INTRODUCTION TO MATLAB – You can find all the Matlab tutorial here. In this tutorial we will discuss about programming in Matlab.
Introduction
So far in these lab sessions, all the commands were executed in the Command Window. The problem is that the commands entered in the Command Window cannot be saved and executed again for several times. Therefore, a different way of executing repeatedly commands with MATLAB is:
1. to create a file with a list of commands,
2. save the file, and
3. run the file.
If needed, corrections or changes can be made to the commands in the file. The files that are used for this purpose are called script files or scripts for short.
This section covers the following topics:
• M-File Scripts
• M-File Functions
M-File Scripts
A script file is an external file that contains a sequence of MATLAB statements. Script files have a filename extension .m and are often called M-files. M-files can be scripts that simply execute a series of MATLAB statements, or they can be functions that can accept arguments and can produce one or more outputs
Examples
Here are two simple scripts.
Example 1
Consider the system of equations:
Find the solution x to the system of equations.
Solution:
• Use the MATLAB editor to create a file: File → New → M-file.
• Enter the following statements in the file:
A = [1 2 3; 3 3 4; 2 3 3];
b = [1; 1; 2];
x = A\b
• Save the file, for example, example1.m.
• Run the file, in the command line, by typing:
>> example1
x = -0.5000
1.5000
-0.5000
When execution completes, the variables (A, b, and x) remain in the workspace. To see a listing of them, enter whos at the command prompt.
Note: The MATLAB editor is both a text editor specialized for creating M-files and a graphical MATLAB debugger. The MATLAB editor has numerous menus for tasks such as saving, viewing, and debugging. Because it performs some simple checks and also uses color to differentiate between various elements of codes, this text editor is recommended as the tool of choice for writing and editing M-files.
There is another way to open the editor:
>>edit
or
>> edit filename.m
to open filename.m.
Example 2
Plot the following cosine functions, y1 = 2 cos(x), y2 = cos(x), and y3 = 0.5 ∗ cos(x), in the interval 0 ≤ x ≤ 2π. This example has been presented in previous Chapter. Here we put the commands in a file.
• Create a file, say example2.m, which contains the following commands:
x = 0:pi/100:2*pi;
y1 = 2*cos(x); y2 = cos(x);
y3 = 0.5*cos(x);
plot(x,y1,’–’,x,y2,’-’,x,y3,’:’)
xlabel(’0 \leq x \leq 2\pi’)
ylabel(’Cosine functions’)
legend(’2*cos(x)’,’cos(x)’,’0.5*cos(x)’)
title(’Typical example of multiple plots’)
axis([0 2*pi -3 3])
• Run the file by typing example2 in the Command Window
Script side-effects
All variables created in a script file are added to the workspace. This may have undesirable effects, because:
• Variables already existing in the workspace may be overwritten.
• The execution of the script can be affected by the state variables in the workspace
As a result, because scripts have some undesirable side-effects, it is better to code any complicated applications using rather function M-file.
M-File functions
As mentioned earlier, functions are programs (or routines) that accept input arguments and return output arguments. Each M-file function (or function or M-file for short) has its own area of workspace, separated from the MATLAB base workspace.
Anatomy of a M-File function
This simple function shows the basic parts of an M-file.
function f = factorial(n)
% FACTORIAL(N) returns the factorial of N.
% Compute a factorial value.
f = prod(1:n);
The first line of a function M-file starts with the keyword function. It gives the function name and order of arguments. In the case of function factorial, there are up to one output argument and one input argument. Table 4.1 summarizes the M-file function.
As an example, for n = 5, the result is,
>> f = factorial(5)
f = 120
Both functions and scripts can have all of these parts, except for the function definition line which applies to function only.
In addition, it is important to note that function name must begin with a letter, and must be no longer than than the maximum of 63 characters. Furthermore, the name of the text file that you save will consist of the function name with the extension .m. Thus, the above example file would be factorial.m.
Table below summarizes the differences between scripts and functions.
Input and output arguments
As mentioned above, the input arguments are listed inside parentheses following the function name. The output arguments are listed inside the brackets on the left side. They are used to transfer the output from the function file. The general form looks like this
function [outputs] = function_name(inputs)
Function file can have none, one, or several output arguments. Table 4.3 illustrates some possible combinations of input and output arguments.
Input to a script file
When a script file is executed, the variables that are used in the calculations within the file must have assigned values. The assignment of a value to a variable can be done in three ways.
1. The variable is defined in the script file.
2. The variable is defined in the command prompt.
3. The variable is entered when the script is executed.
We have already seen the two first cases. Here, we will focus our attention on the third one. In this case, the variable is defined in the script file. When the file is executed, the user is prompted to assign a value to the variable in the command prompt. This is done by using the input command. Here is an example
% This script file calculates the average of points
% scored in three games.
% The point from each game are assigned to a variable
% by using the ‘input’ command.
game1 = input(’Enter the points scored in the first game ’);
game2 = input(’Enter the points scored in the second game ’);
game3 = input(’Enter the points scored in the third game ’);
average = (game1+game2+game3)/3
The following shows the command prompt when this script file (saved as example3) is executed.
>> example3
>> Enter the points scored in the first game 15
>> Enter the points scored in the second game 23
>> Enter the points scored in the third game 10
average = 16
The input command can also be used to assign string to a variable. For more information, see MATLAB documentation
A typical example of M-file function programming can be found in a recent paper which related to the solution of the ordinary differential equation (ODE)
Output commands
As discussed before, MATLAB automatically generates a display when commands are executed. In addition to this automatic display, MATLAB has several commands that can be used to generate displays or outputs.
Two commands that are frequently used to generate output are: disp and fprintf. The main differences between these two commands can be summarized as follows
You can see the complete Matlab Matlab here:
- Introduction to Matlab
- Chapter 2 on Matlab
- Matrix on Matlab
- Array And Linear Operation Matlab
- Programming in Matlab
- Control Flow and Operation in Matlab
- Debugging M-files in Matlab
