Getting Started With Matlab

David Hart
UCS Center for Statistical and Mathematical Computing

Matlab is a computer program for people doing numerical computation. It began as a "MATrix LABoratory" program, intended to provide interactive access to the libraries Linpack and Eispack. These are carefully tested, high-quality programming packages for solving linear equations and eigenvalue problems. The goal of Matlab was to enable scientists to use matrix-based techniques to solve problems, without having to write programs in traditional languages like C and Fortran. More capabilities have been added as time has passed (especially graphics support for X windows).

Matlab has been called "the lingua franca for the exchange of software and algorithms," rapidly displacing Fortran from that position due to its interactive interface, reliable algorithmic foundation, fully extensible environment, and computational speed. Matlab is available for many different computer systems, but UCS makes it available at IUB on Unix systems only. A student edition, for Windows and Macintosh, is available from local bookstores at a very economical price.

This document is intended to help you get started with Matlab, running an X windows system and the Unix operating system. It is intended to be used, while sitting at a computer terminal running Matlab. We will not include output, and many of the commands will fail to convey the intended lesson without it.

We will assume that Unix is familar -- a seperate JumpStart class is offered by UCS, if help with Unix is needed. No familarity with Linear Algebra is required, although some of what we do cannot be fully appreciated, without it.

BASICS

Matlab is available at IUB on Nickel, Copper, Chrome and Cygnus, as well as the UNIX workstations of the Orchard and Ships clusters. To start the program type matlab at the Unix prompt, as follows:

	% matlab

The system should respond (eventually) with:

        Commands to get started: intro, demo, help help
        Commands for more information: help, whatsnew, info, subscribe
        >>

Our first command will make a record of the session, in a file named "session". [The ">>" is Matlab's prompt, you don't need to type it]. Enter:

	>> diary session

Arithmetic uses some fairly standard notation. More than one command may be entered on a single line, if they are seperated by commas.

	>> 2+3
	>> 3*4, 4^2

Powers are performed before division and multiplication, which are done before subtraction and addition.

	>> 2+3*4^2

The arrow keys allow "command-line editing," which cuts down on the amount of typing required, and allows easy error correction. Press the "up" arrow, and add "/2." What will this produce?

	>> 2+3*4^2/2

Parentheses may be used to group terms, or to make them more readable.

	>> (2 + 3*4^2)/2

The equality sign is used to assign values to variables.

	>> x = 3
	>> y = x^2
	>> y/x

If no other name is given, an answer is saved in a variable named "ans."

	>> ans, z=2*ans, ans

Here z was defined in terms of ans. The result was called z, so ans was unchanged.

To get a list of your variables, use one of

	>> who, whos

In Matlab, like C or Fortran, variables must have a value [which might be numerical, or a string of characters, for example]. Complex numbers are automatically available [by default, both i and j are initially aliased to sqrt(-1)]. All arithmetic is done to double precision [about 16 decimal digits], even though results are normally displayed in a shorter form.

	>> a=sqrt(2)
	>> format long, b=sqrt(2)
	>> a-b
        >> format short

To save the value of the variable "x" to a plain text file named "x.value" use

	>> save x.value x -ascii

To save all variables in a file named mysession.mat, in reloadable format, use

	>> save mysession

To restore the session, use

	>> load mysession

To find out about this kind of thing, consult the help system. There's even an HTML version! There's also a "lookfor" command, so that you don't have to guess the topic name precisely.

	>> help
	>> help general
	>> doc

Finally, to stop Matlab and return to the operating system, use

	>> quit

Then, to see the saved files from your session, type

	% more foo
	% more x.value

MATRICES

A matrix is a rectangular array of numbers:

        [ 1 2 3 ]
        [ 4 5 6 ]

defines a matrix with 2 rows, 3 columns, 6 elements. We will refer you to the Math Department for an explanation of the arithmetic of matrices, what they have to do with geometry and equations and why you should care. [The course is called Linear Algebra].

