Chapter 6 User-Defined Functionshung/cs101/chap06.pdfMATLAB Functions M-files are collections of MATLAB statements that stored in a file, called a script file. Script files share the

dr.dcd.h CS 101 /SJC 5th Edition 1

Chapter 6 User-Defined Functions

dr.dcd.h CS 101 Spring 2009 2

MATLAB Functions

M-files are collections of MATLAB statements that stored in a file, called a script file.

Script files share the command window’s workspace, so any variables created by the script files remain in the workspace after the script file finishes executing.

A script file has no input arguments returns no results, but they communicate through the data left behind in the workspace.


MATLAB Functions2

A MATLAB function is a special type of M-file that runs in its own independent workspace.

A MATLAB function receives input data through an input argument list and returns results to the caller through an output argument list.

A MATLAB function should be placed in a file with the same name as the function with an extension of “.m”.


MATLAB Functions3

A function consists of three components

For example:

All functions, whether they are build-in or user-defined functions, are available for any M-file programs

a = cos(x)

output argument function

name input argument


Syntax

The general form of a function as

The function keyword marks the beginning of the function.

If there are only one output argument, the brackets can be dropped.

Input arguments are placeholders for values.

function [outarg1, outarg2, …] =

________funcName(inarg1, inarg2, …) % H1 % other comments Function statements

(return/end)


Syntax2

The comments block following the function statement is called the help block. The first line of it is called the H1. It should contain a one-line summary of the purpose of the function.

H1 will be searched and displaced by lookfor, while the whole help block will be displaced if the command ’help keyword’ is used.

Execution begins at the top and ends when either a return , an end, or the end of the function is reached.


Syntax3

For debugging purpose, it may be useful to print intermediate results to the command window.

However, once you complete your debugging make sure that all your output is suppressed. If you don’t, you’ll see extraneous information in the command window.

It means that you should use ’;’ at the end of

every statement.


Pass-by-Value

When a function is called, MATLAB makes a copy of the actual arguments and passes them to the workspace of the function. If the passed arguments are modified, it won’t affect the original data in the caller.

Command window’s workspace x = 5; … a = abc(x) …

Function abc’s workspace function out=abc(in) … … …

cw.x abc.x

abc.in abc.x


Local Variables

Variables defined in an M-file function, only have meaning inside that program.

The only way to communicate between functions and the workspace, is through the function input and output arguments.


Example 1: dist2(x1,y1,x2,y2)

The distance between two points (x1, y1) and (x2, y2) can be defined as


H1 & Help Block

After applying help and lookfor, function dist2 provides the following results.


Example 2: [x,y]=polar2rect(r,q)

Convert polar coordinates (r,q) to rectangular form (x,y) .


Example 3: [r,q]=rect2polar(x,y)

Convert rectangular coordinates (x,y) to polar form (r,q).


Example 4: Selection Sort

State the problem. Sort the data into ascending or descending order using the selection sort algorithm.

Define the input/output. The inputs are typed by the user, and the outputs are the sorted data written to the command window.

Describe the algorithm.

- Input an array of n values;

- Set the index i equals to1;

- Scan and locate the i-th minimum value;

- Swap the i-th minimum with the i-th element;

- repeat n-1 times.


Example 4: Selection Sort2


Optional Arguments

Some function may support arbitrary numbers of arguments. For example:

plot(x, y);

plot(x, y, ’:ok’);

Plot(x, y, ‘’:ok’, ’MarkerSize’, 3);

Functions that can be used to furnish optional arguments

nargin, nargout, nargchk

varargin, varargout

error, warning

inputname


Optional Arguments2

nargin: the number of actual input arguments.

nargout: the number of actual output arguments.

nargchk: this function returns an error message if too few or too many arguments are used.

- syntax: nargchk(min, max, nargin)

varargin: the list of variables for actual input arguments.

varargout: the list of variables for actual output arguments.


Optional Arguments3

error: display an error message and abort if a fatal error was caught when a function is called. If its input argument is an empty string, then it does nothing.

warning: display a warning message and resume the execution of a function.

inputname: this function returns the actual name of the dummy argument.


Optional Arguments4


Homework Assignment #12

Quiz 6.1

Page 251: 6, 7

This assignment is due by the next week.

Late submission will be penalized.


Global Memory

MATLAB functions can also exchange data with each other with the base workspace through global memory.

Global memory is a specifal type of memory that can be accessed from any workspace.

A global variable is declared as

global variable

The default value for a global variable is empty, so an initialization is required.


Global Memory2

An example: To count how many time a function is called.

An alternative example: Two functions are used to modify a global counter.


Global Memory3


Random Number Generator

It is important to know that the real world does not provide perfect measurements.

So the ideal simulated data needs to add some random noise.

The simulated noise is usually created by a random number generator.

The build-in random number generator function rand generates values in the range of [0.0, 1.0).

Then the generated noise values can be scaled and leveled.


Random Number Generator2

The random numbers are in fact generated by a deterministic function; i.e. the sequence of numbers are fixed.

The function returns a different and apparently random number each time it is called. It is because the function has started at a different starting point internally each time it is called.

The starting point is called a seed and it is set as a global valable.



Example: two set of random numbers in the range of [-0.5, 0.5].


Example 6.4: User Defined

Random Number Generator

A simple random number generator can be defined by the following equation:

n1 = a random seed

ni+1 = mod(8121*ni+28411, 134456)

rani = ni 134456

where 134456 is the total highest number in the sequence and each new ni+1 is set as the

next seed.








Persistent Memory

When a function finishes executing, the workspace created for that function is destroyed, so all local variables within that workspace will disappear.

The next time the same function is called, a new workspace will be created.

Sometimes, it is necessary to preserve local variables between calls to a function.


Persistent Memory2

Persistent memory is a special mechanism to allow local variables to be preserved between calls to a function.

Syntax: persistent variable


Persistent Memory3

To rewrite a previous example: To count how many time a function is called.


Persistent Memory4

How to reset the associated counter?


Homework Assignment #13

6.9 Exercises

Page 268: 6.6, 6.7, 6.14

This homework is for your reference.

Documents

Chapter 6 User-Defined Functionshung/cs101/chap06.pdfMATLAB Functions M-files are collections of MATLAB statements that stored in a file, called a script file. Script files share the