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A GUIDE TO MATLAB Fifth Edition Roger Herz-Fischler Ce livre est d´ edi´ e ` a la m´ emoire de mon ami Albert Badrikian math´ ematicien, homme d’esprit, sportif, et amateur de bons mots Mzinhigan Publishing

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• A GUIDE TO

MATLAB

Fifth Edition

Roger Herz-Fischler

Ce livre est dédiéà la mémoire de mon ami

mathématicien, homme d’esprit, sportif,et amateur de bons mots

Mzinhigan Publishing

• Canadian Cataloguing in Publication Data

Herz-Fischler, Roger

A guide to MATLAB, fifth edition

ISBN 0-9693002-4-7

1. MATLAB (Computer file). 2. Numerical analysis–Data Processing. I. Title

Permission is given to redistribute and modify this work on a strictly non-commercial basis. Edu-cational institutions and student organizations may distribute the whole text or parts of the text ona cost plus basis.

MATLAB is a trademark of The Mathworks, Inc.

MZINHIGAN PUBLISHING340 Second Avenue, Ottawa, Ontario

MZINHIGAN is the word for “book” in the Odawa (Ottawa) dialect of Ojibwa.The logo is “Mzinhigan” written in the related Algonquin language of Cree—asspoken on the west coast of James Bay—using Cree syllabics.

• HOW TO USE THIS GUIDE Before turning the page to the table of contents you shouldconsider the following options:

1. You want to see immediately how MATLAB will solve equations or do eigenvalue problems: Type matlab at the system prompt. Turn to sections III.1 and/or III.2 and type in the examples.

2. You are a first time user and you want to have a good idea of how MATLAB can do straightforward matrix type calculations: Do the sample MATLAB session in section I.1 (10 - 15 minutes). Refer to the table in section II.3 for the MATLAB operations of interest to you. Turn to sections III.1, III.2, III.3 for sample sessions dealing with equations, eigenvalues and matrix models.

3. You want to learn more about MATLAB At the beginning of each section there is a summary together with a quick reference for the commands. The summary is followed by a more detailed explanation of the commands, together with illustrative examples and programs. Read sections I.2, I.3, II.1, II.2. Read sections I.4, I.5, II.4, II.5. Read sections II.7, II.8.

4. You are mainly interested in graphing. The section on the graphing of vectors and matrices (II.6) has been made as independent as possible of the rest of the guide. Graphing of user defined functions (section II.7) requires a knowledge of sections I.3 and II.1.

• INTRODUCTION

"Qu'on ne dise pas que je n'ai rien dit de nouveau: La disposition des matières estnouvelle." - Pascal, Pensées, number 22 (Edition L. Brunschvieg, Hachette, 1950, p. 30)

MATLAB is a program which is especially designed for matrix type operations. The following examples illustrate, without any explanations for the present, some of the range of computations that MATLAB was designed to do.

Pocket type calculator for linear algebra We enter the matrix A:

>> A = [.2 .6;.4 .3]

and find the inverse using the MATLAB function inv( ):

>> A_inv = inv( A )

-1.6667 3.3333 2.2222 -1.1111

Then we check using multiplication:

>> A_inv * A

1 0 0 1

For A as above we find, using the MATLAB function eig( ), the eigenvalues and store them in a diagonal matrix EIG_VAL. At the same time we find the corresponding eigenvectors and store them as the columns of a matrix EIG_VEC:

>> [EIG_VEC , EIG_VAL] = eig( A )

EIG_VEC =

-0.8048 -0.7418 0.5935 -0.6706

EIG_VAL =

-0.2424 0 0 0.7424

• Next, we pull out the first column of EIG_VEC so as to obtain the eigenvector ready for further computations:

>> eigvec1 = EIG_VEC( : , 1 )

-0.8048 0.5935

Programs

At the next level we can program MATLAB to do a sequence of calculations and/or determine when certain conditions are met. If for example we want to determine the first power of P for which all the elements are less than .1, we need only write the following:

P = [.2 .7; .4 .3]n = 1;

while max(max(P^n)) > .1 n = n + 1;end

disp('first index when power

• The cover shows a typical outcome of calculations involving an "initial vector" multiplied by powers of a 3#x 3 "stochastic matrix". Each of the lines represents the values associated with each of 3 states as time evolves. The values were stored in a matrix M and the graph was produced by the simple command:

>> plot( index , M )----

The purpose of this guide is to show the reader how to use MATLAB for the above and other purposes. It is called a guide rather than a manual because of the way it has been organized. The organization is by activity rather than by commands. For quick reference these activities, as well as the associated commands, are listed at the top of each section in which they are discussed. Then the sections continue with a discussion of the commands and examples which illustrate the use of the command with real numbers. References are made to later examples where the function is used again. I have deliberately avoided the use of arcane symbols which, according to my experience with classes, serve mainly to confuse and frustrate users.

My philosophy while writing this guide has been the following:

a. Only those functions and variations which have been found to be most useful should be included; too many functions and too many variations make for confusion and abandon, especially for first time users.

b. Informal suggestions should be made to make life simpler and to point out common errors.

The guide starts off with a sample MATLAB session in Section I.1 so that the first time user can immediately start to do calculations. All the common MATLAB operations and functions are discussed and illustrated in Chapter II, which has been split into eight subsections for quick reference.

Once the user is ready for more sophisticated material, they can consult sections I.2 (the five basic MATLAB symbols) and I.3 (simple loops and functions). With these two sections, the user is ready for advanced manipulations. Sections I.4 (advanced importing and exporting) and I.5 (formatting the output) can be read when needed.

Three special sections at the end give more detailed and advanced examples dealing with the solution of equations, eigenvalue problems, and matrix power models.

Overall, MATLAB produces quick solutions to straightforward matrix problems, via simple transparent commands. For more complicated problems, MATLAB programs tend to be very concise and fairly easy to write (see e.g. the program for the spectral decomposition of a matrix in section III.8). For plotting graphs based on vectors MATLAB is, in my opinion, a sheer delight (see sections II.6 and

• III.3) with the combination of flexibility, simplicity and ease of use being unmatched by any other software package. As far as accuracy is concerned, the reader is invited to look at example 5 of section III.1 where the solution of 1000 non-homogeneous equations in 1000 unknowns is discussed.

This book contains shortened versions of various programs that I have written for my classes. Complete programs can be obtained free of charge by sending an electronic-mail message to me at [email protected] .

EDITORIAL CONVENTIONS

MATLAB functions are given in boldface type, at the principal points of discussion e.g. diary calcul1.dia or inv(A).

System commands e.g. RETURN are given in uppercase letters.

MATLAB programs that I have written are printed in courier type. The numbering starts again in each section, and is referred to by section, e.g program 1 of sectionIII.1. As mentioned above, the programs used to produce the actual output have been printed here in shortened form so as not to obscure the major features.

MATLAB output is also printed in courier type. Sometimes, especially in the earlier sections, my comments have been interspersed with the output so as to provide arunning commentary on what is happening; these are printed in Helvetica as is the main text. Sometimes, italics have been used in the output for clarity. At othertimes, the comments have been put in the programs or added to the MATLAB commands using the MATLAB command % .

The output is that produced by MATLAB version 4.0a (dated December 11, 1992) as installed on the Carleton University UNIX machine called "superior". Various redundant or blank lines have been edited out and spacing has sometimes been changed. When the output involves complex numbers, I have often put braces {###} around them for emphasis.

Example numbering starts again in each section, and is referred to by section, e.g example 1 of section I.2.

I find that it is good practice to indicate vectors and scalars by lower case letters, e.g. "a" or "vect_1" and matrices that are not vectors by upper case letters, e.g. "A" or "STORE" and I have employed this device here.

Another good practice is to use the extensions *.dia and *.gra, so as to easily locate diary and graph files. This practice has been followed here.

TECHNICAL NOTES

MATLAB has implementations on various systems, but any differences are related to how the system works rather than the MATLAB program itself. An "*.m" file

• written in ASCII format for one system will work on another system. The greatest difference between systems will involve the printing out of graphs; see item 3 below.

Most of the basic commands in MATLAB have stabilized, but since MATLAB is being constantly refined, some of the commands have been changed. If you created an "*.m" file on one system and then transferred it to another only to find that it no longer works, it is probably due to a change in syntax. For example olderimplementations used the command eye(A) to create an identity matrix of the same size as the square matrix A. Newer versions however require the command eye( size(A) ); see Section II.2. Some of these changes have been noted in the text, but it is possible that MATLAB will respond to a command with an error message on the particular system that you are using. You should then use the command help axis etc. to find what the proper convention is. For some applications, in particular in connection with plotting and printing, the number of options is very large. I have only listed the most common and most useful options,but the others may be determined by typing help plot, help axis, help title, help print.

There is a summary of the commands at the beginning of each section Thissummary is followed by more detailed explanations of the commands, illustra-tive examples and programs.

