal'
else
error('Unexpected situation')
end
Several functions are helpful for reducing the results of matrix comparisons to
scalar conditions for use with if, including
isequal
isempty
all
any
switch and case
The switch statement executes groups of statements based on the value of a
variable or expression. The keywords case and otherwise delineate the
groups. Only the first matching case is executed. There must always be an end
to match the switch.
The logic of the magic squares algorithm can also be described by
switch (rem(n,4)==0) + (rem(n,2)==0)
case 0
M = odd_magic(n)
case 1
M = single_even_magic(n)
case 2
M = double_even_magic(n)
otherwise
error('This is impossible')
end
4-4
Flow Control
Note Unlike the C language switch statement, MATLAB switch does not
fall through. If the first case statement is true, the other case statements do
not execute. So, break statements are not required.
for
The for loop repeats a group of statements a fixed, predetermined number of
times. A matching end delineates the statements:
for n = 3:32
r(n) = rank(magic(n));
end
r
The semicolon terminating the inner statement suppresses repeated printing,
and the r after the loop displays the final result.
It is a good idea to indent the loops for readability, especially when they are
nested:
for i = 1:m
for j = 1:n
H(i,j) = 1/(i+j);
end
end
while
The while loop repeats a group of statements an indefinite number of times
under control of a logical condition. A matching end delineates the statements.
Here is a complete program, illustrating while, if, else, and end, that uses
interval bisection to find a zero of a polynomial:
a = 0; fa = -Inf;
b = 3; fb = Inf;
4-5
4
Programming
while b-a > eps*b
x = (a+b)/2;
fx = x^3-2*x-5;
if sign(fx) == sign(fa)
a = x; fa = fx;
else
b = x; fb = fx;
end
end
x
The result is a root of the polynomial x3 - 2x - 5, namely
x =
2.09455148154233
The cautions involving matrix comparisons that are discussed in the section on
the if statement also apply to the while statement.
continue
The continue statement passes control to the next iteration of the for loop or
while loop in which it appears, skipping any remaining statements in the body
of the loop. In nested loops, continue passes control to the next iteration of the
for loop or while loop enclosing it.
The example below shows a continue loop that counts the lines of code in the
file magic.m, skipping all blank lines and comments. A continue statement is
used to advance to the next line in magic.m without incrementing the count
whenever a blank line or comment line is encountered:
fid = fopen('magic.m','r');
count = 0;
while ~feof(fid)
line = fgetl(fid);
if isempty(line) | strncmp(line,'%',1)
continue
end
count = count + 1;
end
disp(sprintf('%d lines',count));
4-6
Flow Control
break
The break statement lets you exit early from a for loop or while loop. In nested
loops, break exits from the innermost loop only.
Here is an improvement on the example from the previous section. Why is this
use of break a good idea?
a = 0; fa = -Inf;
b = 3; fb = Inf;
while b-a > eps*b
x = (a+b)/2;
fx = x^3-2*x-5;
if fx == 0
break
elseif sign(fx) == sign(fa)
a = x; fa = fx;
else
b = x; fb = fx;
end
end
x
try - catch
The general form of a try-catch statement sequence is
try
statement
...
statement
catch
statement
...
statement
end
In this sequence the statements between try and catch are executed until an
error occurs. The statements between catch and end are then executed. Use
lasterr to see the cause of the error. If an error occurs between catch and end,
MATLAB terminates execution unless another try-catch sequence has been
established.
4-7
4
Programming
return
return terminates the current sequence of commands and returns control to
the invoking function or to the keyboard. return is also used to terminate
keyboard mode. A called function normally transfers control to the function
that invoked it when it reaches the end of the function. You can insert a return
statement within the called function to force an early termination and to
transfer control to the invoking function.
4-8
Other Data Structures
Other Data Structures
This section introduces you to some other data structures in MATLAB,
including
• “Multidimensional Arrays” on page 4-9
• “Cell Arrays” on page 4-11
• “Characters and Text” on page 4-13
• “Structures” on page 4-16
Multidimensional Arrays
Multidimensional arrays in MATLAB are arrays with more than two
subscripts. One way of creating a multidimensional array is by calling zeros,
ones, rand, or randn with more than two arguments. For example,
R = randn(3,4,5);
creates a 3-by-4-by-5 array with a total of 3x4x5 = 60 normally distributed
random elements.
A three-dimensional array might represent three-dimensional physical data,
say the temperature in a room, sampled on a rectangular grid. Or it might
represent a sequence of matrices, A(k), or samples of a time-dependent matrix,
A(t). In these latter cases, the (i, j)th element of the kth matrix, or the tkth
matrix, is denoted by A(i,j,k).
MATLAB and Dürer’s versions of the magic square of order 4 differ by an
interchange of two columns. Many different magic squares can be generated by
interchanging columns. The statement
p = perms(1:4);
generates the 4! = 24 permutations of 1:4. The kth permutation is the row
vector p(k,:). Then
A = magic(4);
M = zeros(4,4,24);
for k = 1:24
M(:,:,k) = A(:,p(k,:));
end
4-9
4
Programming
stores the sequence of 24 magic squares in a three-dimensional array, M. The
size of M is
size(M)
ans =
4
4
24
.
..
16
3
2
13
16
28
11
13
10
3
8
12
8
3
121
10
1
6
117
13
14
7
8
14
12
106
12
15
6
1
9
10
2
6
11
15
7
4
14
15
16
5
13
16
2
3
8
5
11
10
12
9
7
6
1
4
14
15
15
1
Note The order of the matrices shown in this illustration might differ from
your results. The perms function always returns all permutations of the input
vector, but the order of the permutations might be different for different
MATLAB versions.
