doc

%help <command>
%As an example, to find out more about print:
help print
% when in doubt, try help!

%Matlab can be used as a calculator (with +, -, *, / indicating addition, subtraction, multiplication and division)
 
3*2
(3+5)*15
 
%Exponentiation is done with ^
 
5^2
 
% Square root operation with 'sqrt'
 
sqrt(25)
 
%Matlab already knows the meaning of some variables, eg, pi
 
pi
sin(pi/2)
log(pi) %natural logarithm of pi
log10(pi) %logarithm with base 10
 
%you can use the format command to change the output format for numbers
%for details on format, type
 
help format

%example of long format:
 
format long
pi % displays 3.141592653589793
format short
pi % displays 3.1416

% The array x contains numbers ranging from 0 to 1.0 in increments of 0.25.
% Note that the ";" at the end suppresses the output. Experiment by leaving it out.
 
x=[0,0.25,0.50,0.75,1.00];
 
% typing x (without semi-colon) will output the array
 
x

%The same array can be created with a simpler command. 
%The following syntax with ":" indicates that the array starts from 0
% and is incremented by 0.25 till we reach 1.0. This is much easier for larger
% arrays (such as the array t, which contains 100 numbers).

x=[0:0.25:1.0]
t=[0:0.01:1.0]

%The following (without brackets) also creates an array.

v=0:0.01:1.0

% The function 'linspace' can be used to create an array of 100 equally spaced elements.

v=linspace(0,1.0); note that the default is 100 equally spaced points between 0 and 1.
 
% To create an equally spaced array between 1 and 1.0 with 5 points
 
x=linspace(0,1.0,5); the third argument specifies the number of points.

% Sum of elements of an array
 
sum(x)
 
% Note we already have created an array t before! We create a second array:

x=2*pi*t; This multiplies each element of t by the scalar 2*pi.
 
% y will be an array consisting of sin of the elements of the array x

y=sin(x);

%Sometimes we need to carry out operations on individual elements of an array.
%Let us start with an array x
x=[0,0.25,0.50,0.75,1.00];
%We would like to square each element of the array. In mathlab, this can be done with:
x.^2  % Notice the dot (.) in front of exponentiation (^).

%This returns:
%ans =
%         0    0.0625    0.2500    0.5625    1.0000
%Element-wise operations work on Matrices too!

% We have already created arrays x and y. We can now plot y as a function of x.
plot(x,y)
xlabel('angle'); % will add x-label
ylabel('Amplitude'); % will add y-label
title('Plot of sin(\Theta)') % adds a title. Note \Theta is from the Latex typesetting program.

v1=[1.5,-2,4,10]; % A 1-dimensional array can be handled as a vector.

v2=[3.1,-1,2,2.5]; %Another 1-dimensional array

%The two arrays v1 and v2 are basically vectors with all the vector operations defined.

dot(v1,v2); %dot product of two vectors
 
% Angle between the vectors v1 and v2
% To compute the angle between two vectors, we need the magnitude of the two vectors
 
abs_v1=sqrt(sum(v1.^2)) % Notice we are performing element-wise exponentiation (i.e. .^2)
abs_v2=sqrt(sum(v2.^2))
costheta=dot(v1,v2)/(abs_v1*abs_v2); cosine of the angle between the vectors
 
% An easier way: instead of computing the magnitude yourself, you can use the norm function
 
costheta=dot(v1,v2)/(norm(v1)*norm(v2));
 
%To find out more about norm, do:
 
help norm  % warning: the response is detailed and technical!
theta=acos(costheta); %Now compute the angle (in radians) between the vectors.

% convert radians to degrees
radtodeg(theta)
 
% convert degrees to radians

radtodeg(0.6678)

%A matrix consisting of two rows and 6 columns is defined
%like an array except that the two rows are separated by ";"
%Notice how the semicolon symbol (;) can take on different meanings depending
%on the context. Below you see it used to define a matrix and to suppress the output.
 
x=[1.0,1.2,1.4,1.6,1.8,2.0;2.0,2.2,2.4,2.6,2.8,3.0];

x=

   1.0000    1.2000    1.4000    1.6000    1.8000    2.0000
   2.0000    2.2000    2.4000    2.6000    2.8000    3.0000

x=[1:0.2:2;2:0.2:3]

x =

%  output:   1.0000    1.2000    1.4000    1.6000    1.8000    2.0000
%            2.0000    2.2000    2.4000    2.6000    2.8000    3.0000
 
% Transpose of a matrix is obtained by superscripting a matrix with the prime character, i.e. "'":
 
y=x' % transpose of x

% We extract the last four columns of the second row by "slicing":
x(2,[3:6])

%ans =
%    2.4000    2.6000    2.8000    3.0000

% Random matrix, identity matrix, etc.:

a=rand(2)

%ans =
%    0.8147    0.1270
%    0.9058    0.9134
%    Note: you won't get the same matrix! why? hint is in the name.