Many of files that appear in this book can be found at:people.math.carleton.ca /~rhfischl/

Chapter I Running MATLAB

I.1 A sample MATLAB session

Basics of MATLAB: starting, quitting, entering matrices, “diaries”, lists

of variables, clearing, executing system commands while in MATLAB

I.2 Five special symbols [ ] ( ) ; , :

I.3 Programs, simple loops and functions

i. Creating and importing program and data files

ii. Pauses

iii. “For-loops”

iv. Creating and using your own functions

I.4 Advanced importing and exporting of data techniquesi. Importing data from a file

ii. Long lines of data

iii. Importing data from the keyboard; using the input function

iv. Saving variables to “.mat” files

vi. Saving one variable in ASCII format

I.5 Formatting the output; tables

i. Using the disp command; tables

ii. The number of decimal places in the output

Chapter II. MATLAB operations

II.1 Basic MATLAB matrix and pointwise operations

i. Transpose

iii. Multiplication

• iv. Powers

v. Element by element multiplication

vi. Element by element division

vii. Exponential type (pointwise and matrix)

II.2 Six special matrices

i. identity matrix

ii. matrix of ones

iii. matrix of zeros

iv. matrix of random elements

v. row vectors of indices

vi. Empty matrix

II.3 Common MATLAB functions

i. Linear algebra

ii. Matrix manipulations

iii. Pi, roots, complex numbers, zeros of polynomials

iv. Summing, maximum etc.

v. Real and imaginary parts etc.

vii. Exponential type functions

viii. Trigonometric functions

ix. Timing

II.4 Extracting and Building Matrices

i. Building matrices from vectors

ii. Extracting rows and columns from matrices

iii. Extracting several rows or columns

iv. Replacing a column or row by a vector

v. Building matrices from submatrices

vi. Replacing a block of a matrix by a given matrix

vii. Building matrices recursively by assignment and by con-

catenation; the empty matrix: [ ]

viii. Storing and “fetching” sequences of matrices

ix. Building a diagonal matrix from a vector

x. Building matrices with many zeros

xi. Forming a vector from the elements of a matrix

II.5 Matrix column operations (summing etc.) and sorting

• Column functions.

Sorting

II.6 Graphing vectors and matrices

II.7 Numerical Analysis

i. Defining functions

ii. Finding the roots of a polynomial

iii. Finding the roots of a defined function

iv. Finding the maximum and minimum of a function

v. Finding the definite integral of a function

vi. Plotting defined functions

vii. Curve Fitting

vii. Estimating Values / Interpolation

II.8 Logical operators and advanced programming

i. Relational symbols

ii. Logical operators

iii. Conditional loops

iv. Breaking out of a program; error statements

v. “While loops”

vi. Breaking out of loops

vii. Conditional vectors and matrices

viii. Testing conditions with any and all

ix. Finding indices corresponding to a condition

x. An illustrative example: four methods

Chapter III. Special Topics

III.1 Linear equations

i. General systems

ii. An infinite number of parametric solutions

iii. No solutions

iv. Cramer’s rule

v. Inverses

vi. Using the division operator; ”LU decompositions”

vii. Large systems of equations

viii. Linear independence

III.2 Eigenvalues and eigenvectors

• i. Right eigenvectors and eigenvalues

ii. Left eigenvectors and eigenvalues

iii. Spectral decompositions

III.3 Matrix power models

i. Norms and orthogonalization

ii. Null spaces

iii. Orthogonal transformations

iv. Eigenvalues and eigenvectors

v. The similarity problem, diagonalization

vi. The Perron-Frobenius theorem

• SECTION I.1. BASICS

CHAPTER I: RUNNING OCTAVE/MATLAB

SECTION I.1. BASICS

SUMMARY

If this is your first session with OCTAVE/MATLAB, go through the following OCTAVE/MATLAB session and then do another session with a few of the functions in section II.3.

i. To start OCTAVE/MATLAB.

Type OCTAVE/MATLAB. at the comand line.

ii. To quit OCTAVE/MATLAB.

Type quit.

iii. To save your work in a diary file calcul1.dia.

Open a "diary" using the diary command: diary calcul1a.dia. (the extension .dia helps you identify your diary files)

To toggle the diary: diary off, diary on.

To end the diary session: diary off.

A VERY STRONG SUGGESTION

In principle, do everything without a diary. In practice however, especially when you are learning or when you run a program more than once, or if you run several programs, you are going to spend a lot of frustrating time figuring out what went wrong, what you want to change etc. Diaries permit you to direct the output of different trials or programs to different files. Those files that you want to retain will be small and easily edited.

Suppose that you have a functions tele1.m and tele2.m that you wrote to analyse a telephone system. You want to run them, or debug them, for different sets of values. An efficient way to keep track of what you doing is to create different diaries for different trials and lable them: tele1a.dia, tele1b.dia ..., This way you when you look at your files, you will immediately see that you are dealing with thefirst, second ... diaries for functions tele1.m and tele2.m. You can check these diaries in the middle of a session by using the ! command as in part v. below.

• SECTION I.1. BASICS

iv. Editing in the middle of a session.

Use up arrow, down arrow, delete/backspace, or ENTER commands on the keyboard.

[Note: These commands are very useful as they help avoid the necessity of retyping an entire line. These commands are also valid in window versions.]

v. Returning to the system.

Use ! followed by commands such as DIR, TYPE, LS or by giving the name of a text editor. At the end of the system operation control returns to OCTAVE/MATLAB.

Use % to place comments at the OCTAVE/MATLAB prompt or after OCTAVE/MATLAB commands.

vii. List of variables.

Type whos.

viii. To clear the work space.

Type clear. To clear just some variables: clear A1, ans

ix. To clear the screen (not the work space).

Type clc (= clear command).

x. To list the files in the current directory.

.ls

Notes AND WARNINGS

a. OCTAVE/MATLAB is case sensitive; thus variables A and a are not the same. [If you really do not like case sensitive programs then use casesen off and casesen on to toggle, but first read the next suggestion].

b. To help you distinguish matrices from vectors and scalars it is suggested that you use uppercase for the former and lowercase for the latter, e.g. A or STORE for matrices and a or vect_1 for scalars and vectors.

• SECTION I.1. BASICS

c. Variable names can be any length and may contain an underbar e.g. MATRIX_1. Do not write MATRIX-1, because OCTAVE/MATLAB will think that you aresubtracting. Do not write 1_MATRIX (i.e. don't start the name with a number) as OCTAVE/MATLAB thinks that you are3 dealing with a number. d. Do not use the name of a OCTAVE/MATLAB function for your variables 3because OCTAVE/MATLAB will think that you are trying to evaluate the function and forgot the ( ). Thus since det( ) is a built in OCTAVE/MATLAB function, if you want to designate the determinant of your ou.ls3tput matrix, call it deter or d, but not det! A very common error is to write something similar to: >> sum = 2 + 5 If you next use the OCTAVE/MATLAB function sum( ) you will find that it does not work! >> b = [2 5] >> sum(b) OCTAVE/MATLAB responds with the error message:

" ???? index exceeds matrix dimensions. " Instead of calling your variable sum, call it sum1 or total. Similarly do not use prod for product, or mean for the mean (average). e. OCTAVE/MATLAB functions are all lowercase and use parentheses. The name usually gives a good idea of what the function does e.g inv(A) finds the inverse ofA and rref(A) give the reduced row ech.lselon form of A. Some functions however return a pair of matrices. f. OCTAVE/MATLAB is often able to interpret statements that are incorrect in the sense that they do not stand for what you wanted them to stand for. In particular you can often get away without using parentheses, but not only may the answer be different from what you had intended (see II.1), but also later you won't be able to understand what you had written. So USE PARENTHESES. Always test pro-grams and defined functions on examples for which you can do a hand check. g. OCTAVE/MATLAB is now fairly stable as far as the names of commands are concerned. If you use a command that has been replaced, you will usually see a message that gives you the new form of the command. h. If you quit OCTAVE/MATLAB, then return to OCTAVE/MATLAB and use a diary name that you have previously used, then OCTAVE/MATLAB adds to the previous diary file.

----

A SAMPLE OCTAVE/MATLAB SESSION

Here is a sample OCTAVE/MATLAB session.

First we call up OCTAVE/MATLAB at the system co.lsmmand:

{superior:1} OCTAVE/MATLAB .ls

• SECTION I.1. BASICS

We now open a "diary" file called calcul1.dia in wh.lsich to save the commands and output of the session. If you add the suffix ".dia" it will be easier to locate the diary when you go to edit it.

>> diary calcul1.dia

To enter a matrix to which we assign the variable name A, type in the matrix separating the rows by a semicolon. Push ENTER and OCTAVE/MATLAB automatically types out the variable name and the answer, unless commanded not to do so by the use of a " ; ":

>> A = [.2 .6; .4 .3]

A =

0.2000 0.6000 0.4000 0.3000

I now want to find the inverse matrix which I call A_inv. % inv( ) is the OCTAVE/MATLAB function for finding inverses:

>> A_inv = inv(A)

A_inv =

-1.6667 3.3333 2.2222 -1.1111

Check using multiplication, there is no need to give a variable name. Whenever novariable name has been assigned to an operation OCTAVE/MATLAB assigns the value to the variable ans.:

>> A*A_inv

ans =

1.0000 0.0000 -0.0000 1.0000

I now decide to see what happens if I replace the .4 by .1. Instead of retyping the entire matrix, I use the up arrow until I arrive at the data statement. By using the arrows, DEL or BACKSPACE, I change the .4 to .1. I also decide to change the name of the matrix to A2. When ENTER is pushed the new A2 appears:

>> A2 = [.2 .6; .1 .3]

• SECTION I.1. BASICS

A2 = .ls 0.2000 0.6000 0.1000 0.3000

If I now use the up arrow to go to A_inv and change A to A2 and then push RETURN, the command is repeated. Or I could just retype

>> A2_inv = inv(A2)

Warning: Matrix is singular to working precision.