The statement
sum(M,d)
computes sums by varying the dth subscript. So
sum(M,1)
4-10
Other Data Structures
is a 1-by-4-by-24 array containing 24 copies of the row vector
34
34
34
34
and
sum(M,2)
is a 4-by-1-by-24 array containing 24 copies of the column vector
34
34
34
34
Finally,
S = sum(M,3)
adds the 24 matrices in the sequence. The result has size 4-by-4-by-1, so it looks
like a 4-by-4 array:
S =
204
204
204
204
204
204
204
204
204
204
204
204
204
204
204
204
Cell Arrays
Cell arrays in MATLAB are multidimensional arrays whose elements are
copies of other arrays. A cell array of empty matrices can be created with the
cell function. But, more often, cell arrays are created by enclosing a
miscellaneous collection of things in curly braces, {}. The curly braces are also
used with subscripts to access the contents of various cells. For example,
C = {A sum(A) prod(prod(A))}
produces a 1-by-3 cell array. The three cells contain the magic square, the row
vector of column sums, and the product of all its elements. When C is displayed,
you see
C =
[4x4 double]
[1x4 double]
[20922789888000]
4-11
4
Programming
This is because the first two cells are too large to print in this limited space, but
the third cell contains only a single number, 16!, so there is room to print it.
Here are two important points to remember. First, to retrieve the contents of
one of the cells, use subscripts in curly braces. For example, C{1} retrieves the
magic square and C{3} is 16. Second, cell arrays contain copies of other arrays,
not pointers to those arrays. If you subsequently change A, nothing happens to
C.
You can use three-dimensional arrays to store a sequence of matrices of the
same size. Cell arrays can be used to store a sequence of matrices of different
sizes. For example,
M = cell(8,1);
for n = 1:8
M{n} = magic(n);
end
M
produces a sequence of magic squares of different order:
M =
[
[
[
[
[
[
[
[
4-12
2x2
3x3
4x4
5x5
6x6
7x7
8x8
1]
double]
double]
double]
double]
double]
double]
double]
Other Data Structures
64
2
3
61
60
6
7
57
55
54
12
13
51
50
16
47
46
20
21
43
42
24
40
26
27
37
36
30
31
33
32
34
35
29
28
38
39
25
41
23
22
44
45
19
18
48
49
15
14
52
53
11
10
56
8
58
59
5
4
62
63
1
..
.
9
17
16
2
3
13
5
11
10
8
9
7
6
12
4
14
15
1
8
1
6
3
5
7
4
9
2
1
3
4
2
1
You can retrieve the 4-by-4 magic square matrix with
M{4}
Characters and Text
Enter text into MATLAB using single quotes. For example,
s = 'Hello'
The result is not the same kind of numeric matrix or array you have been
dealing with up to now. It is a 1-by-5 character array.
4-13
4
Programming
Internally, the characters are stored as numbers, but not in floating-point
format. The statement
a = double(s)
converts the character array to a numeric matrix containing floating-point
representations of the ASCII codes for each character. The result is
a =
72
101
108
108
111
The statement
s = char(a)
reverses the conversion.
Converting numbers to characters makes it possible to investigate the various
fonts available on your computer. The printable characters in the basic ASCII
character set are represented by the integers 32:127. (The integers less than
32 represent nonprintable control characters.) These integers are arranged in
an appropriate 6-by-16 array with
F = reshape(32:127,16,6)';
The printable characters in the extended ASCII character set are represented
by F+128. When these integers are interpreted as characters, the result
depends on the font currently being used. Type the statements
char(F)
char(F+128)
and then vary the font being used for the Command Window. Select
Preferences from the File menu to change the font. If you include tabs in lines
of code, use a fixed-width font, such as Monospaced, to align the tab positions
on different lines.
Concatenation with square brackets joins text variables together into larger
strings. The statement
h = [s, ' world']
joins the strings horizontally and produces
h =
Hello world
4-14
Other Data Structures
The statement
v = [s; 'world']
joins the strings vertically and produces
v =
Hello
world
Note that a blank has to be inserted before the 'w' in h and that both words in
v have to have the same length. The resulting arrays are both character arrays;
h is 1-by-11 and v is 2-by-5.
To manipulate a body of text containing lines of different lengths, you have two
choices — a padded character array or a cell array of strings. When creating a
character array, you must make each row of the array the same length. (Pad
the ends of the shorter rows with spaces.) The char function does this padding
for you. For example,
S = char('A','rolling','stone','gathers','momentum.')
produces a 5-by-9 character array:
S =
A
rolling
stone
gathers
momentum.
Alternatively, you can store the text in a cell array. For example,
C = {'A';'rolling';'stone';'gathers';'momentum.'}
creates a 5-by-1 cell array that requires no padding because each row of the
array can have a different length:
C =
'A'
'rolling'
'stone'
'gathers'
'momentum.'
4-15
4
Programming
You can convert a padded character array to a cell array of strings with
C = cellstr(S)
and reverse the process with
S = char(C)
Structures
Structures are multidimensional MATLAB arrays with elements accessed by
textual field designators. For example,
S.name = 'Ed Plum';
S.score = 83;
S.grade = 'B+'
creates a scalar structure with three fields:
S =
name: 'Ed Plum'
score: 83
grade: 'B+'
Like everything else in MATLAB, structures are arrays, so you can insert
additional elements. In this case, each element of the array is a structure with
several fields. The fields can be added one at a time,
S(2).name = 'Toni Miller';
S(2).score = 91;
S(2).grade = 'A-';
or an entire element can be added with a single statement:
S(3) = struct('name','Jerry Garcia',...
