1. Introduction To Matlab Class 3
Instructors: Hristiyan (Chris) Kourtev
and Xiaotao Su, PhD
Double click the matlab icon
When prompted click “Skip”

2. 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)

3. 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 =
d =
9.8480
>> format long
>> d

4. Boolean Expressions == ~= > >= < <= && || equals not equal
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 first e.g. 0 || (4<5) == ~=
>
>=
<
<=
&& ||
equals
not equal
greater than
greater than or equal to
less than
and or

5. 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

6. 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;
%nested loop example
for k = 1:max_loops
disp(‘k1’);
for m = 1:3
disp(‘m’);
end
disp(‘k2’);
% outputs:
% k1 m m m k2 k1 m m m k2 k1 mmm k2 k1 m m m k2 k1 mmm k2

7. 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 condition if (condition) break; % get out of the loop end end

8. Commonly used functions
rand - generates a random decimal number between 0 and 1 e.g or or etc
ceil(num) – returns the next integer bigger than the input e.g. ceil(5.56) 6 or ceil(2.1)  3 or ceil(6)  6
floor(num) – returns the next integer, smaller than the input e.g. floor(0.9)  0 or floor(-0.1)  -1
To generate a random number between 0 & 20: ceil(rand*20)

9. 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 matrix e.g. length(m)  4, length(n)  4, lenth(k)  3
size(mat,dim) – returns all the dimensions of a matrix/vector e.g. size(m)  [1, 4], size(n)  [2, 4], size(k)  [3, 1], size (n, 2)  4

10. 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);

11. Collecting User Input & Using it
Take input from keyboard num1=input('what is the first number?');
Validation checks: - isstr(var) - isnum(var)
Converting from strings to numbers and back - num2str(var) - str2num(var)

12. 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.

13. calling scripts % my_prog.m % double_j.m j = 4;
if(j<7)
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;

14. calling scripts % my_prog.m % double_j.m j = 4;
if(j<7)
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;

15. Less likely to have bugs
Added benefit
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.
% my_prog.m
j = 4;
double_j
if(j<7)
else
half_j
end
disp(j)

16. making programs for all versions of psychtoolbox
%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

17. Functions Functions are similar to scripts in that Differences
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

18. Example of functions %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;

19. Example of functions %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;
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

20. Multiple inputs/outputs
%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;

21. Small Pieces If you do not see a yellow region, in the menu
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

22. For loops using different increments
for i=1:10
disp(i);
end
1, 2, 3, 4, 5, 6, 7, 8, 9, 10
for i=10:-1:1
disp(i);
end
for i=1:2:10
disp(i);
end
10, 9, 8,7,6,5,4,3,2,1
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

23. Task 1 Tips: function d_val = double_me(i) for i=1:10 disp(i); end
create a function that will take any string and spell it backwards
reverse_string(‘stressed’) returns ‘desserts’
Tips:
function d_val = double_me(i)
for i=1:10
disp(i);
end
The length of a string is
length(str_var)

24. Using images in experiments
Images are stored in matlab as
Width x Height x 3 matrix x R G y B

25. file -> matrix and drawing it on the screen
img = imread(‘winter.jpg’, ‘jpg’);
To display an image use the ‘image’ command: image(img);

26. 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

27. Making sounds Tip: To make your own wav files I recommend
[sound, samplerate, samplesize] =
wavread(‘chord.wav’);
wavplay(sound, samplerate); % on PC
sound(sound, samplerate); % if you have a Mac
Tip:
To make your own wav files I recommend
using an application called audacity