Matlab is designed to make matrix manipulation as simple as possible. Every Matlab variable refers to a matrix [a number is a 1 by 1 matrix]. Start Matlab again, and enter the following command.

	>> a = [1,2,3; 4 5 6]

Note that:

the elements of a matrix being entered are enclosed by brackets;
a matrix is entered in "row-major order" [ie all of the first row, then all of the second row, etc];
rows are seperated by a semicolon [or a newline], and the elements of the row may be seperated by either a comma or a space. [Caution: Watch out for extra spaces!]

The element in the i'th row and j'th column of a is referred to in the usual way:

	>> a(1,2), a(2,3)

It's very easy to modify matrices:

	>> a(2,3) = 10

The transpose of a matrix is the result of interchanging rows and columns. Matlab denotes the [conjugate] transpose by following the matrix with the single-quote [apostrophe].

	>> a'
	>> b=[1+i 2 + 2*i 3 - 3*i]'

New matrices may be formed out of old ones, in many ways. Enter the following commands; before pressing the enter key, try to predict their results!

	>> c = [a; 7 8 9]
	>> [a; a; a]
	>> [a, a, a]
	>> [a', b]
	>> [ [a; a; a], [b; b] ]

There are many built-in matrix constructions. Here are a few:

	>> rand(1,3), rand(2)
	>> zeros(3)
	>> ones(3,2)
	>> eye(3), eye(2,3)
	>> magic(3)
	>> hilb(5)

This last command creates the 5 by 5 "Hilbert matrix," a favorite example.

Use a semicolon to suppress output:

        >> s = zeros(20,25);

This is valuable, when working with large matrices. If you forget it, and start printing screenfuls of unwanted data, Control-C is Matlab's "break" key.

To get more information on these, look at the help pages for elementary and special matrices.

	>> help elmat
	>> help specmat

A central part of Matlab syntax is the "colon operator," which produces a list.

	>> -3:3

The default increment is by 1, but that can be changed.

	>> x = -3 : .3 : 3

This can be read: "x is the name of the list, which begins at -3, and whose entries increase by .3, until 3 is surpassed." You may think of x as a list, a vector, or a matrix, whichever you like.

You may wish use this construction to extract "subvectors," as follows.

	>> x(2:12)
	>> x(9:-2:1)

See if you can predict the result of the following.
[Hint: what will x(2) be? x(10)?].

	>> x=10:100;
	>> x(40:5:60)

The colon notation can also be combined with the earlier method of constructing matrices.

	>> a = [1:6 ; 2:7 ; 4:9]

A very common use of the colon notation is to extract rows, or columns, as a sort of "wild-card" operator which produces a default list. The following command produces the matrix a, followed by its first row [with all of its columns], and then its second column [with all of its rows]. What do you think s(6:7, 2:4) does?

	>> a, a(1,:), a(:,2)

        >> s = rand(10,5);  s(6:7, 2:4)

Matrices may also be constructed by programming. Here is an example, creating a "program loop."

	>> for i=1:10, 
	>> 	for j=1:10,
	>> 		t(i,j) = i/j;
	>> 	end
	>> end

There are actually two loops here, with one nested inside the other; they define t(1,1), t(1,2), t(1,3) ... t(1,10), t(2,1), t(2,2) ... , t(2,10), ... t(10,10) [in that order].

	>> t

MATRIX ARITHMETIC

If necessary, re-enter the matrices

	>> a = [1 2 3 ; 4 5 6 ; 7 8 10], b = [1 1 1]'

Scalars multiply matrices as expected, and matrices may be added in the usual way; both are done "element by element."

	>> 2*a, a/4
	>> a + [b,b,b]

Scalars added to matrices produce a "strange" result, but one that is sometimes useful; the scalar is added to every element.

	>> a+1, b+2

Matrix multiplication requires that the sizes match. If they don't, an error message is generated.

