Lab of COMP 406

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Teaching Assistant: Pei-Yuan Zhou

Contact: cspyzhou@comp.polyu.edu.hk

Lab 3: 26 Sep., 2014

Introduction of Matlab (III)

Programming and Scripts

• Find the Matlab under the folder

• 1. Y:\Win32\Matlab\R2012a

• 2. Double click it and open Matlab

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Open Matlab 2012a

• 2-D Plots

>> plot(x, y)

>> xlabel(‘x’); ylabel(‘y’)

>> legend(‘x’, ‘y’) ; title (‘title’)

>> bar(x,y); stairs(x,y);errorbar(x,y,e);stem(x,y)

• 3-D Plots

>> meshgrid; mesh(x,y,z);surf(x,y,z)

>> peaks(N); colormap(hsv)

• Subplots

>> subplot(row,col,num);

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Review

Lecture and Lab Notes

• http://www4.comp.polyu.edu.hk/~csyliu/course/co

mp406/main.html

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Character String

Script

Functions

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Outline

Programming and Scripts

Character String

Script

Functions

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Outline

Programming and Scripts

Character String

• A character string is a sequence of any number of characters enclosed in single quotes. You can assign a string to a variable.

Text1 = 'Hello, world';

• If the text includes a single quote, use two single quoteswithin the definition.

>>Text2 = 'You''re right'

Text2 =

You're right

• Text1 and Text2 are arrays, like all MATLAB variables. Their class or data type is char, which is short for character. (whos)

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Character String

• You can concatenate strings with square brackets, just as you

concatenate numeric arrays.

>>Text3 = [Text1,' - ', Text2]

Text3 =

Hello, world - You're right

• To convert numeric values to strings, use functions, such as num2str

or int2str.

f = 71; %fahrenheit degree

c = (f-32)/1.8; % centi degree

tempText = ['Temperature is ',num2str(c),'C']

tempText =

Temperature is 21.6667C

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Character String

Script

Functions

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Outline

Programming and Scripts

There are two types of *.m file, Scripts and

Functions

The simplest type of MATLAB program is called a

script.

A script is a file with a “.m” extension that contains

multiple sequential lines of MATLAB commands

and function calls.

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Script

To create a script, use the edit command. This

opens a blank file named newscript.m.

>>edit newscript

Go to

File->New->Script

Give a name to script

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Script - Creation

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Script - Building

Typing the sentences in newscript.mPrint the value of each elements in the vector x and shows

whether they are positive or negative.

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Script - Saving

For saving the script, just go to File->Save

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Script - Run

>> newscriptx(1) = 1 is positive

x(2) = 4 is positive

x(3) = -2 is negative or zero

x(4) = 3 is positive

x(5) = -1 is negative or zero

x(6) = -5 is negative or zero

If you want to run it,

typing >> newscript

Then you can see the

output in command

window like right figure

shows.

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Script - Run

>> newscriptx(1) = 1 is positive

x(2) = 4 is positive

x(3) = -2 is negative or zero

x(4) = 3 is positive

x(5) = -1 is negative or zero

x(6) = -5 is negative or zero

If you want to run it,

typing >> newscript

And all generated

variables are listed in

workspace

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Script - Comments

• Whenever you write code, it is a good practice to add comments that

describe the code. Comments allow others to understand your code, and

can refresh your memory when you return to it later. Add comments using

the percentage (%) symbol.

n = 50; % 50 data points

r = rand(n,1);

plot(r) % Draw a line from (0,m) to (n,m)

m = mean(r);

hold on

plot([0,n],[m,m])

hold off

title('Mean of Random Uniform Data')

Ex1

• Create a script, named as “myscript.m”

• The script can draw the plot of function y=sin(x)

when x is in [0,2pi]

• Save your script

• Run the script

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Ex1

• >>edit myscript.m

• The content of script

• Save myscript.m

• myscript

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x=0:0.1:2*pi;

y = sin(x);

plot(x,y);

Script

• Advantage:

• Better for Simple but High Repeatability code

• All generated variables can be stored in workspace – easy

for viewing

• Disadvantage:

• Not support “Input/ Output Arguments”

• The variables stored in workspace, which may be

covered with each other

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“Function” can

improve it

Character String

Script

Functions

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Outline

Programming and Scripts

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Function

• A function is a black box that gets some input andproduces some output. We do not care about the innerworkings of a function.

