Intro To MATLAB CS Fall 2013 Zach Welch

Overview ●Basics ●MATLAB data structures ●Operations ●Useful functions ●Image Processing and other useful things for 534 ●Demo ●Q&A

Accessing MATLAB ●MATLAB is available on the Linux and Windows labs ●On Linux, type “matlab” into the terminal ●Can remotely access via ssh

MATLAB Basics ●Short for MATrix LABoratory ●High level, interpreted language ●Great for data heavy applications “MATLAB is an interactive, matrix-based system for scientific and engineering numeric computation and visualization. You can solve complex numerical problems in a fraction of the time required with a programming Language such as Fortran or C.” ---- MATLAB Primer

MATLAB IDE COMMAND WINDOW Where you type commands Workspace List of your current variables Command History List of previous commands Current Path Filespace

Matrices ●Building a Matrix ○ Explicitly A = [1 2 3 ;4 5 6;7 8 9] ○ Using a Function B = eye(2,3) ■ zeros,ones,rand ●Accessing Elements ○ Matrix indices start at 1,*NOT 0* ○ A(1,2) ->

Matrix Operations ●+Addition ●-Subtraction ●*Matrix Multiplication ●^ Matrix Power ●‘Transpose ●\Left Matrix Division (Solves A*x=B) ●/Right Matrix Division (Solves x*A=B) ●.*Element by Element Multiplication ●./Element by Element Division ●.^ Element by Element Power ●sizeget matrix dimensions ●:access a subset of indices

How Operations Work B = A = A+B = A-B = A*B = A^2 = solves A*x = B A/B = A\B = solves x*A = B

How Per-Element Operations Work A.* B = A.*/ B = A.*^ B = B = A = C = C’ =

Size B = size(B,1) = 3 size(B,2) = 2 [row,col] =size(B) row => 3 col => 2 FUNCTIONS CAN RETURN MULTIPLE ARGUMENTS IN MATLAB Size gets the dimensions of the matrix. size(Matrix, DIM) DIM = 1 -> get row dimension DIM = 2 -> get column dimension

Colon Operator (:) ●Extremely useful operator ○ Generates an array of evenly spaced numbers in a user specified range ●start : increment : stop ●if increment is left out, assumed to be one

Colon Operator (:) ●Used to efficiently access submatrices ○ using no numbers -> all elements in dimension A = A(:,1) = A(2:end,1) = A(1:2:end, 2:2:end) =

●Logical Operators ○ ==is equal to ○, =less/greater than ○ ~not ○ ~=not equal to ●Instead of using brackets, MATLAB uses “end” Flow Control for(int a=0;a<=10;a++){ if( a>3){... … } C for (a=0:10) if (a>3)... … end MATLAB

Flow Control : IF ●If statements in MATLAB are very similar to if statements in other languages ●Notice elseif is one word if (boolean) … elseif (boolean) … else … end

Flow Control : WHILE ●while statements in MATLAB are very similar to while statements in other languages while (boolean)... end

●For is most different from other languages ●Cannot infinite loop in a for loop ●Uses : operator ○ start : increment : stop Flow Control : FOR if(start < stop){ if(inc>0){ for(a=start;a<=stop;a+=inc){ … } else{ if(inc<0){ for(a=start;a>=stop;a+=inc){ … } C for (a=start:inc:stop) … end MATLAB

It seems like I can use these loops as I do in C/C++/Java… Try to AVOID THIS!

Time Cost Comparison Loop vs. No Loop A = rand(1000,1000);B = rand(1000,1000); for i = 1:size(A,1), for j = 1:size(A,2), C(i,j) = A(i,j) + B(i,j); end Using loop: Elapsed time is seconds.

Time Cost Comparison(cont.) Loop vs. no loop C = A + B Elapsed time is seconds. Try to take advantage of matrix/vector structure whenever possible

Useful Functions ●“;” - Suppress output ●who - List current variables ●help - get information on a function ●lookfor - keyword search all function help info ●clear - delete all variables

Writing MATLAB functions ●Matlab code is saved in.m files ●2 kinds of.m files ○ Function.m files ■ Contain a function definition ■ One function per file ■ FILE NAME MUST MATCH FUNCTION NAME ○ Script.m files ■ Contain a list of commands ■ Can be named anything ■ Often used as drivers for functions you have implemented ● Kind of like main in other languages

Writing MATLAB functions ●Structure of a MATLAB function ●Functions Can Return Multiple values ●Make sure you initialize your return variables function returnVal = FunctionName (input1,input2) %Adds two numbers returnVal = input1+input2; end function [return1, return2] = FunctionName (input1,input2) return1 = input1+input2; return2= 0; end

MATLAB can import/export several image formats: –BMP (Microsoft Windows Bitmap) –GIF (Graphics Interchange Files) –HDF (Hierarchical Data Format) –JPEG (Joint Photographic Experts Group) –PCX (Paintbrush) –PNG (Portable Network Graphics) –TIFF (Tagged Image File Format) –XWD (X Window Dump) –raw-data and other types of image data Data types in MATLAB –Double (64-bit double-precision floating point) –Single (32-bit single-precision floating point) –Int32 (32-bit signed integer) –Int16 (16-bit signed integer) –Int8 (8-bit signed integer) –Uint32 (32-bit unsigned integer) –Uint16 (16-bit unsigned integer) –Uint8 (8-bit unsigned integer) Images In MATLAB

Column 1 to 256 Row 1 to 256 o [1, 1] o [256, 256] How to build a matrix (or image)? Intensity Image: row = 256; col = 256; img = zeros(row, col,3); img(100:105, :,1) = 0.5; img(:, 100:105,2) = 1; imshow(img); Images In MATLAB

Image Processing Functions ●img = imread(imageFileName) - reads in an image file (.jpg,.png,etc) and returns : ○ [width,height] sized matrix if grayscale ○ [width,height,3] sized matrix if rgb ●imshow(img) - display an image ●img = im2double(img) - Converts an image (which may be uint8[0- 255] or double[ ]) to double[ ] ●im2uint8 ●imwrite(img,FileName,fileType) - write out an image ●Basic structure is: img = imread(‘input.jpg’); imshow(img); %Do something to the image imshow(img); imwrite(img,’output.jpg’,’jpg’);

Questions?

Demos