MAS 622j matlab help

Originally written by Tom Minka, and modified by Yuan Qi and Ashish Kapoor Sept. 2002

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

Getting started

Open an editor window next to your matlab window. You'll often find yourself mousing text in the editor window and pasting it into the matlab window.

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 continuation code is "..." i.e. if you have a long formula that is several lines long, end each line with ... and continue the formula on the next line.

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

The most useful data strucutre in Matlab is matrix. Scalars is considered 1 by 1 matrices, and strings (delimited by 'single quotes') are considered vectors (which in turn are just skinny matrices). Check out

  sprintf  sscanf  num2str  int2str

Matlab5 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     3

A 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

Plotting and I/O

Matlab is very good at plotting your data. See our plotting examples and/or get matlab's help on:

  plot  grid  hold  drawnow  axis  axes  orient
  subplot  mesh  meshgrid  plot3  rotate3d

Then 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.ps

You can use C style file I/O. Get help on commands

  fopen  fclose  fscanf  fprintf  printf

The 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

Subroutines and objects

Yes, you can write subroutines in matlab. Each subroutine lives in its own file, with a name ending with .m, and you call it by filename. Matlab will sometimes not notice that you have created a new subroutine. Use path(path) to make it rescan the directories.

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!

Thomas P. Minka, tpminka@media.mit.edu