• Functions provide reusable code.

• Functions simplify debugging.

• Functions have private workspaces:

• The only variables in the calling program that can be seenby the function are those in the input list.

• The only variables in the function that can be seen by thecalling program are those in the output list.

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• The black box consists of two parts: the definition of the interfaceby which the user passes data items to and from the function; and the code block that produces the results required by that interface.

Param 2

Param 1

Param n

. . .

Result

Function- Black Box View

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Param 2

Param 1

Param n

. . .

Result

• A function definition consists of the following components:

• A name that follows the same syntactic rules as a variable name;

• A set of 0 or more parameters provided to the function;

• Zero or more results to be returned to the called of the function.

Function- Black Box View

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Function – Template

1. The <return info> section for most functions involves providing

the name(s) of the results returned followed by an = sign. If more

than one result is to be returned, they are defined in a vector like

container. If nothing is to be returned from this function, both the

result list and the = sign are omitted.

2. The <function name> is a name with the same syntactic rules as

a variable name, and will be used to invoke the code body.

function <return info> <function name> (<parameters>)

<documentation>

<code body> % must return the results

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Function – Template

function <return info> <function name> (<parameters>)

<documentation>

<code body> % must return the results

3. The <parameters> section is a comma-separated list of the names of

the data to be provided to the function.

4. The <documentation> section is one or more lines of comments that

describe what the function does and how to call it

• All the documentation lines up to the first non-document line are printed

in the command window when you type the following:

>> help <function name>

• The first line is listed next to the file name in the current directory listing

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• Line1: the Matlab function definition is introduced by the key word function,

followed by the name of the return variable (if any) and the = sign.

• Line2: all comments written immediately after the function header are

available to the command window when you enter:

>> help cylinder

1. function volume = cylinder(height, radius)

2. % function to compute the volume of a cylinder

3. % volume = cylinder(height, radius)

4. base = pi * radius^2;

5. volume = base * height;

Function –An Example

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1. function volume = cylinder(height, radius)

2. % function to compute the volume of a cylinder

3. % volume = cylinder(height, radius)

4. base = pi * radius^2;

5. volume = base * height;

Function –An Example

• Line3: it is a good idea to include in the comments a copy of the function

header line to remind a user exactly how to use this function.

• Line5: you must make at least one assignment to the result variable.

• The function definition needs no end statement. The code body

terminates automatically at the end of the file.

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Function –An Examplefunction p = factorial(n)%FACTORIAL Factorial function.% FACTORIAL(N) is the product of all the integers from 1 to N,% i.e. prod(1:N). Since double precision numbers only have about% 15 digits, the answer is only accurate for N <= 21. For larger N,% the answer will have the right magnitude, and is accurate for % the first 15 digits.%% See also PROD.

% Copyright 1984-2001 The MathWorks, Inc. % $Revision: 1.5 $

if (length(n)~=1) | (fix(n) ~= n) | (n < 0)error('N must be a positive integer');

end

p = prod(1:n);

output argument

name of function

input argument

H1 comment line

other comment lines

executable code

• 1. >> edit function1 % create a function1.m file

• 2. type the code in the file

• function average = function1(vector)

• average = sum(vector)/length(vector);

• 3. save file “function1.m”

• 4. View your function code : >>type function1.m

• 5. call the function:

• >>vector = [1 5 3]

• >>ave = function1(vector)30

Function – Create A Function

• 6. Adding Help & Comments for function

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Function – Create A Function

function average = function1(vector)% FUNCTION1: A simple function with a single help line.%% Usage of this function:% output = function1(input)% "output" is the average of the input vector "input".

% Pei-Yuan, 2014/09/26.

average = sum(vector)/length(vector); % calculate average

• After creating function, the comments (starting from percent (%) symbol) are the content of help

• Type “help + function name”, you can see the help documentation you added

• >> help function1

FUNCTION1: A simple function with a single help line.