'score',70,'grade','C')
Now the structure is large enough that only a summary is printed:
S =
1x3 struct array with fields:
name
score
grade
4-16
Other Data Structures
There are several ways to reassemble the various fields into other MATLAB
arrays. They are all based on the notation of a comma-separated list. If you type
S.score
it is the same as typing
S(1).score, S(2).score, S(3).score
This is a comma-separated list. Without any other punctuation, it is not very
useful. It assigns the three scores, one at a time, to the default variable ans and
dutifully prints out the result of each assignment. But when you enclose the
expression in square brackets,
[S.score]
it is the same as
[S(1).score, S(2).score, S(3).score]
which produces a numeric row vector containing all the scores:
ans =
83
91
70
Similarly, typing
S.name
just assigns the names, one at a time, to ans. But enclosing the expression in
curly braces,
{S.name}
creates a 1-by-3 cell array containing the three names:
ans =
'Ed Plum'
'Toni Miller'
'Jerry Garcia'
And
char(S.name)
4-17
4
Programming
calls the char function with three arguments to create a character array from
the name fields,
ans =
Ed Plum
Toni Miller
Jerry Garcia
Dynamic Field Names
The most common way to access the data in a structure is by specifying the
name of the field that you want to reference. Another means of accessing
structure data is to use dynamic field names. These names express the field as
a variable expression that MATLAB evaluates at run-time. The
dot-parentheses syntax shown here makes expression a dynamic field name:
structName.(expression)
Index into this field using the standard MATLAB indexing syntax. For
example, to evaluate expression into a field name and obtain the values of that
field at columns 1 through 25 of row 7, use
structName.(expression)(7,1:25)
Dynamic Field Names Example. The avgscore function shown below computes an
average test score, retrieving information from the testscores structure using
dynamic field names:
function avg = avgscore(testscores, student, first, last)
for k = first:last
scores(k) = testscores.(student).week(k);
end
avg = sum(scores)/(last - first + 1);
You can run this function using different values for the dynamic field student:
avgscore(testscores, 'Ann Lane', 1, 20)
ans =
83.5000
avgscore(testscores, 'William King', 1, 20)
ans =
92.1000
4-18
Scripts and Functions
Scripts and Functions
Topics covered in this section are
• “Scripts” on page 4-20
• “Functions” on page 4-21
• “Global Variables” on page 4-25
• “Passing String Arguments to Functions” on page 4-26
• “The eval Function” on page 4-27
• “Function Handles” on page 4-28
• “Function Functions” on page 4-28
• “Vectorization” on page 4-31
• “Preallocation” on page 4-31
MATLAB is a powerful programming language as well as an interactive
computational environment. Files that contain code in the MATLAB language
are called M-files. You create M-files using a text editor, then use them as you
would any other MATLAB function or command.
There are two kinds of M-files:
• Scripts, which do not accept input arguments or return output arguments.
They operate on data in the workspace.
• Functions, which can accept input arguments and return output arguments.
Internal variables are local to the function.
If you’re a new MATLAB programmer, just create the M-files that you want to
try out in the current directory. As you develop more of your own M-files, you
will want to organize them into other directories and personal toolboxes that
you can add to your MATLAB search path.
If you duplicate function names, MATLAB executes the one that occurs first in
the search path.
To view the contents of an M-file, for example, myfunction.m, use
type myfunction
4-19
4
Programming
Scripts
When you invoke a script, MATLAB simply executes the commands found in
the file. Scripts can operate on existing data in the workspace, or they can
create new data on which to operate. Although scripts do not return output
arguments, any variables that they create remain in the workspace, to be used
in subsequent computations. In addition, scripts can produce graphical output
using functions like plot.
For example, create a file called magicrank.m that contains these MATLAB
commands:
% Investigate the rank of magic squares
r = zeros(1,32);
for n = 3:32
r(n) = rank(magic(n));
end
r
bar(r)
Typing the statement
magicrank
causes MATLAB to execute the commands, compute the rank of the first 30
magic squares, and plot a bar graph of the result. After execution of the file is
complete, the variables n and r remain in the workspace.
4-20
Scripts and Functions
35
30
25
20
15
10
5
0
0
5
10
15
20
25
30
35
Functions
Functions are M-files that can accept input arguments and return output
arguments. The names of the M-file and of the function should be the same.
Functions operate on variables within their own workspace, separate from the
workspace you access at the MATLAB command prompt.
A good example is provided by rank. The M-file rank.m is available in the
directory
toolbox/matlab/matfun
You can see the file with
type rank
4-21
4
Programming
Here is the file:
function r = rank(A,tol)
%
RANK Matrix rank.
%
RANK(A) provides an estimate of the number of linearly
%
independent rows or columns of a matrix A.
%
RANK(A,tol) is the number of singular values of A
%
that are larger than tol.
%
RANK(A) uses the default tol = max(size(A)) * norm(A) * eps.
s = svd(A);
if nargin==1
tol = max(size(A)') * max(s) * eps;
end
r = sum(s > tol);
The first line of a function M-file starts with the keyword function. It gives the
function name and order of arguments. In this case, there are up to two input
arguments and one output argument.
The next several lines, up to the first blank or executable line, are comment
lines that provide the help text. These lines are printed when you type
help rank
The first line of the help text is the H1 line, which MATLAB displays when you
use the lookfor command or request help on a directory.
The rest of the file is the executable MATLAB code defining the function. The
variable s introduced in the body of the function, as well as the variables on the
first line, r, A and tol, are all local to the function; they are separate from any
variables in the MATLAB workspace.
This example illustrates one aspect of MATLAB functions that is not ordinarily
found in other programming languages — a variable number of arguments.
The rank function can be used in several different ways:
rank(A)
r = rank(A)
r = rank(A,1.e-6)
4-22
Scripts and Functions
Many M-files work this way. If no output argument is supplied, the result is
stored in ans. If the second input argument is not supplied, the function
computes a default value. Within the body of the function, two quantities
named nargin and nargout are available which tell you the number of input
and output arguments involved in each particular use of the function. The rank
function uses nargin, but does not need to use nargout.
Types of Functions
MATLAB offers several different types of functions to use in your
programming.
Anonymous Functions
An anonymous function is a simple form of MATLAB function that does not
require an M-file. It consists of a single MATLAB expression and any number
of input and output arguments. You can define an anonymous function right at
the MATLAB command line, or within an M-file function or script. This gives
you a quick means of creating simple functions without having to create M-files
each time.
The syntax for creating an anonymous function from an expression is
f = @(arglist)expression
The statement below creates an anonymous function that finds the square of a
number. When you call this function, MATLAB assigns the value you pass in
to variable x, and then uses x in the equation x.^2:
sqr = @(x) x.^2;
To execute the sqr function defined above, type
a = sqr(5)
a =
25
Primary and Subfunctions
All functions that are not anonymous must be defined within an M-file. Each
M-file has a required primary function that appears first in the file, and any
number of subfunctions that follow the primary. Primary functions have a
wider scope than subfunctions. That is, primary functions can be invoked from
outside of their M-file (from the MATLAB command line or from functions in
4-23
4
Programming
other M-files) while subfunctions cannot. Subfunctions are visible only to the
primary function and other subfunctions within their own M-file.