OCTAVE/MATLAB has come up with an error message.

I could make more changes, but I have done enough on this problem, so I close the diary

>> diary off

All of a sudden, I remember that I want to find the determinant of A. So I turn the diary on and make sure that I turn it off again at the end. The new material is added at the end of the diary.

>> diary on

In order to continue with the computations, the names of the variables and their dimensions must be known. This is done via the OCTAVE/MATLAB command whos:

>> whos

Name Size Total Complex

A 2 by 2 4 No A2 2 by 2 4 No A2_inv 2 by 2 4 No A_inv 2 by 2 4 No ans 2 by 2 4 No

The variable ans came about from the check on A_inv to which no variable name had been assigned. If we had done this several times only the last ans is kept.

I now proceed

>> determinant = det(A)

• SECTION I.1. BASICS

determinant =

-0.1800

>> diary off % close the diary

We could now start a whole new set of computations; first clear all the variables and then open a new diary:

>> clear

>> diary calcul2.dia

or we could just clear A2 and its inverse

>> clear A2, A2_inv

We now go through a series of calculations using some of the operations of section II.1 or the functions of section I.3. When we are done we close the diary:

>> diary off

At this point I want to see what the results were. There is only one problem, I can'tremember the name of the first diary!! (We could also have used the UP ARROW to find the original diary command.)

To produce a list of *.dia files I type ! at the OCTAVE/MATLAB command line to invoke the system directory commands. For UNIX systems, the directory command is "ls". For DOS systems, use DIR *.dia and for window systems click onthe file listings. >> ! ls *.dia

From the file listing, I remember that the diary name was calcul1.dia.

To actually see what is in the file calcul1.dia, I use either a word processor or a viewing command. At this point, I can either leave OCTAVE/MATLAB or use the system command " ! " :

>> ! {name of text editor} calcul1.dia % invoke the editor

The viewing command in UNIX is "more". For DOS systems use TYPE calcul1.dia:

• SECTION I.1. BASICS

>> ! more calcul1.dia

I can now continue my calculations, with the same or another diary. If I want a clear screen I use clc

>> clc

To leave OCTAVE/MATLAB:

>> quit

The file calcul1.dia can now be edited and printed using a text editor.

• SECTION I.2. FIVE SPECIAL SYMBOLS: [ ] ( ) ; , :

SUMMARY

These five symbols are the most important in MATLAB. They have several uses and are used in conjunction with one another. The summary is grouped by symbol, whereas the example section is grouped by function.

NOTE: In order to make the symbols stand out, space has been left between symbols throughout this guide. This is not necessary when actually using MATLAB,but neither does it affect the result.

i The square bracket [ ]

To build matrices: A = [1 2 ]

ii The parentheses ( )

To indicate functions: B = eig(C)

To extract elements: b = A(1 , 2) for matrices b = a(3) for vectors

To redefine elements: A(1 , 2) = -5

iii. The semicolon ;

To separate the rows of a matrix: A = [1 2 ; 3 4] To place submatrices under one another: C = [A ; B] To suppress printing: A = [1 2 3 4];

iv. The comma ,

To place matrix blocks side by side: B = [vec1 , vec2] To extract elements of a matrix: a = C(2 , 3) To separate functions on a line: a = 1 , B = eig(C)

v. The colon :

1 to 4 by steps of 1: 1 : 4 -1 to 2 by steps of .5: -1 : .5 : 2 5 to 1 in decreasing steps of 1: 5 : -1 : 1

-- DETAILS AND EXAMPLES

i. The use of [ ] to indicate the building of a matrix.

• SECTION I.2. FIVE SPECIAL SYMBOLS: [ ] ( ) ; , :

The use of ; and , as matrix separators.

When we enter the elements of a matrix, we use square brackets, and separate the rows (which can be thought of as row vectors) by a semicolon.

>> A = [1 2 3 ; 4 5 6] % two vectors on separate lines

A = 1 2 3 4 5 6

>> B = [7 8 9 ; 10 11 12]

B = 7 8 9 10 11 12

We can now join the two matrices A and B in two ways to form a new matrix:

We can put B underneath A by using a semicolon.

>> C = [A ; B] % place submatrix B under A

C = 1 2 3 4 5 6 7 8 9 10 11 12

We can put A and B next to one another by using a comma.

>> D = [A , B]

D = 1 2 3 7 8 9 4 5 6 10 11 12

ii. The use of ( ) as a function indicator.

If we wanted to find the rank of C, we would use the rank function rank( ). The dependent variable is placed inside the parentheses.

>> rank_C = rank(C)

rank_C =

• SECTION I.2. FIVE SPECIAL SYMBOLS: [ ] ( ) ; , :

2

iii. The use of ( ) and , to extract or redefine an element.

To find the element in row 2 and column 3 of A above we use parentheses which isthe extraction function indicator and then enter the row and column values separated by the comma:

>> b = A(2 , 3) % row 2 , column 3 6

If a is a vector then all we have to do is give the element number:

>> a = [1 3 5];

>> b = a(2) 3

On the other hand suppose that we wanted to change the element in row 2 and column 3 so that it is -9, while leaving all the other elements alone. There is no need to retype all of A. We just specify the new value.

>> A(2,3) = -9 % redefine row 2, column 3

A = 1 2 3 4 5 -9

The use of the colon to extract submatrices is discussed in subsection v, below and in section II.4

iv. The use of : to indicate the "range" of indices.

The colon could be loosely translated by the term "range". Thus if we wanted a row vector with the integer range -1 to 4 we write:

>> x = [-1 : 4]

-1 0 1 2 3 4

Note that since we are building a vector we use brackets, [ ].

If we wanted to go from -1 to 2 by steps of .5, we would include the .5 in the

• SECTION I.2. FIVE SPECIAL SYMBOLS: [ ] ( ) ; , :

statement. Note that there are now two colons.

>> x_step = [-1 : .5 : 2]

-1.0000 -0.5000 0 0.5000 1.0000 1.5000 2.0000

To generate a vector in reverse order use negative steps.

>> X_dec = [5 : -1 : 1]

X_dec =

5 4 3 2 1

[Note: Writing [5 : 1] would give the empty matrix [ ]; see section II.2.vi].

For examples of the use of the colon, see Section II.1, example 1 and Section II.4

v. The use of : when extracting submatrices.

Subsection iv illustrated the used of the colon as a range indicator. By combining the colon with the extraction function of subsection iv, we can extract rows, columns and submatrices.

To extract the second row of A we fix the row value at 2 and let the column values range over all values as indicated by the colon.

>> row_2 = A(2 , : ) % the entire column range in row 2

4 5 6

To extract the third column of A we fix the column value at 3 and let the row values range over all values as indicated by the colon.

>> col_3 = A( : , 3) % the entire row range in column 3

3 6

If we only wanted to extract columns 2 through 3 of row 2 of A we would limit the column range by use of [2 : 3]

>> row_2_shortened = A(2 , [2 : 3]) % row 2, columns 2 to 3 5 6

• SECTION I.2. FIVE SPECIAL SYMBOLS: [ ] ( ) ; , :

We could also write:

>> indices = [2 : 3]

2 3

and then

>> row_2_shortened = A(2 , indices)

5 6

More examples of the use of the colon are given in section II.4 which deals with extracting and building matrices.

vi. The use of ; to suppress printing.

We reenter B, but this time we put a semicolon after the bracket to suppress printing. Note that the semicolon is also used inside the brackets to separate the rows:

>> B = [7 8 9 ; 10 11 12] ; % outside to suppress printing

To print B we simply type:

>> B

B =

7 8 9 10 11 12

To print just the matrix without "B = " we would type

>> disp(B) % see section I.5.i ].

To suppress printing several variables in a row we put a semicolon after each:

>> B; C;

vii. Using , to place commands on the same line.

If we wanted to find the ranks of C and D, we could write the two commands on separate lines or use a comma.

>> rank_C = rank(C) , rank_D = rank(D)

• SECTION I.2. FIVE SPECIAL SYMBOLS: [ ] ( ) ; , :

rank_C = 2

rank_D =

2

• SECTION I.3. PROGRAMS, SIMPLE LOOPS AND FUNCTIONS

SUMMARY

i. Creating and importing your own program or data file.