%A 3X3 Identity matrix:

b=eye(3) 
 
%output:
%     1     0     0
%     0     1     0
%     0     0     1
 
%Determinant of a matrix obtained with det function (defined only for square matrices)
 
u=[5,11,-3;4,-5,0;7,8,-9]


det(u)


% the answer should be 420.0


% Inverse of a matrix obtained with inv function.


inv(u)


% produces


%    0.1071    0.1786   -0.0357
%    0.0857   -0.0571   -0.0286
%    0.1595    0.0881   -0.1643


%product of a matrix with its determinant should ideally be an identity matrix.
% In practice, it will be close but may not be exact.


u*inv(u)


% produces


% 1.0000 0 0.0000
%  0.0000    1.0000   -0.0000
% -0.0000   -0.0000    1.0000

A=[2 4 3;1 -2 -2;-3 3 2];
B=[4;0;-7];

Solution x=inv(A)B

x=inv(A)*B

% Use of the backslash operator (\) is a more efficient way:

x=A\B

% Let us conside the Taylor series for the sin function
% The series is summation over
% n (from 1 to k) of (-1)^(n-1)x^(2n-1)/(2n-1)\!
% For an arbitrary number of terms k, how do we evaluate sin(x) for a given
% value of x?
% The task is one of carrying out summation of k terms. This is 
% accomplished via the "for" loop. The syntax is:
% for i=1:k
%  .....
%   (do some operations that depend on i)
%  ......
% end
% For the sin series, here is one way to implement the summation:
 sinfunc=0; % initialize the sin function
 for i=1:k
  km1=2*i-1;
  kp1=i-1;
  kcoef=(-1)^kp1;
  sinfunc=sinfunc+kcoef*x.^km1/factorial(km1); %This is the summation step. On the right hand side, The k'th term
                                               % of the series is evaluated and added to the previously stored value
                                               % of sin function. Then sincfunc (left hand side) is replaced with the
                                               % new value on teh right hand side.
 end
 sinfunc   % sinfunc contains the approximation of sin(x)
 err=sin(x)-val; % you can easily calculate the error

x = -pi:.1:pi;
y = sin(x);
p = plot(x,y)

% gca returns the handle to the current axes for the current figure.

set(gca,'XTick',-pi:pi/2:pi)
set(gca,'XTickLabel',{'-pi','-pi/2','0','pi/2','pi'})

% \pi, \leq, \Theta are all from "latex" typesetting system.

xlabel('-\pi \leq \Theta \leq \pi')
ylabel('sin(\Theta)')
title('Plot of sin(\Theta)')

% \Theta appears as a Greek symbol (see String)
% Annotate the point (-pi/4, sin(-pi/4))

text(-pi/4,sin(-pi/4),'\leftarrow sin(-\pi\div4)',... % Notice the line continuation with ellipsis (...)
     'HorizontalAlignment','left')

% Change the line color to red and
% set the line width to 2 points

set(p,'Color','red','LineWidth',2)

%If we have two arrays that define two orthogonal axes, matlab makes it easy to 
%produce a grid from the axes to form the basis for plotting. 
%This is done with the meshgrid command. Let's define the two axes and see how the meshgrid command works.

x=[-8:4:8]; % an array of points on the x-axis [-8 -4 0 4 8]
y=[-8:4:8]; % an array of points on the y-axis [-8 -4 0 4 8]
[X Y]=meshgrid(x,y) % returns two matrices X and Y:

%X =
%    -8    -4     0     4     8
%    -8    -4     0     4     8
%    -8    -4     0     4     8
%    -8    -4     0     4     8
%    -8    -4     0     4     8
%Y =
%    -8    -8    -8    -8    -8
%    -4    -4    -4    -4    -4
%     0     0     0     0     0
%     4     4     4     4     4
%     8     8     8     8     8
 
% Note (X[j],Y[j]) define points in the 2D space defined by the x and y axes.
% Thus, if we have a function defined on x and y, i.e. f(x,y), 
% evaluation of the function over all the ordered pairs of points
% in x and y can now use the whole matrices X and Y instead of looping
% over all the points. See below for an example of how this is done in the
% context of a 3D plot below.

