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28864. Introduction to Matlab programming

Krist V. Gernaey

Outline

Introduction

Matlab syntax: useful commands to get started

Working with vectors and matrices

Matlab scripts and functions

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Matlab programming control

File input / output

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Outline

Introduction

How to use the Matlab help function?




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File input / output

What is Matlab?

The name Matlab stands for MATrix LABoratory

.instructional tool for introductory and advanced courses in mathematics,engineering, and science (e.g. course 28150).

Matlab: basic license (basic functionality) that can be extended with add-on application-specific solutions (toolboxes). Toolboxes are collections ofMatlab functions (M-files) that extend the Matlab environment to solveparticular classes of problems.

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Detailed info: www.mathworks.com

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Free Matlab alternatives

Octave (www.octave.org)

Very similar

Scilab (www.scilab.org)

Syntax differences

Windows supported

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Starting and quitting Matlab

Starting Matlab:

By double-clicking the Matlab icon on the desktop of your Windows

By selecting Matlab from the program list

Quitting Matlab:

Click the Close box in the Matlab desktop.

Select Exit Matlab from the desktop File menu.

Press Ctrl + Q

Type qui t at the Command Window prompt.

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The Matlab desktop (Matlab 6.5)

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The Matlab desktop (Matlab 7.0)

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Outline

Introduction

ow o use e a a e p unc on




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File input / output

How to use the Matlab help function?

The traditional way of getting help:

>>hel p f unct i onname

Example: >>hel p sqr t

Use the Matlab Helpbrowser

>>hel pbrowser

>>hel pwi n f unct i onname

Use Matlab help from the Help menu

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Using the Matlab Start button

Get Help from the toolbar

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Help at www.mathworks.com

Go to Support (http://www.mathworks.com/support/)

Documentation

Information about Matlab based books

Etc.

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Matlab books, introduction to Matlab

Essential Matlab for engineers andscientists (3rd Edition)

Brian Hahn and Dan Valentine (2007)

Elsevier, Paperback, 448 pages

ISBN-13: 978-0-7506-8417-0

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Outline

Introduction




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File input / output

Matlab commands to get started

>>demo % Launches the Matlab Helpbrowser to the Demos tab, containing

information on all demos available in Matlab

>>ver % Displays Matlab, Simulink, and toolbox version information

>>whos % Lists the variables that are currently defined in the Matlab

Workspace, long version with detailed information

>>who % Like whos, but will only list the names of the variables

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>>what % Lists the Matlab specific files in the current directory, sorted by

file type

>>pwd % Lists the name of the current directory

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Matlab commands to get started

>>cl ear % Clear all variables from the memory. As a result, the

Matlab Workspace will be empty

>>cl ear dat a % Clear the variable data from the memory.

>>cl c % Clear the Matlab Command Window

>>cl ose al l % Close all figure windows

These commands are useful at the start of a script, when you want to be

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calculations!

Defining variables in Matlab

Variable names are case sensitive

Y1 and y1 are 2 different variables!!!

Variable names can contain up to 63 characters (as of Matlab 6.5 andnewer)

Use namel engt hmax to check

Variable names must start with a letter followed by letters, digits, andunderscores

OK: a5, x_27, substrate, S_NH, ...

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, ...

If you are not sure whether a variable name is correct, use i svar name to

check

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Keywords are protected!!!

Use i skeyword to check

Matlab function names can be overwritten if you declare a variable with aname that is identical to the name of a function!

The function can be used again after clearing the variable with thefunction name

Some Matlab functions are often overwritten in programming, e.g. i ,j ,hel , s i ze,

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An illustrative example:

E.g.: >> hel p=5;

hel p =

5

>> cl ear hel p

>> hel p

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HELP t opi cs

. . .

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Special variables in Matlab

>>ans % Default variable name for results

>>pi % Value of

>>i nf % Infinity

>>NaN % Not a number, e.g. 0/0

>>i

>>j % i = j = square root of -1

>>r eal mi n % The smallest usable positive real number

>>r eal max % The largest usable positive real number

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Matlab operators

+ Addition

- Subtraction

*

/ Division

\ Left division

^ Power

' Complex conjugate or transpose

( ) Specify evaluation order

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A few tests!

Compare the results of:

>>56/ 8 and >>56\ 8

What is the result of the following statements?:

>> a=5, b=10

>> c=( a+b) 2+b

>> c=c+10

For complex numbers, try the following:

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>>i '

Other Matlab symbols

>> prompt

. . .

, separate statements and data

% start a comment; the comment ends at end of line

; (1) suppress output

(2) used as a row separator in a matrix

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: specify range, for example in a matrix

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Outline

Introduction




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File input / output

Definition of matrices in Matlab

Matlab treats all variables as matrices. For our purposes a matrix can bethought of as an array.

Vectors are special forms of matrices and contain only one row or onecolumn.

Scalars are matrices with only one row and one column

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Matrices and vectors are generated by enclosing a set of numbers insquare brackets

>>x = 1 2 3 4 5 6 7 8 9 10

The following statements are equivalent to the previous one:

>>x = 1: 1: 10

>>x = 1: 10

Defining a column vector:

>>y=[ 1; 2; 3; 4; 5; 6; 7; 8; 9; 10]

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y=x


Defining a 3 by 3 matrix

>>a = [ 1 2 3; 4 5 6; 7 8 9]

Defining a 4 by 2 matrix

>>b = [ 1 2; 3 4; 5 6; 7 8]

Useful commands

>>si ze( a) % Find the size of the matrix a; 2 numbers will

be generated (first number of rows, then number of columns). Checkwith si ze( b)

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>>l ength(y) % Find the length of the vector y

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Selecting matrix elements

>>a = [ 1 2 3; 4 5 6; 7 8 9; 10 11 12] ;

>>aa=a( 4, 2) ;

= ,

>>ac=a( : , 2) ;

The colon operator by itself means that one selects all elements in onedirection. The colon separator can also be used to generate a list ofnumbers.