Use a text editor to create an "m-file", e.g. "program1.m" in which you have typedthe instructions, and data, exactly as you would do at the MATLAB command line.Invoke the program by typing the name without the extension:

>> program1

[Note: Do not start file names with numbers, e.g. "1program.m" is not a valid name.]

ii. Pauses

To have a program pause until return is pushed: pauseTo have a program pause for 10 seconds: pause(10)

iii. "For-loops"

for n = 1 : 5 P^nend

[Note: MATLAB is most efficient when working with vectors(or matrices). Thus whenever possible work with vectorsinstead of using loops. See Section II.1, example 1 for theconciseness and power of the vector approach.]

iv. Creating and using your own functions

Create the file trans1.m which starts off with: function NEW = trans1(OLD)to evaluate a function at the variable A:

>> B = trans1(A).

[Note: For function names, as well as for the variables, do not use the names of MATLAB functions e.g. "det" .

Note: In certain versions you can use capital letters for your functions; e.g. in Section II.3 R( ) is defined to be equal to the MATLAB function sqrt( ).]

--

• SECTION I.3. PROGRAMS, SIMPLE LOOPS AND FUNCTIONS

DETAILS AND EXAMPLES

i. Creating and importing your own program or data file.

Instead of typing in commands at the MATLAB prompt it is often preferable to create a program file which contains the instructions and then call up the program. This is particularly handy when we have several steps to perform in our computation or when we want to perform other calculations which are modifications of a previous one. It makes corrections much simpler and avoids a great deal of frustration caused by typing and other simple errors. When the amount of data to be used in a program is small, as in program.1 of example 1, then the program file will contain both data and instructions. Some files contain just data and are then called up in another program file or via a function (see iv). As far as MATLAB is concerned there is no distinction between a program file and a data file. In both cases we create an "m-file" or "*.m file". If we have two large sets of data that we want to analyse we might put them into two files data1.m and data2.m.In our program file we would then change the call-up portion of the file from data1.m to data2.m. The use of a file that just contains data and is called up by a program is shown in example 2. If we are going to analyse many sets of data, we may prefer to write a function (see iv).

Program vs. function. The main difference between a program and a function, is that with a program all the variables become MATLAB variables (they are listed when we type whos - Section I.1, viii) that can be used later on. For a function only the designated dependent variables become MATLAB variables. This distinction often helps us to decide if we want to write a program or a function.

To write a program or data file we simply use a text editor (to create a pure ASCII file) and type in the instructions and the data. When naming the file, the name portion is arbitrary (except that it must start with a letter and not a number), but the extension must be "m", e.g. program1.m. To call the program one types the name, e.g. program1 of the file, without the extension. Large data sets, saving data and loading data are discussed in more detail in section I.4.

WARNING: Do not use the name of a MATLAB function for your variables because MATLAB will think that you are trying to evaluate the function and forgot the ( ). Thus since det( ) is a built in MATLAB function, if you want to designate the determinant of your output matrix, call it "deter" or "d", but not "det"!

ii. Including a pause in a program.

Example 1 also illustrates the use of pause. When there are many variables the earlier ones will be "gone" before we have had the time to read and check them. So we put either pause in which case MATLAB waits until you push ENTER (or any key), or pause(10) - or pause(15) etc. - in which case MATLAB automatically

• SECTION I.3. PROGRAMS, SIMPLE LOOPS AND FUNCTIONS

resumes after 10 seconds.

Example 1. I used a text editor to type the following program "prog1.m" which contains some data and a sequence of commands to manipulate this data. Note that we can leave a space between lines and put in comments using %.

Program 1

% [prog1.m]

C = [1 2 3 ; 4 5 6] % data

pause(10)

ra_C = rank(C) % find the rank

pause

ech_C = rref(C) % reduced row echelon form

I invoke the program with the command prog1; this loads the variable "C" and then invokes functions which find the rank and reduce the matrix to row echelon form are invoked. The output is assigned to the variables "ra_C" and "ech_C".

>> prog1 % no extension

C = % MATLAB prints C

1 2 3 4 5 6

MATLAB pauses for 10 seconds

ra_C = 2

MATLAB pauses until enter is pushed

ech_C =

1 0 -1 0 1 2

[Note: In this program the matrix C is automatically printed. Forlarge matrices we may want to suppress printing. To do so wewould place a semicolon after the matrix; see Section I.2,vi.

• SECTION I.3. PROGRAMS, SIMPLE LOOPS AND FUNCTIONS

>> C = [1 2 3 ; 4 5 6]; % data

If we needed to see C we would use disp( ); see Section I.5,i:

>> disp(C)

]

Instead of placing the data inside the program we can also place it in a data file C.m

Program 2

% [data1.m] contains only data

C = [1 2 3 ; 4 5 6] % data

We now modify prog.1 so that it calls up the data file data1.m.

Program 3

% [prog1b.m]

data1 % calls up data file [data1.m]

pause(10)

ra_C = rank(C) % find the rank

pause

ech_C = rref(C) % reduced row echelon form

We now call up prog1b:

>> prog1b

C = 1 2 3 4 5 6

ra_C = 2

ech_C =

• SECTION I.3. PROGRAMS, SIMPLE LOOPS AND FUNCTIONS

1 0 -1 0 1 2

When we type whos (Section I.1, viii) to find out what our variables are we see that all variables that appear in prog1b are there

>> whos Name Size Total Complex

C 2 by 3 6 No ech_C 2 by 3 6 No ra_C 1 by 1 1 No

iii. "for - loops".

Often we want to repeat an operation many times and instead of constantly recomputing individual quantities we can use a "for-loop". This consists of a statement of the form:

for n = 1 : 5

instructions to do something

end

Note that for and end are in lower case letters.

Loops are normally created in "m-files" and then called up in MATLAB, although simple ones can be typed in at the prompt. It is suggested that neither i nor j be used for indices since these are also used for imaginary numbers (section II.3.iii) and confusion in reading the program might result.

Example 2. We have a recursive relationship of the form:

nk = Pk * n0 ,

where P is a square matrix and nk and n0 are column vectors. To compute nk, for a sequece of values of k, we create the m-file called power1.m which contains P, n0 and a "for-loop":

Program 4

% [power1.m] compute powers.

% enter initial vector and take the transpose; since the % elements are real we

• SECTION I.3. PROGRAMS, SIMPLE LOOPS AND FUNCTIONS

can use '.

n_0 = [2 3]'

P = [.2 .6 ; .4 .3] % enter the matrix;

%loop startsfor n = 1 : 3 u = P^n * n_0end

We call up "power1.m" by typing power1 without the extension *.m . Notice how for each loop the variable "u" changes its value and is printed out.

>> power1

n_0 =

2 3

P =

0.2000 0.6000 0.4000 0.3000

u =

2.2000 1.7000

u =

1.4600 1.3900

u =

1.1260 1.0010

In the above loop we could have indicated the subscript by adding the index n in the loop:

Program 5

• SECTION I.3. PROGRAMS, SIMPLE LOOPS AND FUNCTIONS

% [program3.m] have the program print the value of n

for n = 1:3 n u = P^n * n_0end

A nicer output can be obtained by suppressing printing using ; and then using disp(n) , disp(u); see section I.5.

Program 6

% [program3.m] suppress "n = " and use disp(n)

for n = 1:3 n; % suppress printing n u = P^n * n_0; % suppress printing u disp(n) , disp(u)end

iv. Creating and using your own functions.

Programs perform a sequence of operations on one set of data. When the same operations are going to be performed over and over it is preferable to define a function. To create a new function we create an "m-file" whose name (without the ".m") is exactly the same as the function. Thus if we want to define a function "trans1( )" we have to create a file "trans1.m"

The first statement - not including comments which start with % - in trans1.m must start off with the word function so that MATLAB knows that it is not dealing with a simple sequence of commands and data as in subsection i. The word function is followed by the statement that defines the dependent variable(s) as trans1( ) of the independent variables. The function name should be written in lower case letters (some older versions will accept upper case). Thus in the file "trans1.m" we start off with:

function NEW = trans1(OLD)

In this case we have one dependent variable, called NEW and one independent variable called OLD, but MATLAB functions can have several dependent and several independent variables as will be seen in the examples below. These variables are all internal variables, i.e. they will not appear directly in the list of MATLAB variables that we obtain when we type whos; see Example 3 for a further discussion of this.

WARNING (second time in this section): Do not use the name of a MATLAB

• SECTION I.3. PROGRAMS, SIMPLE LOOPS AND FUNCTIONS

function for your variables because MATLAB will think that you are trying to evaluate the function and forgot the ( ). Thus since det( ) is a built in MATLAB function, if you want to designate the determinant of your output matrix, call it "deter" or "d", but not "det"!