% Plot of 3D sinc function.
x=[-8:.5:8];
y=[-8:.5:8];
% Look up "meshgrid" in matlab document
% Returns rectangular grid
[X Y]=meshgrid(x,y);
R = sqrt(X.^2 + Y.^2); % Notice how element-wise operation is used.
Z = sin(R)./R;  % Sinc function
surf(Z); % surface plot
mesh(Z); % mesh plot
contour(Z); % contour plot

[X,Y] = meshgrid(-2:.2:2, -2:.2:2);
Z = X .* exp(-X.^2 - Y.^2);
surf(X,Y,Z)

p=[0:0.01:100];
q=2*pi*p;
r=sin(q);

function [mean,stdev] = stat(x)
% x, which is an array of numbers, is the input
% mean and stdev are the return values (outputs)
n = length(x);
mean = sum(x)/n;
stdev = sqrt(sum((x-mean).^2/n));

x=[1.0,1.2,1.4,1.6,1.8,2.0]


[mean stdev]=stat(x)


%returns
%mean=1.50
%stdev=0.3416

function [m,val] = cheby(k,x)
% Chebyshev polynomial recurrent relation
% Relevant to question 2 in assignment_0
% Note: x and k are not vectors.
if k==0
 m=0;
 val=1.0;
elseif k==1
 m=1;
 val=x;
elseif k>=2
 tch(1)=1;
 tch(2)=x;
 for i=3:k+1
  tch(i)=2*x*tch(i-1)-tch(i-2);
 end
 m=i-1;
 val=tch(i);
end

function [sinfunc,err] = sinTaylorSeries(k,x)
% The Taylor series for the sin function
% The series is summation over
% n (from 1 to k) of (-1)^(n-1)x^(2n-1)/(2n-1)\!
% Relevant to question 5 of assignment_0.
% Note x is a vector.
 sinfunc=0;
 for i=1:k
  km1=2*i-1;
  kp1=i-1;
  kcoef=(-1)^kp1;
  sinfunc=sinfunc+kcoef*x.^km1/factorial(km1);
 end
 val=sinfunc;
 err=sin(x)-val;

function [deriv,act,err] = tanderiv(x,h)
% Derivative of tan: (tan(x+h)-tan(x-h))/2h
% relevant to question 4 in assignment_0.
% Note x and h are not vectors.
 y1=tan(x+h);
 y2=tan(x-h);
 deriv=(y1-y2)/(2*h);
 act=sec(x)*sec(x);
 err=abs(deriv-act)/act;

%%The primary functions for loading images is 'imread' and for displaying images is 'imshow'.
 
im=imread('baboon.png');
imshow(im);
 
%% Images are basically two dimensional array of pixels (picture elements) with a third dimension
%% that shows the intensity of pixel. If the image is a gray scale image,, the intensity is a single number
%% at each pixel. If it is a color image, it is stored as a "third dimension" with 3 value
%% each corresponding to the intensity of r (red), g (green) and b (blue) colors. Thus if we do:
 
size(im)
 
%% The result is 512 512 3, which indicates it is a two dimensional array of 512 x  512 pixels with three colors.
%% To get the 'red' intensity, you can do:
 
r=im(:, :, 1);
 
%% This is now a two dimensional array. The color intensity can vary anywhere from 8 bits (256 levels)
%% for grayscale to 24 bits (8 bits for each color) for color images. There are even 30-, 36-, or 48- bit
%% images. As far as Matlab is concerned,. the image once loaded is a three dimensional array with the last 
%% dimension taking on only 3 values. 


max(r(:)),min(r(:))
 
%% will show 255 and 0 (the red color is stored in 8 bits).


imshow(r)
 
%% will show the image in grayscale with intensity corresponding to "red" color intensity in the 
%% original image. You can convert the color image to a grayscale image:
 
img=rgb2gray(im);
imshow(img)
 
This is now a two dimensional image with a grayscale intensity of 8 bits. We have compressed the image
at the expense of color!

%% Many medical images (such as MRI, CT, ultrasound, etc.) are likely to be grayscale images. 
%% They can 10 or 12 bit images and it is convenient to represent them using 16 bits as computers can 16 bits efficiently.
A=imread('Brain-MRI.jpg');
size(A)
imshow(A)
%% We can convert this to an grayscale image with:
B=rgb2gray(A)
%% This will be a 2D array of pixels with gray shades represented by 8 bits (256 shades).
%% We can plot a histogram of color intensity (x-axis intensity, y axis -- number of pixels) with:
imhist(B)
%% We can reduce the variation in the intensity among the pixels (and thereby visually enhance the darker regions)
%% by using the matlab function histeq.
histeq(B)
%%will display the image with formerly darker regions more bright and brighter regions darker.