mas.622j Matlab Help

Getting started

If you're a complete matlab novice, type "intro" or, if you're real fond of splashy colors, type "demo". You can also read help from matlab help menu.

You can also type "helpwin" or "helpdesk" to get a help window. The matlab help command is "help". Try "help general" or just plain "help". The apropos command is "lookfor". Try, e.g. "lookfor tangent" or "lookfor random".

Syntax

The matlab comment character is "%". The semicolon ";" is useful too; it makes the command on that line operate silently. For example, "A = B;" copies matrix B into matrix A. "A = B" does the same thing, but prints out the whole contents of matrix B while copying.

Data structures

  sprintf  sscanf  num2str  int2str

 
>> c = {'joe' 5 [1 2 3]}

c = 

    'joe'    [5]    [1x3 double]

>> c{3}

ans =

     1     2     3

>> s = struct('name', 'joe', 'age', 30)

s = 

    name: 'joe'
     age: 30

>> s.name

ans =

joe

>> s.age

ans =

    30

Plotting and I/O

Some Basic Examples of Variable Manipulations in MATLAB

>> v = ones(1,3)

v =
     1     1     1

>> v = [ 1 1 1 ];

>> w = 2:4;

>> w = [2 3 4];

>> x =  x = [[1];[4];[9]]

x =
     1
     4
     9

>> x = [1 4 9]';

>> v * x

ans =
    14

>> v*3 

ans =
     3     3     3

>> v .* x
??? Error using ==> .*
Matrix dimensions must agree.

>> v .* x'

ans =
     1     4     9

>> y = exp(w) 

y =
    7.3891   20.0855   54.5982

>> z = [v' w' x]

z =
     1     2     1
     1     3     4
     1     4     9

>> w*z

ans =
     9    29    50

Some Basic Examples of Language Structure in MATLAB

%%	help  is a very useful matlab command.  So is  lookfor
%%	try 'help' and 'help general' to see what help topics are available.

>> help if

 IF     Conditionally execute statements.
        The general form of an IF statement is:
                IF variable, statements, END
        The statements are executed if the real part of the variable 
        has all non-zero elements. The variable is usually the result of
        expr rop expr where rop is ==, <, >, <=, >=, or ~=.
        For example:
  
               IF I == J
                  A(I,J) = 2;
               ELSEIF ABS(I-J) == 1
                  A(I,J) = -1;
               ELSE
                  A(I,J) = 0;
               END

>> help for

 FOR    Repeat statements a specific number of times.
        The general form of a FOR statement is:
  
           FOR variable = expr, statement, ..., statement END
  
        The columns of the expression are stored one at a time in
        the variable and then the following statements, up to the
        END, are executed. The expression is often of the form X:Y,
        in which case its columns are simply scalars. Some examples
        (assume N has already been assigned a value).
  
             FOR I = 1:N,
                 FOR J = 1:N,
                     A(I,J) = 1/(I+J-1);
                 END
             END
  
        FOR S = 1.0: -0.1: 0.0, END steps S with increments of -0.1
        FOR E = EYE(N), ... END  sets E to the unit N-vectors.


%% do 'help lang' for more on flow-of-control constructs.
%% do 'help elfun' for more on elementary math functions.
%% do 'help ops' for operators and special characters.
%% do 'help matfun' for matrix functions.
%%
%%  each of these gives a list of topics for further help; e.g.

>> help eig

 EIG    Eigenvalues and eigenvectors.
        EIG(X) is a vector containing the eigenvalues of a square
        matrix X.
 
        [V,D] = EIG(X) produces a diagonal matrix D of
        eigenvalues and a full matrix V whose columns are the
        corresponding eigenvectors so that X*V = V*D.
  
        [V,D] = EIG(X,'nobalance') performs the computation with . . .

Some Basic MATLAB Plotting Examples

This assumes you've gotten matlab started and are staring at a matlab prompt. You can copy and paste text into the matlab window. Do that with each block of statements, one block at a time, so you can see the results of each plot.

%%  make a row vector from -3 to 3
%% you can see the contents of xvals by typing 'xvals' with no trailing ';'
xvals = -3:.25:3;

%% make a row vector of gaussian densities.
yvals = 1/(sqrt(2*pi))*exp(-(xvals.^2)/2);

%%%% The FOR loop below produces the same result as the statement above
%% for i = 1:length(xvals);
%%    yvals(i) = 1/(sqrt(2*pi))*exp(-(xvals(i)^2)/2);
%% end;

%% make a simple 2D plot
plot(xvals, yvals);

%% dress it up a little
clf;
h = axes('position',[.2 .1 .6 .8]);  %% also try "help subplot"
set(h,'fontsize', 7);	  %% type "set(h)" to get huge list of options %%
plot(xvals, yvals), axis([-3.1 3.1 0 .5]), grid on;
xlabel('Standard Deviations'), ylabel('Probablility Density'),
legend('The curve'), text(-.4, .42, 'The peak');
   title('Your Basic Gaussian Density')

%%  make another row vector
zvals = log10(abs(yvals))/2;

%% simple 2-color plot
plot(xvals, yvals, ':', xvals, zvals, '--');
legend('The curve', 'Its log');

%% Clear, then put three plots on the same graph
clf;
plot(xvals, yvals,'-'), hold on,
plot(xvals, zvals,'o'), hold on,
plot(xvals, xvals/7,'+'), hold off;

%% save it in a file in postscript format
print -deps killmeplot.ps 

%% example from 'help quiver'
xord = -2:.2:2;
yord = -2:.2:2;
[x,y] = meshgrid(xord,yord);
z = x .* exp(-x.^2 - y.^2);
[px,py] = gradient(z,.2,.2);
contour(x,y,z),hold on, quiver(x,y,px,py), hold off

%% one kind of 3D plot
plot3(x,y,z), grid on;
% spin it with the mouse
rotate3d on;

%% another kind of 3D plot
mesh(xord, yord, z), view(10,20);
rotate3d on;