>>ad=a( 1: 3, 2) ;

>>ae=a( 2: end, 2) ;

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Matrix algebra

>>x = 1: 10

>>y1 = 2 * x % Multiplies x by a constant

=

>>y3 = y1 + y2 % Adds the two vectors y1 and y2

Matrix multiplication

>>a=[ 1 2; 4 5; 7 8] ;

>>b=[ 0. 5 0. 2 2. 0; 4. 0 2. 0 5. 0];

>>c=a* b % Multiplies the two matrices and puts the result in a

new matrix c

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Matrices, element by element operations

>>a=[ 1 2; 6 9]

=

>>x1=a. 3 % Takes the cube of each element in the square matrix a

>>x2=a 3 % Takes the cube of the matrix a

>>x3=a. *b % Multiplies element by element

a(i,j)*b(i,j) when size(a) = size(b)

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>>x4=a* b % Matrix multiplication

Important!

In Matlab, performing calculations based on vectors is much faster thanwriting a loop construct!

Note that all functions in Matlab, for example sqrt(), will accept scalars,vectors or matrices as inputs

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Outline

Introduction




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File input / output

Matlab scripts

For tasks that are to be repeated:

Save them in a script and execute that script when needed

-,

To create an M-file:

Save all the repetitious Matlab statements in a text file with the .mextension.

To execute the script: enter the file name without the .m extension inthe Matlab command window

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Matlab executes the commands found in the file

Scripts can:

Operate on existing data in the workspace

Can create new data on which to operate

Any variables created in a script remain in the workspace

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Important!

Matlab has a built-in editor that can be used for creating m-files

>>edi t % opens up a new editor window.

Matlab may not find your M-files!

Windows machines: Matlab looks by default for your files in the Workfolder, a subfolder of the MATLAB installation folder.

When keeping files in your own directories.

Change the directory in the MATLAB Command Window to thedirectory where the M-file that you would like to use is located

OR add that directory with your files to the MATLAB searchath

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Matlab functions

M-files that can accept input arguments and return output arguments

- .

Functions operate on variables within their own function workspace,separate from the workspace you access at the Matlab command prompt

Only output arguments of the function become available in theworkspace

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Matlab function example

f uncti on y = squar e( x)

%

' '

%

% Ar gument s:

%

% x ( i nput ) val ue t o be squared

%

% y ( out put ) t he r esul t of t he squar e

%

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y = x*x;

% end of squar e f unct i on

Calling a function from a script

SCRIPT

Define variables Define inputs to thefunction and call the function FUNCTION

Accept inputs

Perform calculations in

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Present resultsextra calculationsplot / save to file

a local workspace

Send outputs to script

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Outline

Introduction




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File input / output

If statement

Conditionally execute statements

i f expr essi on

st at ements

end

Matlab evaluates the expression and, if the evaluation yields logical 1(true) or a nonzero result, executes one or more Matlab commandsdenoted here as statements.

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When you are nesting ifs, each if must be paired with a matching end!

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If statement

When using elseif and/or else within an if statement, the general form ofthe statement is:

i ex r essi on1

st atement s1

el sei f expr essi on2

st atement s2

el se

st atement s3

end

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Switch statement

A means of conditionally executing code

Syntax

_

case case_expr

st atement , . . . , st at ement

case {case_expr 1, case_expr 2, case_expr 3, . . . }


other wi se


end

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Switch statement, example

swi t ch a

case 0

' '=

case {1, 2, 3, 4}

di sp( ' a i s l ess than 5' )

case 6

di sp( ' a i s 6' )

other wi se

di sp( ' a i s out of r ange' )

end

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For loop

Execute code a specified number of times

Syntax

=

st at ement s

end

Example:

a=[ ] ;

f or z=1: 5

even=2*z- 2;

a( z) =even

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end

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While statement

Repeatedly execute statements while condition is true

Syntax

st at ement s

end

repeats statements an indefinite number of times.

expression is usually of the form:

expression rel_op expression

where: rel_op is:

== (equal to), , =, or ~= (not

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More advanced, but useful

Continue

Return

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Example: use break to interrupt while loop

br eak is a very useful statement that allows you to exit a for or while

loop prematurely

Example:

a=10;

whi l e a>2

a=a/ 2;

i f a

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Saving and loading workspace variables

>>save Saves all variables in the Workspace in matlab.mat

>>save FI LENAME Saves all Workspace variables to the binary

- .

>>save FI LENAME X Saves only the Workspace variable X to the

binary MAT-file named FILENAME.mat

>>l oad FI LENAME Retrieves data from the file

FILENAME.mat

If FILENAME has no extension, the .mat extension is assumed bydefault

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Data in text files

>>save FI LENAME. t xt - asci i

Saves all Workspace variables to the ascii text file named FILENAME.txt

. -

Retrieves data from the file FILENAME.txt, which is assumed to be a tab-delimited ascii text file

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01_28864_Matlab_Syntax_E2011