The rank function shown in the section on “Functions” on page 4-21 is an
example of a primary function.
Private Functions
A private function is a type of primary M-file function. Its unique characteristic
is that it is visible only to a limited group of other functions. This type of
function can be useful if you want to limit access to a function, or when you
choose not to expose the implementation of a function.
Private functions reside in subdirectories with the special name private. They
are visible only to functions in the parent directory. For example, assume the
directory newmath is on the MATLAB search path. A subdirectory of newmath
called private can contain functions that only the functions in newmath can
call.
Because private functions are invisible outside the parent directory, they can
use the same names as functions in other directories. This is useful if you want
to create your own version of a particular function while retaining the original
in another directory. Because MATLAB looks for private functions before
standard M-file functions, it will find a private function named test.m before
a nonprivate M-file named test.m.
Nested Functions
You can define functions within the body of any MATLAB M-file function.
These are said to be nested within the outer function. A nested function
contains any or all of the components of any other M-file function. In this
example, function B is nested in function A:
function x = A(p1, p2)
...
B(p2)
function y = B(p3)
...
end
...
end
4-24
Scripts and Functions
Like other functions, a nested function has its own workspace where variables
used by the function are stored. But it also has access to the workspaces of all
functions in which it is nested. So, for example, a variable that has a value
assigned to it by the primary function can be read or overwritten by a function
nested at any level within the primary. Similarly, a variable that is assigned in
a nested function can be read or overwritten by any of the functions containing
that function.
Function Overloading
Overloaded functions act the same way as overloaded functions in most
computer languages. Overloaded functions are useful when you need to create
a function that responds to different types of inputs accordingly. For instance,
you might want one of your functions to accept both double-precision and
integer input, but to handle each type somewhat differently. You can make this
difference invisible to the user by creating two separate functions having the
same name, and designating one to handle double types and one to handle
integers. When you call the function, MATLAB chooses which M-file to
dispatch to based on the type of the input arguments.
Global Variables
If you want more than one function to share a single copy of a variable, simply
declare the variable as global in all the functions. Do the same thing at the
command line if you want the base workspace to access the variable. The global
declaration must occur before the variable is actually used in a function.
Although it is not required, using capital letters for the names of global
variables helps distinguish them from other variables. For example, create an
M-file called falling.m:
function h = falling(t)
global GRAVITY
h = 1/2*GRAVITY*t.^2;
Then interactively enter the statements
global GRAVITY
GRAVITY = 32;
y = falling((0:.1:5)');
4-25
4
Programming
The two global statements make the value assigned to GRAVITY at the
command prompt available inside the function. You can then modify GRAVITY
interactively and obtain new solutions without editing any files.
Passing String Arguments to Functions
You can write MATLAB functions that accept string arguments without the
parentheses and quotes. That is, MATLAB interprets
foo a b c
as
foo('a','b','c')
However, when you use the unquoted form, MATLAB cannot return output
arguments. For example,
legend apples oranges
creates a legend on a plot using the strings apples and oranges as labels. If you
want the legend command to return its output arguments, then you must use
the quoted form:
[legh,objh] = legend('apples','oranges');
In addition, you cannot use the unquoted form if any of the arguments is not a
string.
Constructing String Arguments in Code
The quoted form enables you to construct string arguments within the code.
The following example processes multiple data files, August1.dat,
August2.dat, and so on. It uses the function int2str, which converts an
integer to a character, to build the filename:
for d = 1:31
s = ['August' int2str(d) '.dat'];
load(s)
% Code to process the contents of the d-th file
end
4-26
Scripts and Functions
A Cautionary Note
While the unquoted syntax is convenient, it can be used incorrectly without
causing MATLAB to generate an error. For example, given a matrix A,
A =
0
6
-5
-6
2
20
-1
-16
-10
The eig command returns the eigenvalues of A:
eig(A)
ans =
-3.0710
-2.4645+17.6008i
-2.4645-17.6008i
The following statement is not allowed because A is not a string; however,
MATLAB does not generate an error:
eig A
ans =
65
MATLAB actually takes the eigenvalue of the ASCII numeric equivalent of the
letter A (which is the number 65).
The eval Function
The eval function works with text variables to implement a powerful text
macro facility. The expression or statement
eval(s)
uses the MATLAB interpreter to evaluate the expression or execute the
statement contained in the text string s.
The example of the previous section could also be done with the following code,
although this would be somewhat less efficient because it involves the full
interpreter, not just a function call:
4-27
4
Programming
for d = 1:31
s = ['load August' int2str(d) '.dat'];
eval(s)
% Process the contents of the d-th file
end
Function Handles
You can create a handle to any MATLAB function and then use that handle as
a means of referencing the function. A function handle is typically passed in an
argument list to other functions, which can then execute, or evaluate, the
function using the handle.
