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% Introducing Matlab (adapted from http://www.cns.nyu.edu/~eero and
% http://www.cs.dartmouth.edu/~farid/teaching/cs88/matlab.intro.html)
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% (1) Help and basics
% The symbol "%" is used in front of a comment.
% To get help type "help" (will give list of help topics) or "help topic"
% If you don't know the exact name of the topic or command you are looking for,
% type "lookfor keyword" (e.g., "lookfor regression")
% When writing a long matlab statement that exceeds a single row use ...
% to continue statement to next row.
% When using the command line, a ";" at the end means matlab will not
% display the result. If ";" is omitted then matlab will display result.
% Use the up-arrow to recall commands without retyping them (and down
% arrow to go forward in commands).
% Other commands borrowed from emacs and/or tcsh:
% C-a moves to beginning of line (C-e for end), C-f moves forward a
% character (C-b moves back), C-d deletes a character, C-k deletes
% the line to the right of the cursor, C-p goes back through the
% command history and C-n goes forward (equivalent to up and down arrows),
% tab command completion.
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% (2) Objects in matlab -- the basic objects in matlab are scalars,
% vectors, and matrices...
N = 5 % a scalar
v = [1 0 0] % a row vector
v = [1;2;3] % a column vector
v = v' % transpose a vector
(row to column or column to row)
v = [1:.5:3] % a vector in a specified range:
v = pi*[-4:4]/4 % [start:stepsize:end]
v = [] % empty vector
m = [1 2 3; 4 5 6] % a matrix: 1ST parameter is ROWS
% 2ND parameter is COLS
m = zeros(2,3) % a matrix of zeros
v = ones(1,3) % a matrix of ones
m = eye(3) % identity matrix
v = rand(3,1) % rand matrix (see also randn)
load matrix_data % read data from a file:
% create a file 'matrix_data' containing:
% 2 3 4
% 5 6 7
% 1 2 3
matrix_data
v = [1 2 3]; % access a vector element
v(3) % vector(number)
m = [1 2 3; 4 5 6]
m(1,3) % access a matrix element
% matrix(rownumber, columnnumber)
m(2,:) % access a matrix row (2nd row)
m(:,1) % access a matrix column (1st row)
size(m) % size of a matrix
size(m,1) % number rows
size(m,2) % number of columns
m1 = zeros(size(m)) % create a new matrix with size of m
who % list of variables
whos % list/size/type of variables
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% (3) Simple operations on vectors and matrices
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% (A) Pointwise (element by element) Operations:
% addition of vectors/matrices and multiplication by a scalar
% are done "element by element"
a = [1 2 3 4]; % vector
2 * a % scalar multiplication
a / 4 % scalar multiplication
b = [5 6 7 8]; % vector
a + b % pointwise vector addition
a - b % pointwise vector addition
a .^ 2 % pointise vector squaring (note .)
a .* b % pointwise vector multiply (note .)
a ./ b % pointwise vector multiply (note .)
log( [1 2 3 4] ) % pointwise arithmetic operation
round( [1.5 2; 2.2 3.1] ) % pointwise arithmetic operation
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% (B) Vector Operations (no for loops needed)
% Built-in matlab functions operate on vectors, if a matrix is given,
% then the function operates on each column of the matrix
a = [1 4 6 3] % vector
sum(a) % sum of vector elements
mean(a) % mean of vector elements
var(a) % variance
std(a) % standard deviation
max(a) % maximum
a = [1 2 3; 4 5 6] % matrix
mean(a) % mean of each column
max(a) % max of each column
max(max(a)) % to obtain max of matrix
max(a(:)) % or...
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% (C) Matrix Operations:
[1 2 3] * [4 5 6]' % row vector 1x3 times column vector 3x1
% results in single number, also
% known as dot product or inner product
[1 2 3]' * [4 5 6] % column vector 3x1 times row vector 1x3
% results in 3x3 matrix, also
% known as outer product
a = rand(3,2) % 3x2 matrix
b = rand(2,4) % 2x4 matrix
c = a * b % 3x4 matrix
a = [1 2; 3 4; 5 6] % 3 x 2 matrix
b = [5 6 7]; % 3 x 1 vector
b * a % matrix multiply
a' * b' % matrix multiply
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%(4) Saving your work
save mysession % creates mysession.mat with all variables
save mysession a b % save only variables a and b
clear all % clear all variables
clear a b % clear variables a and b
load mysession % load session
a
b
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%(5) Relations and control statements
% Example: given a vector v, create a new vector with values equal to
% v if they are greater than 0, and equal to 0 if they less than or
% equal to 0.
v = [3 5 -2 5 -1 0] % 1: FOR LOOPS
u = zeros( size(v) ); % initialize
for i = 1:size(v,2)
if( v(i) > 0 )
u(i) = v(i);
end
end
u
v = [3 5 -2 5 -1 0] % 2: NO FOR LOOPS
u2 = zeros( size(v) ); % initialize
ind = find( v>0 ) % index into >0 elements
u2(ind) = v( ind )
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%(6) Creating functions using m-files:
% Functions in matlab are written in m-files. Create a file called
% 'thres.m' In this file put the following:
function res = thres( v )
u = zeros( size(v) ); % initialize
ind = find( v>0 ) % index into >0 elements
u(ind) = v( ind )
v = [3 5 -2 5 -1 0]
thres( v ) % call from command line
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%(7) Plotting
x = [0 1 2 3 4]; % basic plotting
plot( x );
plot( x, 2*x );
axis( [0 8 0 8] );
x = pi*[-24:24]/24;
plot( x, sin(x) );
xlabel( 'radians' );
ylabel( 'sin value' );
title( 'dummy' );
gtext( 'put cursor where you want text and press mouse' );
figure; % multiple functions in separate graphs
subplot( 1,2,1 );
plot( x, sin(x) );
axis square;
subplot( 1,2,2 );
plot( x, 2.*cos(x) );
axis square;
figure; % multiple functions in single graph
plot( x,sin(x) );
hold on;
plot (x, 2.*cos(x), '--' );
legend( 'sin', 'cos' );
hold off;
figure; % matrices as images
m = rand(64,64);
imagesc(m)
colormap gray;
axis image
axis off;
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%(8) Working with Images
[I,map]=imread('trees.tif'); % read a TIFF image
figure
imshow(I,map) % display it as indexed image w/colormap
I2=ind2gray(I,map); % convert it to grayscale
figure
imagesc(I2,[0 1]) % scale data to use full colormap
% for values between 0 and 1
colormap('gray') % use gray colormap
axis('image') % make displayed aspect ratio proportional
% to image dimensions
I=imread('photo.jpg'); % read a JPEG image into 3D array
figure
imshow(I)
rect=getrect; % select rectangle
I2=imcrop(I,rect); % crop
I2=rgb2gray(I2); % convert cropped image to grayscale
imagesc(I2) % scale data to use full colormap
% between min and max values in I2
colormap('gray')
colorbar % turn on color bar
pixval % display pixel values interactively
truesize % display at resolution of one screen pixel
% per image pixel
truesize(2*size(I2)) % display at resolution of two screen pixels
% per image pixel
I3=imresize(I2,0.5,'bil'); % resize by 50% using bilinear
% interpolation
I3=imrotate(I2,45,'bil','same'); % rotate 45 degrees and crop to
% original size
I3=double(I2); % convert from uint8 to double, to allow
% math operations
imagesc(I3.^2) % display squared image (pixel-wise)
imagesc(log(I3)) % display log of image
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