Once the name of the function and symbols for the dependent and independent variables have been decided the next step is to give an algorithm which relates the variables:

Example 3. We have a fixed matrix F = [-2 2 ; 3 1] and we want to take various 2 x2 matrices, square them and then add F. The function program would be:

Program 7% [trans1.m]

function NEW = trans1(OLD)

F = [-2 2; 3 1]; % suppress printing

% give the formula. add ; to avoid printing twiceNEW = (OLD)^2 + F;

To use this function we have to assign a variable name to the independent variable that whose functional value we are going evaluate. We do not have to use OLD as in the program and can use any variable name, e.g. A:

>> A = [1 2; 3 4];

Next, we a assign a variable name, e.g. B, to the value of the function (we do not have to use the name NEW) as follows

>> B = trans1(A)

B =

5 12 18 23

It is preferable, but not necessary, to assign a variable to the value of the function. If we do not then the value is assigned to the variable ans as follows:

>> trans1(A)

ans =

5 12

• SECTION I.3. PROGRAMS, SIMPLE LOOPS AND FUNCTIONS

18 23

Internal variables. Note that in the function trans1.m the variables OLD, NEW, F are variables which are internal to the function; they will not appear when we typewhos (Section I.1, viii). Sometimes we do not want to keep a quantity as a MATLABvariable, but we want to print out the value, perhaps as a check when we run the program. The quantities can also be introduced as internal variables as shown in the next example:

Example 4. Suppose that as we go along in the above calculation we want to check the determinant of A to make sure that it is non-zero:

Program 8

% [trans2.m] % prints out determinant as part of function call

function NEW = trans2(OLD)

F = [-2 2; 3 1]; % suppress printing

det_check = det(OLD) % do not suppress printing!!

% alternative method of printing out the valuedisp(det(OLD))

NEW = (OLD)^2 + F;

We run the program and then type whos:

>> B = trans2(A)

det_check = -101

B = 5 12 18 23

>> whos Name Size Total Complex A 2 by 2 4 No B 2 by 2 4 No

Note that the quantity det_check has been printed out, but does not appear as a variable. The value of det_check was printed out because we did not put a semicolon. An alternative method, as shown in the program, is to use the disp

• SECTION I.3. PROGRAMS, SIMPLE LOOPS AND FUNCTIONS

function. The value assigned to NEW by the function is listed under the variable B.

Functions of several independent variables. MATLAB functions can also be functions of several independent variables. Suppose that instead of a fixed adder in example 3, we want to have a variable adder. We simply include the adder as a variable in the defined function as in the following example.

Example 5.Program 9

% [trans3.m] - illustrates a function of two variables

NEW = (OLD)^2 + ADDER; % give the formula do not print out.

To use this function we need to define the adder, using any symbol:

>> C = [1 -2; 0 1]

We designate the dependent variable as D:

>> D = trans3(A , C)

8 8 15 23

Several dependent variables. In calculus a true function can only have one dependent variable, but a MATLAB function can compute and store several variables. For the function trans2( ) in example 4, we computed the determinant of the input matrix as we went along and even had the value printed out, but we did not assign a variable name to it. As we saw in example 4 the quantity does notappear in the "whos" list and thus cannot be used again. If we want to save the quantity as a variable then we proceed as in the following example:

Example 6. Suppose that in example 5 we had wanted to compute the rank of the output matrix, and also to assign a variable name to it. In this case, there are two dependent variables, which we designate as NEW and rank_new. We place both ofthese variable names inside square brackets on the first line of the m-program. Note that the two variables do not need to have the same dimensions, i.e. we are not forming a true matrix:Program 10

% [trans4.m] - illustrates a multi-variable output% modification of program 5

• SECTION I.3. PROGRAMS, SIMPLE LOOPS AND FUNCTIONS

function [NEW, rank_new] = trans4(OLD)

F = [-2 2; 3 1]; %suppress printing

NEW =(OLD)^2 + F; % give the formula do not print out.

rank_new = rank(NEW);

At the MATLAB prompt we define our dependent variables. In this case we will assign the function value NEW to a variable whose name is also NEW. We assign the function value rank_new to a variable whose name is different namely lin_ind.

>> [NEW, lin_ind] = trans4(A) % A was entered above.

NEW =

5 12 18 23

lin_ind =

2

>> Z = trans4(A)

then the value of Z would be that of the first of the variables (NEW) and the second variable lin_ind would not have appeared.

No independent variables. It is possible to define a function with no independent variables. In the following example we use the input function, which is discussed in I.4, to obtain that value we use in the computation.

Program 11

% [squa_it.m] - a function with zero variables

function c = squa_it;

N = input(' what is N? ');c = N^2;

We could have obtained exactly the same value for c if we had not put in the function statement, in which case we would have had an ordinary MATLAB program. In the function format only c becomes a MATLAB variable, whereas with

• SECTION I.3. PROGRAMS, SIMPLE LOOPS AND FUNCTIONS

the program format both N and C become variables that can be used later on.

Functions with one or several dependent variables. For some built-in MATLAB functions there is the possibility of assigning either one or two dependent variables. An example of this is the function diag( ) which is discussed in detail in sections I.3.1 and III.2. If we type:

>> EIG_VAL = eig(A)

then we will obtain a diagonal matrix whose diagonal elements are the eigenvalues of A. But if we type:

>> [EIG_VEC , EIG_VAL] = eig(A)

we obtain two matrices, the first one having the eigenvectors of A stored as columns and the second again being a diagonal matrix whose diagonal elements are the eigenvalues of A.

• SECTION I.4. ADVANCED IMPORTING AND EXPORTING TECHNIQUES

SUMMARY

[Typical names of files are indicated in the summary by italics; note that in the save and load commands discussed in iv and v, the extension .mat is added automatically by MATLAB. The expression "m-file" refers to a file with the extension *.m]

i. Importing data.

Use a word editor to create an "m-file" e.g. import1.m in which you have entered the variables and data exactly as you would at the MATLAB command line.

ii. Long lines of data.

The data is enclosed in brackets [ ] as is the case with all matrices of data. At the end of each line we type ellipses ... , push ENTER, then continue the data on thenext line. This applies to "m-files" as well as at the MATLAB command.

[Warning! Leave a space after the last number, e.g. 5 ...Otherwise MATLAB thinks that you have put decimal points.]

iii. Importing data from the keyboard.

vect_1 = input('type in your first vector: ');

for string variables use the form: comment = input('type your comment: ' , 's' );

A string input is made without ' ', e.g yes (and not 'yes').

iv. Saving variables to a "mat-file" called results.mat.

To save all variables: save results (no extension).To save several specific variable: save results A1 a[Note: there is no comma anyplace in the statement].

vi. Saving one variable A in ascii format to a file data1

>> save data1 A /ascii /double.

[Note and warning: As discussed in section I.1 output can be saved in a "diary". Although the output from such a diary can be placed (by use of a text editor) in an

• SECTION I.4. ADVANCED IMPORTING AND EXPORTING TECHNIQUES

m-file and then loaded into MATLAB, this is not advisable when decimals are present and high accuracy is desired. A variable saved in a mat-file is saved to a high degree of accuracy, whereas a variable saved in a diary has been permanently truncated!

ASCII output (see vi) can only be done one variable at a time and the accuracy may not be as high as with mat-files.]

----DETAILS AND EXAMPLES

i. Importing data.

For small data sets one can enter the numbers at the MATLAB control line. For larger sets of data and for situations where one wants to make repeated changes, it is better to use a text editor to create an ASCII file which contains the data. When naming the file, the name is arbitrary, but the extension must be ".m", e.g. data1.m. To call the program one types the name, e.g. data of the file, without theextension.

The concept of m-files was discussed in section I.3.i in connection with programs. From the viewpoint of MATLAB a program file is no different from a data file. The concept of putting data in an m-file is repeated here so that a comparison can be made with other methods discussed later in this section. Recall also that in section I.3.iii, we saw how user defined functions were also created by putting the instructions in a file with the extension .m . The difference between function files and program/data m-files is that a function file starts off with the word "function". Functions, via function files, operate on variables, whereas m-files contain a once only sequence of instructions.

Example 1. A simple data file.

The file contains the names of two variables: a matrix and ascalar, and the corresponding data. Note that the variable nameMAT1 is not the same as the file name. However it could be the same ifwe wished.

Program 1

% [import.m]

MAT1 = [1 2 3; 4 5 6]; % the semicolon suppresses printing

a = sin( pi/2 ); % sin( ) and pi are MATLAB functions

To run this program in MATLAB we simply type the name of the file, without the

• SECTION I.4. ADVANCED IMPORTING AND EXPORTING TECHNIQUES

extension, at the command line.

>> import1

To check that the variables are there:

>> whos

Name Size Total ComplexMAT1 2 by 3 6 Noa 1 by 1 1 No

ii. Long lines of data.

For long strings of data we want to be able to continue data entry on the next line.To do so, type three or more ellipses "... ", push ENTER and continue the data on the next line. Make sure that you put a semi-colon if you do not want a huge massof data to be printed out!

Ellipses can be used in "m-files" as well as at the MATLAB command.

Program 2

% [dots.m]

% place the data in brackets; add ellipses at the end% of each line of data. But make sure that you leave a space% between the last number and the ellipses.% place a semicolon after the data to suppress printing.% there is no need to have the same number of elements in each row.

a = [1 3 ... 4 ];

s = sum(a)

>> dots % call up the program "dots.m"; no extension

a =

1 3 4

s =

8

• SECTION I.4. ADVANCED IMPORTING AND EXPORTING TECHNIQUES

If we have large sets of data and want to have the same number of elements in each row we can place ... in the first row.

a = [ ... % first row is empty * * * ... % !! space after the last number * * * ... ]; % ! closing bracket

[Warning: Leave a space between the last number and the ellipses; otherwise MATLAB thinks that you have put a decimal point!]

