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Basics of MATLAB 2- Programming in MATLAB By DR. Wafaa Shabana Email: wrs_coms@yahoo.com
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Table of Contents: 1. Introduction 2. MATLAB m-files. 3. Flow control in MATLAB. 4. Image Processing using MATLAB 5. Questions? 6. References
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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. ansDefault variable name for results % start comment which ends at end of line
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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~=
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MATLAB supports three logical operators. not~% highest precedence and&% equal precedence with or or |% equal precedence with and
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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
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Some of useful MATLAB commands whoList 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
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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.
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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 and 2- function files.
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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
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Example (1): %this is our first m.file %saved as first.m %plot a graph of a cos function clear t = 0: 0.001:10; y = cos (2*pi*t); plot(t,y) title(‘My first graph’) xlabel(‘time, sec’) ylabel(‘values of y’)
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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 first 3-You have the choice of operating the debugger from the Editor window that displays your program, from the MATLAB command line, or both.
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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?
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* 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.
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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)
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input Request user input Syntax evalResponse = input(prompt) strResponse = input(prompt, 's') disp Display text or array Syntax disp(X)
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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.
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3. Control flow in MATLAB: Decision in MATLAB: 1- if statement: if (logical statement) block of code to execute if logical statement is true end 2- if …………else statement: if (logical statement) block of code to execute if logical statement is true else block of code to execute if logical statement is false end
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3- Nested if ------else statement: if (logical statement 1) block of code to execute if logical statement 1 is true elseif (logical statement 2) block of code to execute if logical statement 2 is true elseif (logical statement n) block of code to execute if logical statement n is true else block of code to execute if all logical statements are false end
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4- switch statement: switch evaluative expression case value_1 block of code to execute if the evaluative expression is value_1 case value_2 block of code to execute if the evaluative expression is value_2 case value_n block of code to execute if the evaluative expression is value_n otherwise block of code to execute if the evaluative expression is not any of the values end
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2- Iteration in MATLAB: 1- for statement: for i = 1:10 % execute these commands end 2- while statement: while x <= 10 % execute these commands end
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break statement: Terminate execution of for or while loop Syntax break return statement: Return to invoking function Syntax return continue statement: Pass control to next iteration of for or while loop Syntax continu e
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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 3 4- Intensity image: [0, 1] or uint8 5- Indexed images: m x 3 color map
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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’)
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Check the Image in Memory whos Name Size Bytes Class ans 291x240 69840 uint8 array Grand total is 69840 elements using 69840 bytes uint8 [0, 255] uint16 [0, 65535] double [0, 1]
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Resizing Images B = imresize(A, scale) B = imresize(A, [mrows ncols])
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For example: clear close all y= imread('pic2.jpg'); I1 = imresize(y, 0.5); I2 = imresize(y,[256 256]); figure imshow(y) figure imshow(I1) figure imshow(I2)
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im2bw Convert image to binary image, based on threshold Syntax BW = im2bw(I, level) BW = im2bw(X, map, level) BW = im2bw(RGB, level) rgb2gray Convert RGB image or colormap to grayscale Some useful commands:
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image Display image object imwrite Write image to graphics file Syntax imwrite(A,filename,fmt) e.g. imwrite(I, ‘wafaa’, ‘gif’)
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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.
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6. References: You can read the following references: 1- “The Basics of MATLAB” Jeffrey O. Bauer 2- www.mathworks.comwww.mathworks.com 3. “MATLAB tutorial for beginners” Jyotirmay Gadewadikar http://arri.uta.edu/acs/ Need further help? Contact me
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Thank you for your attendance and hope you have not wasted your time.
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