Construct a function handle in MATLAB using the at sign, @, before the
function name. The following example creates a function handle for the sin
function and assigns it to the variable fhandle:
fhandle = @sin;
You can call a function by means of its handle in the same way that you would
call the function using its name. The syntax is
fhandle(arg1, arg2, ...);
The function plot_fhandle, shown below, receives a function handle and data,
generates y-axis data using the function handle, and plots it:
function x = plot_fhandle(fhandle, data)
plot(data, fhandle(data))
When you call plot_fhandle with a handle to the sin function and the
argument shown below, the resulting evaluation produces a sine wave plot:
plot_fhandle(@sin, -pi:0.01:pi)
Function Functions
A class of functions called “function functions” works with nonlinear functions
of a scalar variable. That is, one function works on another function. The
function functions include
• Zero finding
• Optimization
4-28
Scripts and Functions
• Quadrature
• Ordinary differential equations
MATLAB represents the nonlinear function by a function M-file. For example,
here is a simplified version of the function humps from the matlab/demos
directory:
function y = humps(x)
y = 1./((x-.3).^2 + .01) + 1./((x-.9).^2 + .04) - 6;
Evaluate this function at a set of points in the interval 0 ≤ x ≤ 1 with
x = 0:.002:1;
y = humps(x);
Then plot the function with
plot(x,y)
100
90
80
70
60
50
40
30
20
10
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
4-29
4
Programming
The graph shows that the function has a local minimum near x = 0.6. The
function fminsearch finds the minimizer, the value of x where the function
takes on this minimum. The first argument to fminsearch is a function handle
to the function being minimized and the second argument is a rough guess at
the location of the minimum:
p = fminsearch(@humps,.5)
p =
0.6370
To evaluate the function at the minimizer,
humps(p)
ans =
11.2528
Numerical analysts use the terms quadrature and integration to distinguish
between numerical approximation of definite integrals and numerical
integration of ordinary differential equations. MATLAB quadrature routines
are quad and quadl. The statement
Q = quadl(@humps,0,1)
computes the area under the curve in the graph and produces
Q =
29.8583
Finally, the graph shows that the function is never zero on this interval. So, if
you search for a zero with
z = fzero(@humps,.5)
you will find one outside the interval
z =
-0.1316
4-30
Scripts and Functions
Vectorization
One way to make your MATLAB programs run faster is to vectorize the
algorithms you use in constructing the programs. Where other programming
languages might use for loops or DO loops, MATLAB can use vector or matrix
operations. A simple example involves creating a table of logarithms:
x = .01;
for k = 1:1001
y(k) = log10(x);
x = x + .01;
end
A vectorized version of the same code is
x = .01:.01:10;
y = log10(x);
For more complicated code, vectorization options are not always so obvious.
For More Information See “Improving Performance and Memory Usage” in
the MATLAB Programming documentation for other techniques that you can
use.
Preallocation
If you can’t vectorize a piece of code, you can make your for loops go faster by
preallocating any vectors or arrays in which output results are stored. For
example, this code uses the function zeros to preallocate the vector created in
the for loop. This makes the for loop execute significantly faster:
r = zeros(32,1);
for n = 1:32
r(n) = rank(magic(n));
end
Without the preallocation in the previous example, the MATLAB interpreter
enlarges the r vector by one element each time through the loop. Vector
preallocation eliminates this step and results in faster execution.
4-31
4
Programming
4-32
5
Creating Graphical User
Interfaces
What Is GUIDE? (p. 5-2)
Introduces GUIDE, the MATLAB graphical user
interface design environment
Laying Out a GUI (p. 5-3)
Briefly describes the GUIDE Layout Editor
Programming a GUI (p. 5-6)
Introduces callbacks to define behavior of the GUI
components
5
Creating Graphical User Interfaces
What Is GUIDE?
GUIDE, the MATLAB graphical user interface development environment,
provides a set of tools for creating graphical user interfaces (GUIs). These
tools greatly simplify the process of designing and building GUIs. You can use
the GUIDE tools to
• Lay out the GUI.
Using the GUIDE Layout Editor, you can lay out a GUI easily by clicking
and dragging GUI components — such as panels, buttons, text fields,
sliders, menus, and so on — into the layout area. GUIDE stores the GUI
layout in a FIG-file.
• Program the GUI.
GUIDE automatically generates an M-file that controls how the GUI
operates. The M-file initializes the GUI and contains a framework for the
most commonly used callbacks for each component — the commands that
execute when a user clicks a GUI component. Using the M-file editor, you
can add code to the callbacks to perform the functions you want.
5-2
Laying Out a GUI
Laying Out a GUI
Starting GUIDE
Start GUIDE by typing guide at the MATLAB command prompt. This
displays the GUIDE Quick Start dialog box, as shown in the following figure.
From the GUIDE Quick Start dialog box, you can
• Create a new GUI from one of the GUIDE templates — prebuilt GUIs that
you can modify for your own purposes.
• Open an existing GUI.
5-3
5
Creating Graphical User Interfaces
The Layout Editor
When you open a GUI in GUIDE, it is displayed in the Layout Editor, which
is the control panel for all of the GUIDE tools. The following figure shows the
Layout Editor with a blank GUI template.
Layout Area
Component
Palette
You can lay out your GUI by dragging components, such as panels, push
buttons, pop-up menus, or axes, from the component palette, at the left side
of the Layout Editor, into the layout area. For example, if you drag a push
button into the layout area, it appears as in the following figure.
5-4
Laying Out a GUI
You can also use the Layout Editor to create menus and set basic properties
of the GUI components.
To get started using the Layout Editor and setting property values, see
“Creating a GUI” in the MATLAB documentation.
5-5
5
Creating Graphical User Interfaces
Programming a GUI
After laying out the GUI and setting component properties, the next step is
to program the GUI. You program the GUI by coding one or more callbacks
for each of its components. Callbacks are functions that execute in response
to some action by the user. A typical action is clicking a push button.
A GUI’s callbacks are found in an M-file that GUIDE generates
automatically. GUIDE adds templates for the most commonly used callbacks
to this M-file, but you may want to add others. Use the M-file Editor to edit
this file.
The following figure shows the Callback template for a push button.
To learn more about programming a GUI, see “Creating a GUI” in the
MATLAB documentation.
5-6
6
Desktop Tools and
Development
Environment
Desktop Overview (p. 6-2)
Access tools, arrange the desktop, and set preferences.
Command Window and Command
History (p. 6-5)
Run functions and enter variables.
Help Browser (p. 6-7)
Find and view documentation and demos.
Current Directory Browser and Search Manage and use M-files with MATLAB.
Path (p. 6-10)
Workspace Browser and Array Editor
(p. 6-12)
Work with variables in MATLAB.
Editor/Debugger (p. 6-14)
Create and debug M-files (MATLAB programs).
M-Lint Code Check and Profiler
Reports (p. 6-16)
Improve and tune your M-files.
Other Development Environment
Features (p. 6-19)
Interface with source control systems, and publish M-file
results.