	>> a*b, b*a
	>> b'*a
	>> a*a', a'*a
	>> b'*b, b*b'

To perform an operation on a matrix element-by-element, precede it by a period.

	>> a^2, a.^2
	>> a.*a, b.*b
	>> 1 ./ a
	>> 1./a.^2

One of the main uses of matrices is in representing systems of linear equations. If a is a matrix containing the coefficients of a system of linear equations, x is a column vector containing the "unknowns," and b is the column vector of "right-hand sides," the constant terms, then the matrix equation a x =b represents the system of equations. Matlab provides a very efficient mechanism for solving linear equations:

	>> x = a \ b

This can be read "x equals a-inverse times b." To verify this assertion, look at

	>> a*x, a*x - b

Change b, and do the problem again.

	>> b = [1 1 0]'
	>> x = a\b
	>> a*x, a*x - b

If there is no solution, a "least-squares" solution is provided [a*x - b is as small as possible]. Enter

	>> a(3,3) = 9

[which makes the matrix singular] and do those again. [Use the up-arrow, to recall the commands without retyping them].

There is a related problem, to solve x a = b (given a and b), which is done with

	>> x = b / a

This can be read "B times A-inverse." Again, if there is no solution, a least-squares solution is found.

MATRIX FUNCTIONS

There are a number of builtin matrix functions, for example the determinant, rank, nullspace, and condition number.

	>> det(a)
	>> rank(a)
	>> norm(a)
	>> null(a)

Enter

	>> a(3,3) = 10

[which makes the matrix nonsingular] and do those again.

Other valuable functions find the inverse, eigenvalues and eigenvectors of a matrix.

	>> h=hilb(5)
	>> cond(a)
	>> inv(h)
	>> eig(h)

The "eig" function has two forms of output. The last command produced a vector of eigenvalues. The next command produces two matrices, the first containing the eigenvectors as its columns, and the second containing the eigenvalues, along its diagonal.

	>> [v,d]=eig(h)

The matrix, h, times the first eigenvector, v(:,1), should equal the first eigenvalue, d(1,1), times that same eigenvector.

	>> h*v(:,1)
	>> d(1,1)*v(:,1)

"Round-off error" is a primary concern in numerical computing. Matlab does numerical computation, which is to say, it works with limited precision; all decimal expansions are truncated at the sixteenth place [roughly speaking]. Even if this is acceptable for any single calculation, its effects may accumulate with unacceptable consequences. The machine's round-off, the smallest distinguishable difference between two numbers as represented in Matlab, is denoted "eps".

	>> help eps
	>> eps

We can check the assertion just made about eigenvectors and eigenvalues, as follows.

	>> h*v(:,1) - d(1,1)*v(:,1)

This is "the zero vector, modulo round-off error."

GRAPHICS

Matlab has outstanding graphics capabilities [you must be using a terminal which supports graphics, to use them]. Start with

	>> x = -10:.1:10;
	>> plot( x.^2 )
        >> figure
	>> plot( x, x.^2 )
        >> figure
	>> plot( x.^2, x )

Note that x must be assigned values, before the plot command is issued [although you could use plot( (-10 : .1 : 10) .^ 2 ) if you really really wanted to].

	>> plot( x, x.*sin(x) )
	>> plot( x.*cos(x), x.*sin(x) )
	>> comet( x.*cos(x), x.*sin(x) )

Functions of two variables may be plotted, as well, but some "setup" is required!

	>> [x y] = meshgrid(-3:.1:3, -3:.1:3);
	>> z = x.^2 - y.^2;
	>> mesh(x,y,z)
	>> surf(x,y,z)
	>> contour(z)
	>> help slice

There's a very interesting example, in the help page for slice; use the mouse to cut and paste it to the matlab prompt.

The following commands bring up lists of useful graphics commands [each has a help page of its own].

