Basics of MATLAB 2- Programming in MATLAB

ByDR. Wafaa Shabana

Email: wrs_coms@yahoo.com

Table of Contents:

1. Introduction2. MATLAB m-files.3. Flow control in MATLAB.4. Image Processing using MATLAB5. Questions?6. References

1. Introduction:

• Recall that:• Everything in MATLAB is a matrix !• The MATLAB environment is command oriented somewhat

like UNIX. • If a statement is terminated with a semicolon ( ; ), no results

will be displayed. Otherwise results will appear before the next prompt.• ans Default variable name for results• % start comment which ends at end of line

The colon operator ( : ) to specify range •MATLAB supports six relational operators.

Less Than <Less Than or Equal <=Greater Than >Greater Than or Equal >=Equal To ==Not Equal To ~=

• MATLAB supports three logical operators.

not ~ % highest precedenceand & % equal precedence with oror | % equal precedence with and

• MATLAB is case-sensitive Programming language.• Variable names can contain up to 63 characters (as of

MATLAB 6.5 and newer)• Variable names must start with a letter followed by letters,

digits, and underscores• Examples: my_name , day2 % valid variable names Invalid variable name: my-name %minus not allowed 2day %must start with letter %x % not allowed character @room %not allowed character

Some of useful MATLAB commands•who List known variables•whos List known variables plus their size•help >> help sqrt Help on using sqrt•lookfor >> lookfor sqrt Search for

keyword sqrt in m-files•what >> what a: List MATLAB files in a:•clear Clear all variables from work space•clear x y Clear variables x and y from work space•clc Clear the command window (clear the screen)•save saves the workspace load loads a saved workspace or data file

2. MATLAB m-files:

•There is no doubt that you can do a lot of work in the command window. However, when the amount of instructions needed to complete a task increases or you need to re-execute a block of instructions several times, the command window is a poor choice. The best choice is to write a MATLAB program.

•MATLAB programs are stored as text files. •The files can be loaded into the MATLAB command environment where they are then interpreted and executed.

MATLAB supports two types of programs: 1- script files and2- function files.

Script files consist of sequences of instructions stored in text files. The files are commonly referred to as m-files because of the .m extension used.•An m-file can be created by any text editor or word processor. •If a word processor is used, make sure to save the file as a text file.• MATLAB has its own built-in text editor. •To launch it, type the command edit in the command window

Example (1):%this is our first m.file%saved as first.m%plot a graph of a cos functioncleart = 0: 0.001:10;y = cos (2*pi*t);plot(t,y)title(‘My first graph’)xlabel(‘time, sec’)ylabel(‘values of y’)

Remark:1- Before trying to run your program, make sure that its file is on the MATLAB path. 2- Use the which function to see if your program is on the path:

which first3-You have the choice of operating the debugger from the Editor window that displays your program, from the MATLAB command line, or both.

Function files:• The main difference between a script and a function is that a function

accepts input from and returns output to its caller, whereas scripts do not.

• You define MATLAB functions in a file that begins with a line containing the function key word.

• Question:Can we define a function within a script file or at the command window?

* Functions always begin with a function definition line for example function y = average(x)

• The function ends either with the first matching end statement, the occurrence of another function definition line, or the end of the file, whichever comes first.

Example:

• function y = area(x1,x2)• %this function computes the area of a rectangle• %input: length x1 and width x2• %output: area y• x1 =input('Enter the width:');• x2 =input('Enter the length:');• y = x1 * x2;• disp(‘The area is %d’, y)

inputRequest user input

SyntaxevalResponse = input(prompt)strResponse = input(prompt, 's')

dispDisplay text or array

Syntaxdisp(X)

• If the function is supposed to return more than one value, the syntax is:

function [output_1,output_2,…]= function_name(input_1, input_2,…)

Question:What is the difference between MATLAB functions and C-

functions?Note that:• The use of the editor is pretty straight-forward. The

advantage of using MATLAB’s editor is that color coding and formatting is built in.

3. Control flow in MATLAB:

Decision in MATLAB:1- if statement:if (logical statement) block of code to execute if logical statement is trueend2- if …………else statement:if (logical statement) block of code to execute if logical statement is trueelse block of code to execute if logical statement is falseend

3- Nested if ------else statement:if (logical statement 1)

block of code to execute if logical statement 1 is trueelseif (logical statement 2)

block of code to execute if logical statement 2 is trueelseif (logical statement n)

block of code to execute if logical statement n is trueelse

block of code to execute if all logical statements are false

end

4- switch statement:switch evaluative expressioncase value_1

block of code to execute if the evaluative expression is value_1case value_2

block of code to execute if the evaluative expression is value_2case value_n

block of code to execute if the evaluative expression is value_notherwise

block of code to execute if the evaluative expression is not any of the valuesend

2- Iteration in MATLAB:

1- for statement:for i = 1:10 % execute these commandsend

2- while statement:while x <= 10

% execute these commandsend

break statement:Terminate execution of for or while loop

Syntaxbreak

return statement:Return to invoking function

Syntaxreturn

continue statement:Pass control to next iteration of for or while loop

Syntaxcontinue

4. Image Processing in MATLAB:Image processing toolbox is one of the most useful toolboxes in MATLAB.This toolbox supports a wide range of image processing operations.There are five types of images in MATLAB: 1- Binary image : {0, 1}{black white}

2- Gray-scale image: [0,255] {shades of gray}3- True color image: m x n x 34- Intensity image: [0, 1] or uint85- Indexed images: m x 3 color map

1- Reading an image: I = imread(‘image name’)It reads an image from an image file and store it In a matrix I

2- displaying an image:imshow(‘image_name’)

3- image size

size(‘image_name’)

Check the Image in Memory

< Name, Size, Bytes, Class >whos

Name Size Bytes Class ans 291x240 69840 uint8 arrayGrand total is 69840 elements using 69840 bytes

•uint8 [0, 255]•uint16 [0, 65535]•double [0, 1]

Resizing Images

• B = imresize(A, scale)• B = imresize(A, [mrows ncols])

For example:

clearclose ally= imread('pic2.jpg');I1 = imresize(y, 0.5);I2 = imresize(y,[256 256]);figureimshow(y)figureimshow(I1)figureimshow(I2)

im2bwConvert image to binary image, based on threshold

SyntaxBW = im2bw(I, level)BW = im2bw(X, map, level)BW = im2bw(RGB, level)

rgb2grayConvert RGB image or colormap to grayscale

Some useful commands:

imageDisplay image object

imwriteWrite image to graphics file

Syntaximwrite(A,filename,fmt)

e.g. imwrite(I, ‘wafaa’, ‘gif’)

5. Questions?(Mini project in MATLAB):

1- Encoding step:Write a m-function that hides some information (e.g. : your name) into a given image.(watermarked image)

2- Decoding Step:Write an m-function that receives the watermarked image and extract the water mark from it.

6. References:You can read the following references:

•1- “The Basics of MATLAB” Jeffrey O. Bauer•2- www.mathworks.com•3. “MATLAB tutorial for beginners” Jyotirmay Gadewadikar http://arri.uta.edu/acs/

Need further help? Contact me

• Thank you for your attendance and hope you have not wasted your time.