Introduction To Matlab Class 3

1. Introduction To MatlabClass 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 = 9.8479847498749847984 d = 9.8480 >> format long >> d d = 9.847984749874986

4. 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)

5. Loops (for and while) • For loopfor index = from:to % do somethingend • While loopwhile(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; end %nested loop example for k = 1:max_loops disp(‘k1’); for m = 1:3 disp(‘m’); end disp(‘k2’); end % outputs: % k1 m mm k2 k1 m mm k2 k1 mmm k2 k1 m mm 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 endend

8. 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)

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 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

10. Multiple Input/Output Functions • Functions can have more than one input and more than one outpute.g. s = size(mat, dim); • Storing returned values in 2 or more separate variablese.g. [x, y] = size(mat); • Storing returned values in a vector/cell arraye.g. vals = size(mat);

11. 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)

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 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;

14. 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;

15. 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) double_j else half_j end disp(j)

16. making programs for all versions of psychtoolbox %draw_stuff.mclear 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 • 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. double_me*’s variables i = 4 d_val = 8 double_me**’s variables i = 8 d_val = 16 my_prog’s variables f = 4 then 16 k = 8 i = 6

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 • 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 • create a function that will take any stringand 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 • [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/