1

The Hong Kong Polytechnic UniversityIndustrial Centre

MatLABLesson 7 : M-file Programming I

Edward Cheung

email: icec@polyu.edu.hk

Room W311g

2008

2

Types of M-file

• Script M-file doesn’t accept input argument No return output argument Operates on data in the workspace only Useful for the automation of a series of commands that

frequently needed – like macro

• Function M-files Accept input argument and provide output return argument Internal variables are local by default Useful for building function

• Can be include in search path

• fileSet path or >> pathtool

3

Example of Script File

%script M-file%This program shows how to create a script filestart_th=0;end_th=2*pi;th=start_th:.1:end_th;x=sin(th);y=cos(th);plot(x,y);axis equaltitle('Sine vs. Cosine','fontsize',18)xlabel('Sine','fontsize',16)ylabel('Cosine','fontsize',16)

4

Example of Script File (cont.)

>> save file as script_test.m>> script_test % run script file

5

Functions

• Types of functions Built-in function User-defined function

• Function files are created and edited like script files using the editor/debugger window

• fileNewM-file

• First line must be the function’s definition

• Syntax:- function [output_arg]=fun_name (input_arg) [] is not required for single output argument

6

Variables

• Local The arguments and variables are valid for the function file

only

• Global We can make a variable recognized in different function

files by using >>global=variable_name

• All the variables in a function file are local

• Good practice is to minimize the use of global variable to avoid conflict with other programmes sharing the environment.

7

Example of Functions in Modular Arithmetic

Remainder & Modular Functions >> mod(38,3)ans = 2>> rem(38,3)ans = 2>> rem(38,-3) % rem (x,y) has the same sign as x (36/3)

ans = 2>> mod(38,-3) %mod(x,y) has the same sign as y (39/3)

ans = -1> which modC:\matlab12\toolbox\matlab\elfun\mod.m

8

Creating Function M-files

• Must start with a function declaration in first line• Function output=my_function(x)• % suppose we want to create a function to calculate sine

square.• Enter the following in a new m-file and save the file as

sin_square.mfunction x=sin_square(y)x=sin(y)*sin(y)

>> which sin_square %function exist?>> type sin_square %type to screen if needed

>> sin_square(pi/8) %use function>> u=sin_square(pi/8)u = 0.1464

9

Application Example: Determine Triangle Area

)(For

))()((

)(2

1

bac

csbsassArea

cbas

• There are different methods to calculate the area of a triangle depending on known parameters

• The Huron formula (Greek 1st century) can be used if the sides of the triangle are known

• where

s= semiperimeter

Area=area of ABC

10

Example on Function with More Variables% Calculates Area of a Triangle % Huron Formula :Input 3 sides% Usage: triangle_area(a,b,c)% area=triangle_area(a,b,c)% Example:- % triangle_area(3,4,5)% Area=triangle_area(6,8,10)function x=triangle_area(a,b,c)s=1/2*(a+b+c);x=sqrt(s*(s-a)*(s-b)*(s-c));

You can get help by >>help triangle_area

% print comments after the function declaration

11

In-line Functions

• Use when a simple function is needed for many times within a programme

• Reside in the same programme file• Name=inline(‘expression’,’arg1’,’arg2’…)• Create an inline object based on character string

Example:>> sin2_cos2=inline('sin(x)^2+cos(y)^2','x','y')sin2_cos2 = Inline function: sin2_cos2(x,y) = sin(x)^2+cos(y)^2>> which sin2_cos2sin2_cos2 is a variable.>> u=sin2_cos2(pi/2,pi/2)u = 1

12

Anonymous Function & Function Handle

• User can define a function at the command line or in M-file using a file handle without an associated file. It is available after MatLab v.6. A function handle is a data type in MatLab containing all the

information for referencing a function. Created by using the @ operator or the str2func function

• Syntax:- handle = @(arglist)anonymous_function

For example, to create a square anonymous function,

y = x2

>> y=@(x)x^2; >> y(4) ans = 16

13

Example of Two Variables Anonymous Function

g=@(x,y) (x.^2 + y.^2);ezsurf(g);shading flat;title('f(x,y)=x^2+y^2');

-5

0

5

-5

0

5

0

20

40

60

80

x

f(x,y)=x2+y2

y

• Use function handle to pass function into function for new applications as it is more efficient than string operations. Use inline function if MatLab flags an error.

14

Flow Control

• There are 4 flow control statements in MatLab

• If statement

• Switch statement

• While loops

• For loops

• The if statement and the while loops use relational or logical operation to determine programme flow

15

Loops

• For loop Use when the number of passes are known ahead of time

• While loop Use when the loop must terminate when a specific condition

is met.

• Automatic indentation is available on the editor/debugger when pressing the enter key until an end statement is reached.

16

Relational Operators

Relational Operators Meaning

< Less than

<= Less than or equal

> Greater than

>= Greater than or equal

== Equal to

~= Not equal to

17

Example on Relational Operator

Example 1:a=[1 2 4 6 2];b=[2 1 4 5 2];a== b % check if a equals ba>b % check if a greater than b

Example 2:x=1:5;y=4; % y is a scaler x<y % ans is a vectorans = 1 1 1 0 0

• For a comparison to be true for an entire matrix, every element must be true(i.e. all equals to 1).