6
Desktop Tools and Development Environment
Desktop Overview
Use desktop tools to manage your work in MATLAB. You can also use
MATLAB functions to perform the equivalent of most of the features found in
the desktop tools.
The following illustration shows the default configuration of the MATLAB
desktop. You can modify the setup to meet your needs.
Menus change, depending on the tool you
are currently using.
View or change the
current directory.
Click to move Command Window
outside of desktop (undock).
For More Information For an overview of the desktop tools, watch the video
tutorials, accessible by typing demo matlab desktop. For complete details, see
the MATLAB Desktop Tools and Development Environment documentation.
6-2
Desktop Overview
Arranging the Desktop
These are some common ways to customize the desktop:
• Show or hide desktop tools via the Desktop menu.
• Resize any tool by dragging one of its edges.
• Move a tool outside of the desktop by clicking the undock button
tool’s title bar.
in the
• Reposition a tool within the desktop by dragging its title bar to the new
location. Tools can occupy the same position, as shown for the Current
Directory and Workspace browser in the preceding illustration, in which
case, you access a tool via its tab.
• Change fonts and other options by using File -> Preferences.
6-3
6
Desktop Tools and Development Environment
Start Button
The MATLAB Start button provides easy access to tools, demos, shortcuts, and
documentation. Click the Start button to see the options.
For More Information See “Desktop” in the MATLAB Desktop Tools and
Development Environment documentation.
6-4
Command Window and Command History
Command Window and Command History
Command Window
Use the Command Window to enter variables and to run functions and M-file
scripts.
Run functions and
enter variables at the
MATLAB prompt.
MATLAB displays the
results.
Press the up arrow key to recall a statement you previously typed. Edit the
statement as needed and then press Enter to run it. For more information
about entering statements in the Command Window, see “Controlling
Command Window Input and Output” on page 2-28.
For More Information See “Running Functions” in the MATLAB Desktop
Tools and Development Environment documentation for complete details.
6-5
6
Desktop Tools and Development Environment
Command History
Statements you enter in the Command Window are logged in the Command
History. From the Command History, you can view previously run statements,
as well as copy and execute selected statements. You can also create an M-file
from selected statements.
Timestamp
marks the start
of each session.
Select one or more
entries and
right-click to copy,
evaluate, or create
an M-file from the
selection.
To save the input and output from a MATLAB session to a file, use the diary
function.
For More Information See “Command History” in the MATLAB Desktop
Tools and Development Environment documentation, and the reference page
for the diary function.
6-6
Help Browser
Help Browser
Use the Help browser to search for and view documentation and demos for all
your MathWorks products. The Help browser is an HTML viewer integrated
into the MATLAB desktop.
To open the Help browser, click the help button
in the desktop toolbar.
The Help browser consists of two panes, the Help Navigator, which you use to
find information, and the display pane, where you view the information. These
are the key features:
• Contents tab — View the titles and tables of contents of the documentation.
• Index tab — Find specific index entries (selected keywords) in the
documentation.
• Search tab — Look for specific words in the documentation.
• Demos tab — View and run demonstrations for your MathWorks products.
While viewing the documentation, you can
• Browse to other pages — Use the arrows at the tops and bottoms of the pages
to move through the document, or use the back and forward buttons in the
toolbar to go to previously viewed pages.
• Bookmark pages — Use the Favorites menu.
• Print a page — Click the print button in the toolbar.
• Find a term in the page — Click the find icon ( ) in the toolbar.
• Copy or evaluate a selection — Select text, such as code from an example,
then right-click and use a context menu item to copy the selection or evaluate
(run) it.
6-7
6
Desktop Tools and Development Environment
Tabs in the Help Navigator pane provide different ways to find information.
Use the close box to hide
the pane.
6-8
Drag the separator bar to adjust
the width of the panes.
View documentation in the
display pane.
Help Browser
Other Forms of Help
In addition to the Help browser, you can use help functions. To get help for a
specific function, use the doc function. For example, doc format displays
documentation for the format function in the Help browser.
To see a briefer form of the documentation for a function, type help followed by
the function name. The resulting help text appears in the Command Window.
It shows function names in all capital letters to distinguish them from the
surrounding text. When you use the function names, type them in lowercase or
they will not run. Some functions actually consist of both uppercase and
lowercase letters, and the help text clearly indicates that. For those functions,
match the case used in the help function.
Other means for getting help include contacting Technical Support
(http://www.mathworks.com/support) and participating in the newsgroup for
MATLAB users, comp.soft-sys.matlab.
For More Information See “Help for Using MATLAB” in the MATLAB
Desktop Tools and Development Environment documentation, and the
reference pages for the doc and help functions.
6-9
6
Desktop Tools and Development Environment
Current Directory Browser and Search Path
MATLAB file operations use the current directory and the search path as
reference points. Any file you want to run must either be in the current
directory or on the search path.
Current Directory
A quick way to view or change the current directory is by using the current
directory field in the desktop toolbar, shown here.
To search for, view, open, and make changes to MATLAB related directories
and files, use the MATLAB Current Directory browser. Alternatively, you can
use the functions dir, cd, and delete. Use the Visual Directory and Directory
Reports to help you manage M-files.
Change the
directory here.
This field only
appears here
when the Current
Directory browser
is undocked from
the desktop.
Double-click a
file to open it in
an appropriate
tool.
6-10
Search for files and content within text files.
For Visual Directory and Directory Reports
Current Directory Browser and Search Path
For More Information See “File Management Operations” in the MATLAB
Desktop Tools and Development Environment documentation, and the
reference pages for the dir, cd, and delete functions.
Search Path
MATLAB uses a search path to find M-files and other MATLAB related files,
which are organized in directories on your file system. Any file you want to run
in MATLAB must reside in the current directory or in a directory that is on the
search path. When you create M-files and related files for MATLAB, add the
directories in which they are located to the MATLAB search path. By default,
the files supplied with MATLAB and other MathWorks products are included
in the search path.
To see which directories are on the search path or to change the search path,
select File -> Set Path and use the resulting Set Path dialog box.
