MATLAB Tutor, Part 6

MATLAB Tutor

Part 6: M-Files and User-defined Functions

A good way to "automatically" execute a sequence of often-used commands without having to go to the trouble of typing them in each time is to use an m-file. An m-file is a plain text file that you can create with any text editor. In it, you can put the same kind of commands that you type in at a MATLAB prompt.

Typing the m-file's name at a prompt simply successively executes the commands in the file. It is just as if you had typed them in at individual MATLAB prompts.

Use a text editor to create a file that contains the following lines. Save the file in your working directory under the name tester.m.

d2r = pi/180
a = sin(30*d2r)
b = (1 + a)/4

In MATLAB, at the prompt, enter

tester

There will be occasions when you will need to define your own specialized functions which are not built into MATLAB. You do this by using a special kind of "function" m-file. As with standard functions like sin(x), a function m-file accepts input arguments and returns outputs.

An example function m-file is given below. It accepts the input argument x and returns y = fun(x). Note that the first line contains the key word "function" and describes inputs and outputs. The output value must be assigned within the "program" to the variable y which appears before the equals sign on the first line of the file. The m-file must be given the same name as the function, namely fun.m.

Use a text editor to create a file that contains the following lines. Save in your working directory under name fun.m.

function y = fun(x)
d2r = pi/180 ;
y = 3*cos(x*d2r) - 1 ;

In MATLAB, at the prompt, enter

fun(45)

Enter

z = 30
a = fun(z)+ 8

User-defined functions can be plotted as easily as built-in ones. We will use the plot commands we learned in Part 5 of this Tutorial.

Enter

x = -10: 0.5: 20;
y = fun(x);
plot(x, y)

Enter

fplot('fun(x)', [- 10, 20], '-o')

You can define functions that take inputs of more than one variable. Output can likewise be more than one variable. Both input and output variables can be vectors.