[Note: Nothing prevents you from using a semicolon to form amatrix. For example:

>> F = [4 5 6 ... % ellipses to continue row 1 7 8; % end of row 1 9 10 11 12 13] % row 2 all on one line

F =

4 5 6 7 8 9 10 11 12 13

iii. Inputing data from the keyboard

One can also import data directly from the keyboard. This is especially useful in connection with a program that we want to check or run several times and which involves small sized matrices. By using the keyboard, we can avoid switching to a text editor in order to change the data.

To invoke the keyboard, we assign a variable, e.g. vect_1 to the MATLAB function input(' '). Inside the apostrophes, we place a prompt message:

>> vect_1 = input(' type in your first vector: ')

Note that for clarity a colon has been included in the prompt message and a little space has been left after the colon.

The input can also be a string, i.e. text, and to indicate this we add , 's' :

>> comment = input('type your comment: ' , 's');

When we input the text we do not put the apostrophes around it; i.e if the input asks yes or no? we type yes, not 'yes'.

Example 2. Program 3 illustrates the use of input(' ') to input two variables and

• SECTION I.4. ADVANCED IMPORTING AND EXPORTING TECHNIQUES

to input a comment.

Note that I have not placed a semicolon after vect_1 and vect_2 and so MATLAB prints out the values. I could have placed a semicolon and then used disp(vect_1).This however has been done with the variable "comment"; a semicolon has been placed at the end of the line which defines "comment" and then disp( comment ) has been used.

The example shows that when we multiply a column vector by a row vector we obtain a matrix of rank 1. We input vect_1 as a row vector and then take its transpose by using the true transpose operator .' ; see section II.3.i for a discussion of the transpose operators.

Program 3

%[keyb1.m]

vect_1 = input(' type in your first vector: ')vect_2 = input(' type in your second vector: ')

% multiply the transpose of vect_1 and vect_2; find the rank% .' is the true transpose; see section II.3.i

col_x_row = (vect_1).' * (vect_2); ran_1 = rank(col_x_row) ;

comment = input(' type your comment: ', 's');disp(comment)

Invoke this program by typing the file name keyb1 without the extension. Boldfacein the following indicates what the prompt text is. Italics in the following indicates what is typed in at the keyboard.

>> keyb1

type in your first vector: [1 2]

vect_1 =

1 2

type in your second vector: [3 4]

vect_2 =

3 4

• SECTION I.4. ADVANCED IMPORTING AND EXPORTING TECHNIQUES

col_x_row =

3 4 6 8

ran_1 =

1

type your comment: each row is a multiple of first

each row is a multiple of first

In example 2 we imported two variables "vect_1" and "vect_2", and then calculated the values of two variables "col_x_row" and "ran_1". If we wanted to save all four variables for later use, the simplest and best manner is to save them to a "mat-file". This is a file written in machine language in which the variables aresaved to a very high degree of accuracy. In principle, one could take the contents of a "diary" that has been created (section I.1), put them into an "m-file" (as in subsection i above) and then call up the "m-file". Not only would that way require a lot of work, but it also has the added inconvenience that accuracy is lost because the output appears in truncated form in the "diary". If you need to save the variables in ascii format, e.g. for export to another program, see vi below and the save ... ascii command; the precision may be less however.

To place all the current variables into a "mat-file" called results we give the command save and follow this by the name (without the extension ".mat")

>> save results

MATLAB will automatically assign the extension .mat so that in the directory there will be a file results.mat.

If during another MATLAB session we want to have access to the variables we use the load command:

Sometimes we only want to save a few of the variables. Supose that we only wanted to save "vect_1" and "col_x_row" to a mat-file results2.mat. We could either first clear "vec_2" and "ran_1" by using:

• SECTION I.4. ADVANCED IMPORTING AND EXPORTING TECHNIQUES

>> clear vec_2, ran_1

and then typing:

>> save results2

or we could use save with the name of the file results2 followed by the list of the variables that we want to save. Notice that there is no comma anyplace in the statement.

>> save results2 vec_1 ran_1

To re-obtain the variables from results2, we use the load command:

To find what variables were stored in results2, and what their dimensions are, we use whos:

>> whos

Name Size Total Complexvec_1 1 by 2 2 Noran_1a 1 by 1 1 No

We can now display the variables that we want by typing their names or by using disp( ):

>> vec_1 % "vec_1 =" is printed>> disp(ran_1a) % only the value is printed

Note: It is not possible to load just one of the variables from results. Just load and clear the ones that you do not want.

vi. Saving one variable A in ascii format to a file data1

To save the variable "A" in ascii format to a file data1, we use save with the option/ascii:

>> save data1 A /ascii /double.

The option /double is usually added to ensure a high degree of accuracy.

• SECTION I.5. FORMATTING THE OUTPUT

SUMMARY

i. Using disp() to avoid "A = ".disp(A) gives A without the statement, " A = "disp(' A is a matrix ') gives the statement typed between the apostrophes.Use disp(' ') to skip a line.

ii. The number of decimal places in the output.For 14-15 places: format long 57.27564927611034to return to the standard fixed point format (4 places): format short 57.2756floating point (scientific) notation: format short e 5.7276e+001 format long e 5.727564927611034e+001converting matrices to the + { } - format

format +[Note: Recent versions of MATLAB have more commands for specifying the format:e.g.

format bank 57.27 (2 decimal places, for money!)

sprintf This command is based on C syntax and can be used to control the number of decimal places more precisely. String statements (such as in disp('hello') ) can also be included:

a = 57.27564927611034 sprintf( ' %10.5f ' , a ) = 57.27564 [5 decimal places; the 10 indicates the printing is being done in a minimum of 10 spaces with the "4" being placed on the extreme right.]

using e instead of f will give the answer in floating point notation.

format hex the output will be in hexadecimal format.

type help format, help fprintf for details of what is available on the system that you are using]

----DETAILS AND EXAMPLES

i. Using disp() to avoid "A = "; construction of simple tables.

MATLAB output is usually in the form: "A = " followed by the value of A. To print out just the value of A we use disp( ) :

• SECTION I.5. FORMATTING THE OUTPUT

>> disp(A)

The display command is also used to produce alphanumeric text. The text to be displayed is enclosed within apostrophes. Thus if we enter:

>> disp(' A is a matrix ')

we obtain:

A is a matrix

The use of disp( ) is illustrated in the following two methods of producing a table of sines and cosines. There are various ways of producing the individual results and these are also illustrated. In the first method we produce individual row vectors of values one at a time and print them out; this method is shown first in step by step format and then in a program. In the second method, we compute the vectors, take their transposes and create a matrix whose columns are these transposes. We then ask MATLAB to display the matrix.

Example 1. Producing the output in rows.

We first form a vector "values" that gives the range of values that interests us.

>> values =[0 pi/4 pi/2]; % ; to suppress printing

>> disp(values)

0 0.7854 1.5708

Next, we calculate the sines of the values in "values" by using the MATLAB vector function sin( ). MATLAB trigonometric and other functions are discussed in detail in section II.3. We use both sin( ) and disp( ) in the same statement

>> disp( sin( values ) )

0 0.7071 1.0000

For the sine, we did not assign a symbol, but we do so for the cosine values. A common error is to use a function name for a variable, e.g to let cos = cos(values). Note that we have used the underbar in the variable name "cos_variables", because the use of a minus sign, e.g. "cos-values" would make MATLAB think that we want to subtract!

>> cos_values = cos(values);>> disp(cos_values)

• SECTION I.5. FORMATTING THE OUTPUT

1.0000 0.7071 0.0000

Finally we produce a title for our table. To indicate that we are dealing with an alphanumeric quantity we enclose it in apostrophes. Some space is left after the first comma to shift the title over to the right:

>> label = ' table of sines and cosines';>> disp(label)

table of sines and cosines % MATLAB response

Above, the commands were given on different lines, but they could also have been put on one line using the comma. Since the line is too long we use ellipses toindicate that we want to continue on the next line, although in this case it is just as simple to push ENTER and go to the next command line. Note how we use disp(' '), i.e. the message is empty, in order to have MATLAB skip a line

>> disp(label) , disp(values) , disp(sin( values )) , ... disp(' ') , disp(cos_values)

table of sines and cosines

0 0.7854 1.5708 0 0.7071 1.0000 1.0000 0.7071 0.0000

Instead of doing the above steps individually we can write a short program "disp2.m".

Program 1% [disp2.m]

x = [0:.2:1]; % ; to suppress printingdisp(' first row values; second is sine; third is cosine')disp(' ') % empty message to leave a spacedisp(x)disp( sin(x) ) disp( cos(X) )

We now call up "disp2.m". Note that we just type "disp2" and not "disp2.m"

>> disp2

first row is values; second is sine; third is cosine

0 0.2000 0.4000 0.6000 0.8000 1.0000

• SECTION I.5. FORMATTING THE OUTPUT

0 0.1987 0.3894 0.5646 0.7174 0.84151.0000 0.9801 0.9211 0.8253 0.6967 0.5403

Example 2. Producing the output in columns.

