introduction to matlab class 3

Introduction To MatlabClass 3

Instructors: Hristiyan (Chris) Kourtevand Xiaotao Su, PhD

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Unit Name

Variables

• Integers m = 5; % = [5]• Doubles (Floating pt) n = 7.382;• Character strings c1 = ‘beep’ ; % = [‘b’, ‘e’, ‘e’, ‘p’]

c2 = ‘4’;• Arrays of numbers arr1 = [4, 5, 8, m];

arr2 = [m, n, 5.6, 0];• Arrays of strings str1 = [c1; ‘blob’]; % same dimen.• Concatenating arrays of numbers arr3 = [arr1, arr2];• Concatenating strings str2 = [c1,c2];• Matrices mat1 = [4, 5; 6, 7];

mat2 = [arr1; arr2]; % same dimen.• Cell Arrays (later on)


Unit Name

Accuracy of Displayed results

• Usually numerical values are rounded to 4 digits after the decimal

• Use “format long” and “format short” to display actual and short values respectively

>> d = 9.8479847498749847984d = 9.8480>> format long >> dd = 9.847984749874986


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Boolean Expressions

• Boolean operands

• Boolean expressions either return 1 for true e.g. 5 == 5 or 0 for false e.g. 5 > 9

• Put expressions in parentheses so they get evaluated firste.g. 0 || (4<5)

== ~= > >= < <= && ||equals not

equalgreater than

greater than or equal to

less than

greater than or equal to

and or


Unit Name

Loops (for and while)

• For loop

for index = from:to% do something

end

• While loop

while(condition)% do something% change something that affects value of “condition”

end


Unit Name

Loops (for and while) -- examples

max_loops = 5;for index = 1:max_loops

disp(index);end

counter = 1;while(counter < max_loops)

disp(counter);counter = counter + 1;

end

%nested loop example

for k = 1:max_loops

disp(‘k1’);

for m = 1:3

disp(‘m’);

end

disp(‘k2’);

end

% outputs:

% k1 m m m k2 k1 m m m k2 k1 mmm k2 k1 m m m k2 k1

mmm k2


Unit Name

The “do-while” loop

• General syntax in most languages (does NOT exist in Matlab):do {

//run some code} while (condition)

• How to do it in matlab:while(1) % loop forever

% run some code here% check conditionif (condition)break; % get out of the loopend

end


Unit Name

Commonly used functions

• rand - generates a random decimal number between 0 and 1e.g. 0.3456 or 0.9993 or 0.0013 etc

• ceil(num) – returns the next integer bigger than the inpute.g. ceil(5.56) 6 or ceil(2.1) 3 or ceil(6) 6

• floor(num) – returns the next integer, smaller than the inpute.g. floor(0.9) 0 or floor(-0.1) -1

• To generate a random number between 0 & 20: ceil(rand*20)


Unit Name

Commonly used functions -- continued

m = [1, 2, 3, 4]; n = [1, 2, 3, 4; 5, 6, 7, 8]; k = [9; 8; 0];

• length(mat) – returns the length of a vector or a matrixe.g. length(m) 4, length(n) 4, lenth(k) 3

• size(mat,dim) – returns all the dimensions of a matrix/vectore.g. size(m) [1, 4], size(n) [2, 4],

size(k) [3, 1], size (n, 2) 4


Unit Name

Multiple Input/Output Functions

• Functions can have more than one input and more than one output

e.g. s = size(mat, dim);

• Storing returned values in 2 or more separate variables

e.g. [x, y] = size(mat);

• Storing returned values in a vector/cell array

e.g. vals = size(mat);


Unit Name

Collecting User Input & Using it

• Take input from keyboardnum1=input('what is the first number?');

• Validation checks: - isstr(var)- isnum(var)

• Converting from strings to numbers and back- num2str(var)- str2num(var)


Unit Name

Calling scripts within scripts

• This is done to modularize code• Modular code is useful because you can

– reuse the same piece of code in many different programs– have the same piece of code called many times in one program– Only have to debug that piece of code once and then be able to rely on

it to work the same way all the time.