18

Find

• find searches a matrix and identifies which elements in that matrix meet certain criterion.>> x =[7 14 21 28 35]>> find(x>20) % get index of elementans = 3 4 5>> z=find(x>21) % save indices into zz = 4 5

19

Example on using find

% ex_find1% Given a matrix x, find & print all elements

with x>50.%x=[20 30 40;89 9 88;33 90 54];disp('Given a matrix x=')disp(x)element=find(x>50);[row,col]=find(x>50);disp('Element in matrix x with value greater than

50 are as follows:-')fprintf('x(%d,%d)=%d\n',[row,col,x(element)]')

20

Logical Operators

Logical Operator Meaning

~ Not

& And

| Or

Xor Exclusive or

•Logical operator works element-wise on matrices

•Boolean operation: 1=true and 0=false

21

Truth Table

Input Not And Or Xor

x y ~x x&y x|y xor(x,y)

0 0 1 0 0 0

0 1 1 0 1 1

1 0 0 0 1 1

1 1 0 1 1 0

22

The if & if/else Structure

• The if statement evaluates a logical expression and executes a group of statement when the expression is trueIf condition

StatementsEnd

• The if/else allows user to execute a series of statement if a condition is true and to skip to the else if the condition is false. Example:-

x=input('x=')if x>0 y=log(x)else disp('input must be positive')end

23

The elseif Structure

• The elseif structure allows user to check multiple criteria while keeping the code easy to read.

• For example:-If age<11 disp(‘You cannot get any ID card’)elseif age<17 disp(‘You may have a children ID card’)else age>21 disp(‘You may have a adult ID card’)End

• In this example, the programme steps thru the structure until it find a true statement, execute & exit or until it encounters the else

24

Example on Function with Logic Control% Calculates Area of a Triangle % Huron Formula :Input 3 sides% Usage: triangle_area(a,b,c)% area=triangle_area(a,b,c)% Example:- % triangle_area(3,4,5)% Area=triangle_area(6,8,10)function x=triangle_area(a,b,c)if a<0|b<0|c<0 error('Input a,b,c must be positive')elseif (a+b)<c|(a+c)<b|(b+c)<a warning('The sum of any two sides must be greater

than the third side')else s=1/2*(a+b+c); x=sqrt(s*(s-a)*(s-b)*(s-c));End% Save as triangle_area1.m and try (a,b,c) = (1,1,2.1)

25

Array Indexing - Use an Array as an Index to an Array

• MATLAB supports a type of array indexing that uses one array as the index into another array.

• Example:- Array B indexes into elements 1, 3, 6, 7, and 10 of array A.A = 5:5:50A = 5 10 15 20 25 30 35 40 45 50

B = [1 3 6 7 10];

A(B) % syntax of Array Indexingans = 5 15 30 35 50

Hence, the numeric values of array B designate the intended elements of A.

26

Array Indexing – Logical Indexing

A Simple Example on logic operations:->> q=1; r=2;>> q>r>> q<r

• We can use this logic operation in matrix for element selection. Logical indexing designates array elements of a matrix based on their position in an indexing array. This is a logical masking operation. True element in the indexing array is being used as a positional index.

27

Example on Logical Indexing

• Example:- Eliminate elements for A>0.5. Creates logical array B that satisfies the condition A > 0.5,

and uses the positions of ones in B to index A. This is known as logical indexing:

Steps:-A = rand(3) B = A > 0.5 % set elements of B to 1 ifA(B) = 0

• The following is a short-hand logical indexing expression:-A(A > 0.5) = 0

• We can test if B is logical using islogic(B)

28

Debugger

• Environment is created when functions are invoked and returned; variables are generated, modified and destroyed.

• We can check the programme state using a debugger.

• Debugger in MatLab available in two interfaces; command line interface (CLI) and graphical user interface (GUI)

• Using the debugger typically involved the following:- Set one or more breakpoints where the debugger will stop

execution• Breakpoint can be line number or a condition such as stop on

error, warning, etc.

Execute the programme and when the programme stops, you can examine the environment and troubleshoot your programme

29

Example on the use of debugger

30

Debugging

• When MATLAB reaches a breakpoint, it enters debug mode. The prompt changes to a K>> and, on the PC, the debugger window becomes active.

• All MATLAB commands are allowed at the prompt.

• To resume M-file function execution, use DBCONT or DBSTEP. To exit from the debugger, use DBQUIT.

• For example: to clear and set breakpoint at line 8 dbstop in ex_sumprimes at 8 dbclear in ex_sumprimes at 8

• Others: help debug

31

Hints on Exercise 6.3

• Generate 4 spheres using sphere function from MatLab

• Cut the spheres

• Commands that you may needed:-

>> sphere

>> hold on

>> colormap(winter)

>> camlight

>> lighting phong

>> axis equal

220

20

20 rzzyyxx

32

Plottools, Property Editor & Inspector

>> a=randn(1000,2);hist(a)>> propedit % or from Desktop Menu>> inspect % bring up inspector>> propertyeditor % see desktop menu