Usage of this function:

output = function1(input)

"output" is the average of the input vector "input".

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Function – Create A Function

• 1. The maximum identifier length is 63.

• len = namelengthmax => returns the maximum length allowed for MATLAB identifiers.

• The maximum identifier length is different for different version of MATLAB

• 2. The name of function can be different from name of M-file. (It is better to use the same name)

• The name of M-file is used for calling function

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Function – *Maximum identifier Length

Function – Input and Output

• A function can have multiple inputs and outputs

• The following func2 has two inputs and two outputs

• Run func3.m

• >> [a,b] = func3 ([1 2 3], [4 5 6 7 8])

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function [ave1, ave2] = func2(vector1, vector2);ave1 = sum(vector1)/length(vector1);ave2 = sum(vector2)/length(vector2);

Function – Input and Output

• nargin and nargout can be used to control the number

of inputs and outputs.

• nargin: returns the number of input arguments that were

used to call the function.

• nargout: returns the number of output arguments that

were used to call the function

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• Then you can change the func2.m into func3.m

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Function – Input and Output

function [ave1, ave2] = func4(vector1, vector2)

if nargin == 1, % only one inputave1 = sum(vector1)/length(vector1);

end

if nargout == 2, % two outputsave1 = sum(vector1)/length(vector1);ave2 = sum(vector2)/length(vector2);

end

• Then you can change the func2.m into func3.m

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Function – Input and Output

>> [a, b] = func4([1 2 3], [4 5 6 7 8])

a = 2

b = 6

>> c = func4([1 3 5 7 9])

c = 5

The output is different based on

different calling method

Function – Nested Function

function out = func4(x)

r = reciproc(x);

out = sum(r);

% Definition for subfunctions

function output = reciproc(input)

output = 1./input;

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>> func4([1 2 3])

ans = 1.8333

Calculate the sum of reciprocal of all elements in vector

Ex1

Problem: write a function called strsearch that takes a string s and a character c, and returns the number of occurrences of c in s and the index of the first occurrence.

Hint & Pseudo code:

Function [cnt, pos] = strsearch( s, c)

For each character of s in reverse order

If character is equal to c

increment the counter

save the index39

function [ cnt, pos ] = strsearch( s, c )

%STRSEARCH find the number of occurrences of a character in a

string

pos = 0;

cnt = 0;

n = length(s);

for i = n:-1:1, % from n to 1, and minus 1 for each loop

if ( s(i) == c ),

cnt = cnt + 1;

pos = i;

end

end 40

Ex1 - answer

Ex1 - answer

[ a, b ] = strsearch( 'abccdecfac', 'c' )

a =4

b =3

a = strsearch( 'abccdecfac', 'c' )

a =4

strsearch( 'abccdecfac', 'c' )

ans =4

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Ex2 – Nested Function

Problem: write a function called mystats that find mean and median with internal functions.

Hint:

Creation: function [avg, med] = mystats(u)

The functions you can use:

avg = mean(u,n) % u is the vector, n is the length of u

med = median (u,n)

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function [avg, med] = mystats(u)%MYSTATS Find mean and median with internal functions.n = length(u);avg = mean(u,n);med = median(u,n);function a = mean(v,n) % Subfunction to calculate average.

a = sum(v)/n;function m = median(v,n) % Subfunction to calculate median.

w = sort(v);if rem(n,2) == 1

m = w((n+1)/2);else

m = (w(n/2)+w(n/2+1))/2;end

Ex2 - answer

• >> [a,m]=newscript([1 2 3 4 11 12 13])

a =

6.5714

m =

4

• >> [a,m]=newscript([1 2 3 4 12 13])

a =

5.8333

m =

3.500044

Ex2 - answer

Summary

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• Both scripts and functions are saved as m-files.

• Functions are special m-files that receive data through input

arguments and return results through output arguments.

• Scripts are just a collection of MATLAB statements.

• Functions are defined by the function statement in the first

line.

• Scripts use the global workspace but functions have their own

local independent workspaces.

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Contact: cspyzhou@comp.polyu.edu.hk

Lab 3: 26 Sep., 2014