Matlab on Athena . Getting started . Matlab variables . Matlab language . Plotting Examples . Useful Subroutines

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".

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.

sprintf sscanf num2str int2strMatlab5 and 6 added the very useful "cell array" and "struct" data types. A cell array is just like a matrix except each entry can be any data type, not just a number. For example:

>> c = {'joe' 5 [1 2 3]} c = 'joe' [5] [1x3 double] >> c{3} ans = 1 2 3A struct is similar except its contents are addressable by name only.

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

plot grid hold drawnow axis axes orient subplot mesh meshgrid plot3 rotate3dThen you'll want to print your plot to a file. To print the current plot to a postscript file, type

orient tall %% this line is optional print -deps myfilename.psYou can use C style file I/O. Get help on commands

fopen fclose fscanf fprintf printfThe matlab parser is not too clever, so don't put "-" in a filename; poor matlab will think you want to subtract!

Matlab has a native data file format, plus it can save and load data from ASCII files. See the stock answers and check out

save filename.dat -ascii load filename

In matlab, subroutines can be associated with specific classes to simulate
"methods" for object-orientation. To make a method for class
`myclass`,
all you have to do is put the subroutine in a subdirectory called `@myclass`.

You must always have a subroutine in `@myclass` which is just
called `myclass`. This subroutine is called to create instances
of the class. Get matlab's help on `class`. If you redefine a class,
e.g. by editing `@myclass/myclass`, Matlab will complain unless
you clear the old definition with `clear myclass`. Don't ask me
why.

`clear x` can also be used to remove the variable *x* from
the workspace, e.g. to reclaim memory. Careful: `clear` alone removes
all variables!

>> 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

%% 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 . . .

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;