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Intro To MATLAB

Intro To MATLAB. CS 534 - 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

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Intro To MATLAB

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  1. Intro To MATLAB CS 534 - Fall 2013 Zach Welch

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

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

  4. 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

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

  6. 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) -> 2 1 2 3 4 5 6 7 8 9 1 0 0 0 1 0

  7. 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 • size get matrix dimensions • : access a subset of indices

  8. How Operations Work -.3077 .3846 .1538 .6923 -1 4 2 1.5 7 10 15 22 -2 1 -2 -2 13 13 29 27 1 2 3 4 3 1 5 6 4 3 8 10 A/B = A^2 = A*B = A+B = A-B = A\B = A = B = solves A*x = B solves x*A = B

  9. How Per-Element Operations Work C’ = C = 1 3 5 7 9 11 13 15 1 2 243 4096 3 2 15 24 .333 2 .6 .666 3 1 5 6 1 2 3 4 1 5 9 13 3 7 11 15 A .*/ B = A .*^ B = A .* B = B = A =

  10. Size Size gets the dimensions of the matrix. size(Matrix, DIM) DIM = 1 -> get row dimension DIM = 2 -> get column dimension 1 2 3 4 7 8 B = size(B,1) = 3 size(B,2) = 2 FUNCTIONS CAN RETURN MULTIPLE ARGUMENTS IN MATLAB [row,col] =size(B) row => 3 col => 2

  11. 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

  12. Colon Operator (:) • Used to efficiently access submatrices • using no numbers -> all elements in dimension A(1:2:end, 2:2:end) = A = 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 A(:,1) = 1 5 9 13 2 4 10 12 A(2:end,1) = 5 9 13

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

  14. 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

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

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

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

  18. 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 • end • Using loop: Elapsed time is 1.125289 seconds.

  19. Time Cost Comparison(cont.) • Loop vs. no loop • C = A + B • Elapsed time is 0.002346 seconds. • Try to take advantage of matrix/vector structure whenever possible

  20. 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

  21. 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

  22. 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

  23. Images In MATLAB • 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)

  24. Images In MATLAB [1, 1] • 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); o Row 1 to 256 o Column 1 to 256 [256, 256]

  25. 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[0.0 - 1.0]) to double[0.0-1.0] • 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’);

  26. Questions?

  27. Demos

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