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

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)

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

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

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

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

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

Commonly used functions rand - generates a random decimal number between 0 and 1 e.g. 0.3456 or 0.9993 or 0.0013 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)

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

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);

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)

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.

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;

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;

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)

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

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

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;

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

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;

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

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

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)

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

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

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

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 http://audacity.sourceforge.net/