	>> help plotxy
	>> help plotxyz
	>> help graphics

To print Matlab graphics, just enter "print" at the Matlab prompt; the current figure window will be sent to the printer. On some systems, it is necessary to set the environment variable PRINTER, before starting Matlab. This is done by typing, at the Unix prompt:

	% setenv PRINTER=myprinter
                                           [for C shell]

	$ PRINTER=myprinter; export PRINTER 
                                           [for B shell]

We don't normally do printing during classes!

SCRIPTS AND FUNCTIONS

Matlab statements can be prepared with any editor, and stored in a file for later use. The file is referred to as a script, or an "m-file" (since they must have names of the form foo.m). Writing m-files will make you much more productive.

Using your favorite editor, create the following file, named sketch.m:

	[x y] = meshgrid(-3:.1:3, -3:.1:3);
	z = x.^2 - y.^2;
	mesh(x,y,z);

Then start Matlab from the directory containing this file, and enter

	>> sketch

The result is the same as if you had entered the three lines of the file, at the prompt.

You can also enter data this way: if a file named mymatrix.m in the current working directory contains the lines

	A = [2 3 4; 5 6 7; 8 9 0]
	inv(A)
	quit

then the command

	>> mymatrix

reads that file, generates A and the inverse of A, and quits matlab [quitting is optional]. You may prefer to do this, if you use the same data repeatedly, or have an editor that you like to use. You can use Control-Z to suspend Matlab, then edit the file, and then use "fg" to bring matlab back to the foreground, to run it.

Matlab may be ran in "batch mode," in a very similar way. If a file named "test.in" contains the [non-graphics] commands you want processed, at the Unix prompt type:

	% matlab < mymatrix.m > homework.out

This is read, "Run matlab, with input from test.in, and output to test.out." The input file does not need to be named "something-dot-m," but it must end with "quit"!

Functions are like scripts, but are compiled the first time they are used in a given session, for speed. Create a file, named sqroot.m, containing the following lines.

	function sqroot(x)
	% Compute square root by Newton's method

	% Initial guess
	xstart = 1;

	for i = 1:100
		xnew = ( xstart + x/xstart)/2;
		disp(xnew);
		if abs(xnew - xstart)/xnew < eps, break, end;
		xstart = xnew;
	end

Save this file, start Matlab, and enter the commands

	>> format long
	>> sqroot(19)

A good exercise would be to create the STAT function described in the help file for function. Note that

	>> stat(x)

and

	>> [m,sd] = stat(x)

produce different results.

The "m-files" which came with Matlab provide lots of examples! To find their location, use

	>> path

This will also lead you to some really nifty demos.

FOR FURTHER INFORMATION

Manuals for statistical and mathematical software are kept in the Swain and SPEA Libraries, and at the Center for Statistical and Mathematical Computing, 618 East Third Street.

Some books which may be useful are

Matlab Primer, by Kermit Sigmon [CRC Press, 1994],

Matrices and Matlab, a Tutorial, by Marvin Markus [Prentice-Hall, 1993],

Solving Problems in Scientific Computing Using Maple and Matlab, ed. Walter Gander and Jiri Hrebicek [Springer-Verlag, 1993].

The http-based [Mosaic] window that appears as a result of the "doc" command contains a link to the MathWorks Home Page, which offers answers to Frequently Asked Questions, and more.

This document, as well as other information, is also available from the UCS Stat/Math Center's WWW home page; connect to http://www.indiana.edu/~statmath . The Matlab at Indiana University page should be especially useful.

If you are affiliated with Indiana University, you may also get help from the Stat/Math Center by sending e-mail to statmath or by phoning 855-4724.

Please send comments and suggestions to: statmath@indiana.edu
URL http://www.indiana.edu/~statmath/smdoc/Matlab.html
Last Modified on Mon Dec 09 13:57:47 1996 EST
Copyright 1995-96, Indiana University.
Permission to use this document is granted so long as the author is acknowledged and notified.