Unit Name

calling scripts

% my_prog.m

j = 4;

double_j

if(j<7)

double_j

else

half_j

end

disp(j)

% double_j.m

% doubles the value of j

j = j*2;

% half_j.m

% cuts j in half

j = j/2;


Unit Name

Added benefit

% my_prog.m

j = 4;

double_j

if(j<7)

double_j

else

half_j

end

disp(j)

If double_j and half_j were much more

complicated programs the benefit to seperating

them out into separate scripts makes our code

•Shorter

•Simpler to read

•Less likely to have bugs

You can also have your program perform

largely different behaviors based upon

different conditions.


Unit Name

%draw_stuff.m

clear all;

screen_setup

while( …)

…

screen(window, ‘FillRect’ …

flip

end

clear screen

%screen_setup

---Determine Operating System---

if(oldmac|win)

Set up screen variables

for old mac and windows

else

Set up screen for OS X

end

if(osx ==1)

Screen(window,'Flip');

else

Screen('CopyWindow', window, window_ptr);

Screen('CopyWindow', blank, window);

Screen(window_ptr, 'WaitBlanking');

end

making programs for all versions of psychtoolbox


Unit Name

Functions

• Functions are similar to scripts in that– they are separate from your main body of code– used to perform one coherent task– make your code neater

• Differences– You pass it specific variable(s) and it returns specific variables(s)– The variables within it are not accessible outside the function– The variables outside the function are not accessible inside the function


Unit Name

%my_prog.m

f = 4;

k = double_me(f);

i = 6;

f = double_me(k);

disp(f);

disp(i);

function d_val = double_me(i)

%double_me.m

%doubles any value passed to it

d_val = i*2;

Example of functions


Unit Name

%my_prog.m

f = 4;

k = double_me(f);

i = 6;

f = double_me(k);

disp(f);

disp(i);


%double_me.m

%doubles any value passed to it

d_val = i*2;

Example of functions

•Notice, i was set to 6 in my_prog, and i was used in double_me,

but the two references didn’t effect eachother.

•Also each call to a function creates a separate set of variable

references for that call.

my_prog’s variables

f = 4 then 16

k = 8

i = 6

double_me*’s variables

i = 4

d_val = 8

double_me**’s

variables

i = 8

d_val = 16


Unit Name

%my_prog2.m

f = 9;

[a, b] = double_times(f, 4);

c = double_times(f, 4);

disp(a);

disp(b);

disp(c);

function [d_val, t_val] = double_times(i, fact)

%double_times.m

%doubles any value passed to it and multiplies

d_val = i*2;

t_val = i*fact;

Multiple inputs/outputs


Unit Name

Small Pieces

• Starting with matlab version 7.0 you can execute small chunks of code

• This is called cells (nothing to do with cell arrays)• %% mark off the beginning and end of cell region• Cell regions are seen as yellow • Pressing ctrl/cmd + return causes the workspace to execute the

command in the active cell

If you do not see a yellow region, in the menu

bar select Cell->Enable Cell Mode


Unit Name

For loops using different increments

for i=1:10

disp(i);

end

1, 2, 3, 4, 5, 6, 7, 8, 9, 10

for i=1:2:10

disp(i);

end

1, 3, 5, 7, 9

for i=2:2:10

disp(i);

end

2, 4, 6, 8, 10 for i=10:-2:1

disp(i);

end

10, 8, 6, 4, 2

for i=10:-1:1

disp(i);

end

10, 9, 8,7,6,5,4,3,2,1


Unit Name

Task 1

• create a function that will take any stringand spell it backwards

• reverse_string(‘stressed’)returns ‘desserts’

for i=1:10

disp(i);

end

The length of a string is

length(str_var)

Tips:



Unit Name

Using images in experiments• Images are stored in matlab as Width x Height x 3 matrix

RG

B

x

y


Unit Name

file -> matrix and drawing it on the screen

• img = imread(‘winter.jpg’, ‘jpg’);

• To display an image use the ‘image’ command:image(img);


Unit Name

Making sounds

• A sound is a 2 x N matrix where N is the number of bits that make up the sound file and the two channels are for left and right


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Making sounds

• [sound, samplerate, samplesize] = wavread(‘chord.wav’);

wavplay(sound, samplerate); % on PCsound(sound, samplerate); % if you have a Mac

Tip:

To make your own wav files I recommend

using an application called audacity

http://audacity.sourceforge.net/

introduction to matlab class 3

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