Alternatively, you can use the path function to view the search path, addpath
to add directories to the path, and rmpath to remove directories from the path.
For More Information See “Search Path” in the MATLAB Desktop Tools
and Development Environment documentation, and the reference pages for
the path, addpath, and rmpath functions.
6-11
6
Desktop Tools and Development Environment
Workspace Browser and Array Editor
The MATLAB workspace consists of the set of variables (named arrays) built
up during a MATLAB session and stored in memory. You add variables to the
workspace by using functions, running M-files, and loading saved workspaces.
Workspace Browser
To view the workspace and information about each variable, use the
Workspace browser, or use the functions who and whos.
To delete variables from the workspace, select the variables and select Edit ->
Delete. Alternatively, use the clear function.
The workspace is not maintained after you end the MATLAB session. To save
the workspace to a file that can be read during a later MATLAB session, select
File -> Save, or use the save function. This saves the workspace to a binary file
called a MAT-file, which has a .mat extension. You can use options to save to
different formats. To read in a MAT-file, select File -> Import Data, or use the
load function.
For More Information See “MATLAB Workspace” in the MATLAB Desktop
Tools and Development Environment documentation, and the reference pages
for the who, clear, save, and load functions.
6-12
Workspace Browser and Array Editor
Array Editor
Double-click a variable in the Workspace browser, or use openvar
variablename, to see it in the Array Editor. Use the Array Editor to view and
edit a visual representation of variables in the workspace.
View and change values
of array elements.
Arrange the display of
array documents.
Use document bar to view other variables you have open in the Array Editor.
For More Information See “Viewing and Editing Workspace Variables with
the Array Editor” in the MATLAB Desktop Tools and Development
Environment documentation, and the reference page for the openvar function.
6-13
6
Desktop Tools and Development Environment
Editor/Debugger
Use the Editor/Debugger to create and debug M-files, which are programs you
write to run MATLAB functions. The Editor/Debugger provides a graphical
user interface for text editing, as well as for M-file debugging. To create or edit
an M-file use File -> New or File -> Open, or use the edit function.
Comment selected lines and specify
indenting style using the Text menu.
Set breakpoints
where you want
execution to
pause so you
can examine
variables.
Hold the
cursor over a
variable and
its current
value appears
(known as a
data tip).
Use the document bar to access other
documents open in the Editor/Debugger.
6-14
Find and replace text.
Arrange the display of
documents in the Editor.
Editor/Debugger
You can use any text editor to create M-files, such as Emacs. Use preferences
(accessible from the desktop File menu) to specify that editor as the default. If
you use another editor, you can still use the MATLAB Editor/Debugger for
debugging, or you can use debugging functions, such as dbstop, which sets a
breakpoint.
If you just need to view the contents of an M-file, you can display the contents
in the Command Window using the type function.
For More Information See “Editing and Debugging M-Files” in the
MATLAB Desktop Tools and Development Environment documentation, and
the function reference pages for edit, type, and debug.
6-15
6
Desktop Tools and Development Environment
M-Lint Code Check and Profiler Reports
MATLAB provides tools to help you manage and improve your M-files,
including the M-Lint Code Check and Profiler Reports.
M-Lint Code Check Report
The M-Lint Code Check Report displays potential errors and problems, as well
as opportunities for improvement in your M-files. The term “lint” is used by
similar tools in other programming languages such as C.
Access the M-Lint Code Check Report and other directory reports from the
Current Directory browser. You run a report for all files in the current
directory. Alternatively, you can use the mlint function to get results for a
single M-file.
Directory reports
In MATLAB, the M-Lint Code Check Report displays a message for each line
of an M-file it determines might be improved. For example, a common M-Lint
message is that a variable is defined but never used in the M-file.
6-16
M-Lint Code Check and Profiler Reports
The report
displays a line
number and
message for
each potential
problem or
improvement
opportunity.
Click a line
number to open
the M-file in the
Editor at that
line.
For More Information See “Tuning and Managing M-Files” in the
MATLAB Desktop Tools and Development Environment documentation, and
the reference page for the mlint function.
6-17
6
Desktop Tools and Development Environment
Profiler
MATLAB includes the Profiler to help you improve the performance of your
M-files. Run a MATLAB statement or an M-file in the Profiler and it produces
a report of where the time is being spent. Access the Profiler from the Desktop
menu, or use the profile function.
For More Information See “Tuning and Managing M-Files” in the
MATLAB Desktop Tools and Development Environment documentation, and
the reference page for the profile function.
6-18
Other Development Environment Features
Other Development Environment Features
Additional development environment features include
• Source Control — Access your source control system from within MATLAB.
•Publishing Results — Use the Editor’s cell features to publish M-files and
results to popular output formats including HTML and Microsoft Word. You
can also use MATLAB Notebook to access MATLAB functions from within
Microsoft Word.
For More Information See “Source Control” and “Publishing Results” in
the MATLAB Desktop Tools and Development Environment documentation.
