1.6.2 Function Files

1.6.2 Function Files

Function files are m-files that are used to create new MATLAB functions.

Variables defined and manipulated inside a function file are local to the function,

and they do not operate globally on the workspace. However, arguments

may be passed into and out of a function file.

The general form of a function file isfunction variable(s) = function_name (arguments)

% help text in the usage of the function

%

.

.

end

To illustrate the usage of function files and rules for writing m-file function, let

us study the following two examples.

Example 1.3

Write a function file to solve the equivalent resistance of series connected resistors,

R1, R2, R3, …, Rn.

Solution:

MATLAB Script

function req = equiv_sr(r)

% equiv_sr is a function program for obtaining

% the equivalent resistance of series

% connected resistors

% usage: req = equiv_sr(r)

% r is an input vector of length n

% req is an output, the equivalent resistance(scalar)

%

n = length(r); % number of resistors

req = sum (r); % sum up all resistors

end

The above MATLAB script can be found in the function file equiv_sr.m,

which is available on the disk that accompanies this book.

Suppose we want to find the equivalent resistance of the series connected resistors

10, 20, 15, 16 and 5 ohms. The following statements can be typed in the

MATLAB command window to reference the function equiv_sr

a = [10 20 15 16 5];

Rseries = equiv_sr(a)

diary

The result obtained from MATLAB is

© 1999

Rseries =

66

Example 1.4

Write a MATLAB function to obtain the roots of the quadratic equation

ax2 + bx + c = 0

Solution:

MATLAB Script

function rt = rt_quad(coef)

%

% rt_quad is a function for obtaining the roots of

% of a quadratic equation

% usage: rt = rt_quad(coef)

% coef is the coefficients a,b,c of the quadratic

% equation ax*x + bx + c =0

% rt are the roots, vector of length 2

% coefficient a, b, c are obtained from vector coef

a = coef(1); b = coef(2); c = coef(3);

int = b^2 - 4*a*c;

if int > 0

srint = sqrt(int);

x1= (-b + srint)/(2*a);

x2= (-b - srint)/(2*a);

elseif int == 0

x1= -b/(2*a);

x2= x1;

elseif int < 0

srint = sqrt(-int);

p1 = -b/(2*a);

p2 = srint/(2*a);

x1 = p1+p2*j;

x2 = p1-p2*j;

end

rt =[x1;

x2];

end