In the first method, the output is in rows, but if the range of values is above nine or ten, then the rows will not fit on one line of print. So, instead of working exclusively with row vectors we take the transpose of the vectors, create a matrix whose columns are these vectors and then print the matrix. This method is followed in the program "disp3.m". Note how we compute the variables, but do not print them immediately. It is sometimes difficult to place the title exactly and it is often simpler to make a final adjustment with a text editor. Since the entries are all real, we can use the operator ' for transpose instead of .' (see section II.1). In the program we first use the semicolon (section I.2.v) to generate a domain vector with values going form 0 to 1 by steps of .2.

Program 2% [disp3.m]

y = ( [0 : .2 : 1] )'; % ' change to a column vectorsi = sin(y); % ; to suppress printingco = cos(y);A = [y , si , co]; % put in a matrix by means of `,'

disp(' x sin(x) cos(x) ') % display the titledisp(' ') % empty message, leaves a spacedisp(A) % display the matrix

We now call the program:

>> dispro3 % type filename - without the extension .m

x sin(x) cos(x)

0 0 1.0000 0.2000 0.1987 0.9801 0.4000 0.3894 0.9211 0.6000 0.5646 0.8253 0.8000 0.7174 0.6967 1.0000 0.8415 0.5403

One should not spend too much time lining up a title inside the disp(' '); it is usually simpler to do it later with a text editor.

ii. The number of decimal places in the output.

• SECTION I.5. FORMATTING THE OUTPUT

MATLAB usually shows the results to five digits; to display more places use format long:

>> s1 = 81*sin(pi/4)

57.2756

>> format long>> s1 % type s1 to display it

57.27564927611034

To display the answer in "floating point" notation use `format short e' or `format long e'

>> format short e>> s1

5.7276e+001

>> format long e>> s1

5.727564927611034e+001

>> format short>> s1

57.2756

For some matrices we may want to just see if elements are negative, zero or positive. Use the "format +" command. MATLAB leaves a very small space where the terms are 0.

>> format +>> C = [-2 0 5]

- + % There is a space between the - and +

• SECTION II.1. BASIC MATLAB OPERATIONS

SUMMARY

ia. Transpose of a matrix with real elements: '

ib. Transpose of a matrix with complex elements: .'

ii. Addition: A + B ; A + 5 adds 5 to each element.

iii. Multiplication: A*B; 2*A

iv. Powers: A^3

v. Element by element multiplication: (A).* B

[Note the dot; use parentheses to avoid problems and confusion. A and B must have the same dimensions.]

vi. Element by element division: (A)./B

[Note the dot; use parentheses to avoid problems and confusion. A and B must have the same dimensions.]

vii. Element by element powers: scalar a, vector x: (a).^x ; (x).^a ; (x).^x

[Note the dots; use parentheses to avoid problems and confusion. Vectors aand x must have the same dimensions.]

viii. For very large matrices:

See the examples in section III.1.vii

ix. Use parentheses!

MATLAB interprets 5/2*4 as (5/2)*4. Is this what you wanted or was it 5/(2*4)?---

DETAILS AND EXAMPLES

>> A = [1 2; 3 4]

1 2

• SECTION II.1. BASIC MATLAB OPERATIONS

3 4i. Transpose of a real matrix: ' or .'

>> TRANS = A'

1 3 2 4

[Note: See section II.3 for examples involving complex numbers and the differencebetween ' (conjugate transpose) and .' (true transpose).

>> S = A + A'

2 5 5 8

To add 5 to each element of A there is no need to define a matrix of the same dimensions as A, and having all entries equal to 5. MATLAB interprets A + 5 in thissense:

>> B = A + 5

6 7 8 9

iii. Multiplication: *

>> A * B

22 25 50 57

>> 5*A

5 10 15 20

>> L = (1/5)*A

0.2000 0.4000 0.6000 0.8000

iv. Powers: ^

• SECTION II.1. BASIC MATLAB OPERATIONS

>> A^3

37 54 81 118

Element by element operations

v. Element by element multiplication.

To multiply each element of A by the corresponding element of B use .* . Note the dot which distinguishes pointwise multiplication from regular matrix multiplication. It is advisable to get in the habit of using parentheses, even thoughthey are not always required, to avoid problems and confusion that arise with numbers. A and B must have the same dimensions.

>> D = (A).*B

6 14 24 36

vi. Element by element division.

To divide each element of A by the corresponding element of B use (A)./B. A and Bmust have the same dimensions. To remember the direction of the slash in ./ think of 6/2 = 3. Division tends to be confusing so use many parentheses.

>> (A)./B

0.1667 0.2857 0.3750 0.4444

To divide each element of A by the number 5, use (A)./5. Note that this is equivalent to doing element by element division of A by a matrix all of whose elements equal 5.

>> F3 = (A)./5

0.2000 0.4000 0.6000 0.8000

To divide each element of A into the number 5, use (5)./A. Note that this is equivalent to doing element by element division of a matrix all of whose elementsequal 5, by the matrix A

>> F =(5)./A

• SECTION II.1. BASIC MATLAB OPERATIONS

5.0000 2.5000 1.6667 1.2500

See the function "normalize.m" in section III.3, example 1 where pointwise divisionis used to normalize a positive matrix so that the row sums are all 1.

Example 1. The sum and products of the first eight odd numbers.

% [div.m] % Illustrate term by term division of a vector% Sum and product of the reciprocals of the first 8 odd% numbers.% note how the upper index of the vector a is a variable % that has been calculated on the previous line.

odd_8 = 1 + (8-1)*2; % 8th odd number

a = [ 1: 2 : odd_8]; % I.2,iv

% Note the dot and the ( ) in the following!!recip_a = (1)./a;

sum_recip = sum(recip_a); % II.3,iv

prod_recip = prod(recip_a); % II.3,iv

disp('The eighth odd number: '), disp(odd_8)disp('The first eight odd numbers:'), disp (a)

disp('The reciprocals:'), disp(recip_a)disp('Their sum and product are: ')disp(sum_recip), disp(prod_recip)

The output of this program is:

The eighth odd number: 15

The first eight odd numbers:

1 3 5 7 9 11 13 15

The reciprocals:

1.0000 0.3333 0.2000 0.1429 0.1111 0.0909 0.0769 0.0667

• SECTION II.1. BASIC MATLAB OPERATIONS

Their sum and product are:

2.0218 4.9333e-007

vii. Element by element powers

>> x = [-1: .5 : 1] % section I.2,v

-1.0000 -0.5000 0 0.5000 1.0000

Suppose that we want to raise each element of x to the third power. We use the point-wise power operator .^ and write (x).^3. As with multiplication it is a good habit to use many parentheses.

>> (x).^3

-1.0000 -0.1250 0 0.1250 1.0000

On the other hand, if we want to use the elements of the vector x as powers to which we raise the number 3 then we write (3).^x.

>> (3).^x

0.3333 0.5774 1.0000 1.7321 3.0000

To show what can happen if we do not put parentheses consider:

>> 3.^x

This is ambiguous since we are perhaps referring to the number 3.0. MATLAB responds with:

Error using ^Matrix must be square.

One can also raise each element of a vector to a power determined by the elements of another vector of the same size.

>> (x).^x

-1.0000 {0.0000 - 1.4142i} 1.0000 0.7071 1.0000

The element is the complex number obtained by evaluating (-.5)^(-.5): >> (-.5)^(-.5)

• SECTION II.2. FIVE SPECIAL MATRICES

SUMMARY

[Note: It is for the commands in this section that MATLAB has changed the most. Because of this the newer syntax is given first and then this is followed by the older syntax. If the newer command does not work, then try the older form or type help eye etc. One difference that is common to the first four forms is that theterm size is now needed; e.g before one could write ones(A) to obtain an identity matrix with the same dimensions as A, but now one must write ones( size(A) ).]i. Identity matrix.

eye(3) or eye( size(A) ), if A is already defined and square.

older versions: eye(A). ii. Matrix of ones.

ones(1 , 5): a vector of length 5 of ones.ones(3 , 5): 3 rows, 5 columns.ones(3): 3 x 3 square matrix of ones.ones( size(A) ): If A is already defined.

older versions: ones(A)iii. Matrix of zeros.

zeros(1 , 5): a vector of length 5 of zeros.zeros(3 , 5): 3 rows, 5 columns.zeros(3): 3 x 3 square matrix of zeros.zeros( size(A) ): If A is already defined.

older versions: zeros(A) iv. Matrix of random elements.

random elements in [0,1) ("uniform generator")

rand or rand(1) - one random numberrand(1 , 5): a vector of length 5 of random numbers.rand(3 , 5) - 3 rows, 5 columns.rand(3) - 3 x 3 square matrix of random numbersrand( size(A) ) - If A is already defined.

older versions: rand(A)

To have a different set of random numbers corresponding to the initial seed 9956295 : rand('seed' , 9956295)

Repeating this command will result in the same initial seed as the first time that this command was issued.

• SECTION II.2. FIVE SPECIAL MATRICES

To print out the present value of the seed: rand('seed') [newer versions only].