6-19
6
Desktop Tools and Development Environment
6-20
Index
Symbols
: operator 2-7
axis
labels 3-49
titles 3-49
A
axis function 3-48
algorithms
vectorizing 4-31
animation 3-71
annotating plots 3-16
ans function 2-4
application program interface (API) 1-3
Array Editor 6-13
array operators 2-22
arrays
cell 4-11
character 4-13
columnwise organization 2-24
creating in M-files 2-15
deleting rows and columns 2-17
elements 2-10
generating with functions and operators 2-14
listing contents 2-10
loading from external data files 2-15
and matrices 2-21
multidimensional 4-9
notation for elements 2-10
preallocating 4-31
structure 4-16
variable names 2-10
arrow keys for editing commands 2-30
aspect ratio of axes 3-48
AVI files 3-71
axes
managing 3-48
visibility 3-49
B
bit map 3-59
break function 4-7
built-in functions
defined 2-12
C
callbacks 5-6
case function 4-4
catch function 4-7
cell arrays 4-11
char function 4-15
character arrays 4-13
characteristic polynomial 2-21
colon operator 2-7
colormap 3-54
colors
lines for plotting 3-41
Command History 6-6
command line
editing 2-30
Command Window 6-5
complex numbers
plotting 3-43
concatenation
defined 2-16
of strings 4-14
constants
special 2-12
Index-1
Index
continue function 4-6
continuing statements on multiple lines 2-30
control keys for editing commands 2-30
current directory 6-10
Current Directory browser 6-10
D
data source 3-27
debugging M-files 6-14
deleting array elements 2-17
demos
running from the Start button 6-4
desktop for MATLAB 1-7
desktop tools 6-1
determinant of matrix 2-19
diag function 2-4
documentation 6-7
E
editing command lines 2-30
Editor/Debugger 6-14
eigenvalue 2-20
eigenvector 2-20
elements of arrays 2-10
entering matrices 2-3
erase mode 3-71
eval function 4-27
executing MATLAB 1-6
exiting MATLAB 1-6
exporting graphs 3-35
expressions
evaluating 4-27
examples 2-13
using in MATLAB 2-10
Index-2
F
figure function 3-46
figure tools 3-7
figure windows 3-46
with multiple plots 3-46
find function 2-27
finding object handles 3-69
fliplr function 2-6
floating-point numbers 2-11
flow control 4-2
for loop 4-5
format
of output display 2-28
format function 2-28
function functions 4-28
function handles
defined 4-28
using 4-30
function keyword 4-22
function M-files 4-19
naming 4-21
function of two variables 3-52
functions
built-in, defined 2-12
defined 4-21
how to find 2-11
running 6-5
variable number of arguments 4-22
G
global variables 4-25
graphical user interface
creating 5-1
laying out 5-3
programming 5-6
Index
graphics
files 3-60
Handle Graphics 3-62
objects 3-63
printing 3-60
grids 3-49
GUIDE 5-1
line continuation 2-30
line styles of plots 3-41
load function 2-15
loading arrays 2-15
local variables 4-22
log of functions used 6-6
logical vectors 2-26
H
M
Handle Graphics 3-62
defined 1-3
finding handles 3-69
Help browser 6-7
help functions 6-9
hold function 3-44
magic function 2-8
I
if function 4-2
images 3-58
imaginary numbers 2-10
K
keys for editing in Command Window 2-30
L
legend
adding to plot 3-40
legend function 3-40
library
mathematical function 1-3
lighting 3-55
limits
axes 3-48
magic square 2-4
markers 3-41
MAT-file 3-58
mathematical function library 1-3
mathematical functions
listing advanced 2-11
listing elementary 2-11
listing matrix 2-11
MATLAB
application program interface 1-3
desktop 1-7
executing 1-6
exiting 1-6
history 1-2
language 1-3
mathematical function library 1-3
overview 1-2
quitting 1-6
running 1-6
shutting down 1-6
starting 1-6
MATLAB user newsgroup 6-9
matrices 2-18
creating 2-14
entering 2-3
Index-3
Index
matrix 2-2
antidiagonal 2-6
determinant 2-19
main diagonal 2-5
multiplication 2-18
singular 2-19
swapping columns 2-8
symmetric 2-18
transpose 2-4
mesh plot 3-52
M-files
and toolboxes 1-2
creating 4-19
editing 6-14
for creating arrays 2-15
function 4-19
script 4-19
Microsoft Word and access to MATLAB 6-19
movies 3-73
multidimensional arrays 4-9
multiple data sets
plotting 3-40
multiple plots per figure 3-46
multivariate data
organizing 2-24
N
newsgroup for MATLAB users 6-9
Notebook 6-19
numbers 2-10
floating-point 2-11
O
object properties 3-65
Index-4
objects
finding handles 3-69
graphics 3-63
online help
viewing 6-7
operators 2-11
colon 2-7
output
controlling format 2-28
suppressing 2-30
overlaying plots 3-44
P
path 6-11
plot function 3-38
plots
editing 3-16
plotting
adding legend 3-40
adding plots 3-44
basic 3-38
complex data 3-43
complex numbers 3-43
contours 3-44
editing 3-16
functions 3-38
line colors 3-41
line styles 3-41
lines and markers 3-41
mesh and surface 3-52
multiple data sets 3-40
multiple plots 3-46
overview 3-2
tools 3-9
PostScript 3-61
preallocation 4-31
Index
presentation graphics 3-29
print function 3-60
Printing 3-33
printing
example 3-33
graphics 3-60
Profiler 6-18
Property Editor
interface 3-19
Property Inspector 3-18
Q
quitting MATLAB 1-6
R
return function 4-8
revision control systems
interfacing to MATLAB 6-19
running functions 6-5
running MATLAB 1-6
S
scalar expansion 2-25
scientific notation 2-10
script M-files 4-19
scripts 4-20
search path 6-11
semicolon to suppress output 2-30
shutting down MATLAB 1-6
singular matrix 2-19
source control systems
interfacing to MATLAB 6-19
special constants
infinity 2-12
not-a-number 2-12
specialized graphs 3-45
Start button 6-4
starting MATLAB 1-6
statements
continuing on multiple lines 2-30
executing 4-27
strings
concatenating 4-14
structures 4-16
subplot function 3-46
subscripting
with logical vectors 2-26
subscripts 2-6
sum function 2-4
suppressing output 2-30
surface plot 3-52
switch function 4-4
symmetric matrix 2-18
T
text
entering in MATLAB 4-13
TIFF 3-61
title
figure 3-49
toolboxes 1-2
tools in the desktop 6-1
transpose function 2-4
try function 4-7
Index-5
Index
V
variables 2-10
global 4-25
local 4-22
vectorization 4-31
vectors 2-2
logical 2-26
preallocating 4-31
version control systems
interfacing to MATLAB 6-19
visibility of axes 3-49
W
while loop 4-5
windows for plotting 3-46
windows in MATLAB 1-7
wireframe
surface 3-52
Word and access to MATLAB 6-19
word processing access to MATLAB 6-19
workspace 6-12
Workspace browser 6-12
X
xor erase mode 3-71
Index-6
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