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 ●  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) -> 2

1 2 3 4 5 6 7 8 9

1 0 0 0 1 0

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

How Operations Work

3 1 5 6 B =

1 2 3 4 A =

4 3 8 10 A+B =

-2 1 -2 -2 A-B =

13 13 29 27 A*B =

7 10 15 22 A^2 =

solves A*x = B -.3077 .3846 .1538 .6923 A/B =

-1 4 2 1.5 A\B = solves x*A = B

How Per-Element Operations Work

3 2 15 24 A .* B =

.333 2

.6 .666 A .*/ B =

1 2 243 4096 A .*^ B =

3 1 5 6 B =

1 2 3 4 A =

1 3 5 7 9 11 13 15

C =

1 5 9 13 3 7 11 15

C’ =

Size 1 2 3 4 7 8

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 1 2 3 4 5 6 7 8 9 10 11 12

13 14 15 16

A =

1 5 9

13

A(:,1) =

5 9

13

A(2:end,1) =

2 4 10 12

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 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 end Using loop: Elapsed time is 1.125289 seconds.

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

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[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’);

Questions?

Demos