See the discussion for details and problems associated with the seed.

[note: for testing purposes use the built in seed or a fixed seed; for serious work pick your seed at random, e.g. via randomly picked telephone numbers.]

normal random numbers with mean 0 and variance 1

In the newer versions the commands are the same as for random numbers in [0,1)except that rand is replaced by randn, i.e. "n" is added on. A seed change for the normal generator does not change the seed for the [0,1) generator. To return to the [0,10 generator one simply uses the rand( ) commands given above.

newer versions: randn(1,5), rand('seed' , 127269), randn('seed' , 0), randn('seed')

older versions:to switch to the normal generator: rand('normal')to return to the [0,1) generator: rand('uniform')to change the seed: rand('seed', 7626)v. Row vector of indices.

[-1 : .5 : 2] gives -1 to 2 by steps of .5vi. The empty matrix [ ].--

DETAILS AND EXAMPLES

i. Identity matrices.

MATLAB uses the function eye( ) to define the identity matrix.

>> IDENT = eye(2)

IDENT = 1 0 0 1

If however, we have already defined a square matrix A and we want an identity matrix of the same size, we can simply type eye(A):

>> A = [4 -1 ; 3 0]

4 -1 3 0

• SECTION II.2. FIVE SPECIAL MATRICES

>> eye(size(A))

1 0 0 1

The use of eye(size (A)) is very useful in multiple operations:

>> A - 3*eye(size(A))

1 -1 3 -3

ii. Matrix of ones.

A matrix all of whose elements equal 1 is defined using the function ones(2 , 3).

>> a = ones(3 , 1) % column vector of ones

1 1 1

If the matrix is square one can write ones(2):

>> E = ones(2)

1 1 1 1

If A has already been defined we can use ones( size(A) ):

>> E = ones( size(A) )

1 1 1 1

A vector of 1's is useful in constructing a matrix all of whose rows are equal to a given vector:

>> b = [2 4 6];

>> a*b % a = ones(3,1); see above

2 4 6 2 4 6 2 4 6

• SECTION II.2. FIVE SPECIAL MATRICES

This idea is used in the function "normaliz.m" which is defined in section III.3 program 1.

iii. Matrix of zeros.

A matrix all of whose elements equal 0 is defined using the function zeros(2 ,3).

>> B = zeros(2 , 3)

0 0 0 0 0 0

If the matrix is square one can use zeros(2).

>> D = zeros(2)

0 0 0 0

If A has already been defined we can use zeros( size(A) ):

>> F = zeros( size(A) )

0 0 0 0

A vector of zeros is very useful in predefining the size of a matrix. We can then fill in various positions with non-zero elements. Suppose that B is a 2 x 3 matrix of zeros obtained, as above, by zeros(2 , 3) as above. If we redefine the element in row 2 and column 3 to be 5, then we obtain a matrix of the same dimensions as B,but with a 5 replacing the 0 in row 2, column 3.

>> B(2 , 3) = 5 % see section I.2.iii

B =

0 0 0 0 0 5

The use of a predefined shape helps to avoid confusion and error statements. Thus you may think that you are working exclusively with row vectors, but since MATLAB tends to work with column vector --e.g. when finding the eigenvalues of amatrix--you may end up with incompatible matrices. This idea is exploited in section II.4, viii and the program "rand1.m" which is discussed in the section II.6.

• SECTION II.2. FIVE SPECIAL MATRICES

iv. Matrix of random elements.

A matrix whose elements are "random" (i.e. are numbers between 0 and 1 which act statistically as if they were picked by chance) is defined using the function rand( ). The examples here use the older commands, for the newer ones and also additional commands see the summary.

To obtain single elements we use rand( ) or simply rand:

>> rand

0.2190

>> rand(1)

0.0470

To obtain a random vector of length 3:

>> rand(1 , 3)

0.2190 0.0470 0.6789

If the matrix is square one can just write rand(2).

>> rand(2)

0.6793 0.3835 0.9347 0.5194

If A has already been defined we can use rand( size(A) ).

>> rand( size(A) )

0.5017 0.7013 0.4919 0.9217

Seeds: The basic MATLAB random number generator is based on a multiplicative congruential generator, i.e., a relationship of the form:

Zn = a* Zn-1 mod m

The initial value Z0 is called the seed or the initial seed.

In the context of MATLAB the Zn are also "seeds" (see below) although it would bebetter to think of them as the present value of the seed.

• SECTION II.2. FIVE SPECIAL MATRICES

In some versions of MATLAB the following values of a and m are used:

a = 75 = 16807, m = 231 -1 = 2147483647

The actual implementation of the algorithm depends upon the machine and is based on various numerical "tricks". If you write and test your program on say a PC, and then switch to a big UNIX machine, the numbers that you obtain might bedifferent. This in itself is not a problem; however difficulties might arise at the level of testing your program (see below)

Once the values of the Zn are obtained we obtain our random numbers, which fall in the interval [0,1) by division:

xn = Zn/m

Random numbers are used in simulations and in Monte Carlo evaluations, and they are also used to generate data for the purpose of testing statistical procedures. They are also quite useful for making up numerical examples involving matrices (see Sections III.1.vii and III.4).

For the purpose of testing programs we would like to be able to always obtain the same set of random numbers. On the other hand when actually running an experiment on a computer we want to make sure that we obtain a completely newset of random numbers. For otherwise our simulation would not be completely random. In order to replicate the sequence of random numbers we simply reset the seed (which we can consider as the new initial seed). Suppose that we decide to use 9956295 (the telephone number of NSERC, the Natural Sciences and Engineering Research Council of Canada), we use the command:

>> rand('seed', 9956295) % set the seed

>> rand % generate a random number 0.9216

Now we reset the seed to the same value 9956295

>> rand('seed', 9956295)>> rand % generate a random number 0.9216 % the same number before

[Note: MATLAB has a built in inItial seed. On Unix version 5.3.1.29215a (R11.1) the value is 931316785. In order to avoid obliging the user to retype this number each time, MATLAB switches to this value when 0 is used as the seed.

Using the function rand( ) is a very quick way of generating matrices for testing.

• SECTION II.2. FIVE SPECIAL MATRICES

In section III.1.vii we test how long it takes for MATLAB to solve equations by generating 1000 equations in 1000 unknowns where the coefficients and constants are picked at random. This only takes a few seconds whereas typing in 1,001,000 numbers would take slightly longer.

v. Vectors of Indices.

The concept of building a row vector of indices has already been discussed in section I.2.iv, in connection with the semicolon, but a brief summary is repeated here for reference and also to emphasize that we are creating a type of matrix.

To obtain the index set -1, 0,... , 4 we write:

>> x = [-1 : 4]

-1 0 1 2 3 4

To obtain the index set -1, -.5, 0, .5, ... , 4 we write:

>> x_step = [-1 : .5 : 2]

-1.0000 -0.5000 0 0.5000 1.0000 1.5000 2.0000

To obtain the index set 5, 4, 3, 2, 1 (i.e. in reverse order) we write:

>> X_dec = [5 : -1 : 1]

5 4 3 2 1

(Note: Writing [5 : 1] would result in MATLAB responding with the empty matrix [ ]).

vi. The empty matrix [ ].

This matrix is often very useful because it can serve as the initial matrix when building up matrices by concatenation, see section II.4.ix. The empty matrix is treated by MATLAB in exactly the same way it treats any other matrix.

>> length( [ ] )

0

>> A = [ ]*[ ] A = [ ]

• SECTION II.2. FIVE SPECIAL MATRICES

As the last note in v. above shows, an incorrect statement may lead to MATLAB returning the empty matrix.

• SECTION II.3. COMMON MATLAB FUNCTIONS

SUMMARY

In the following A represents a matrix, square where required, and x, y vector (rowor column). Some of the functions are discussed in detail in other sections and for these only a short example or the function are given together with a reference.

Note that in general MATLAB functions, e.g. sum( ), operate on the columns of a matrix; to work with rows we use the ordinary transpose of A which is written A' if all the entries are real or (A).' . The commands are in usually written in terms of a matrix A, but they apply equally well to a vector v.

i. Linear algebra.

Examples of the following functions are given in the details section.

The solution of equations using rref( ), inv( ), COEFF\const and lu( ) is discussed in detail in section III.1.

The computation of eigenvalues and eigenvectors is discussed in detail in section III.2.

ordinary transpose when all entries are real: A' ordinary (true) transpose with complex entries: (A).' conjugate transpose: A' inverse: inv(A) reduced row echelon form (Gauss-Jordan): rref(A) determinant: det(A) eigenvalues as a column vectors of a matrix A: eig(A) eigenvectors (right) & eigenvalues in a diagonal matrix: [VEC , VAL] = eig(A) eigenvectors (left) & eigenvalues in a diagonal matrix: first write: [VEC_L , VAL] = eig(A.') then: VEC_L = (VEC_L).' dot product (program 1): dot(x,y); cross product (program 2): cross(x,y)

ii. Matrix manipulations.

Short examples of the following functions are given in the details section. More involved examples are given in section II.4 (extracting and building matrices) and throughout chapter III.

transpo
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