How to write EXTERNAL FUNCTIONS and SCRIPTS
EXTERNAL FUNCTIONS
==================
Matlab allows you to write your own functions. You can execute these just
like any other function in matlab. These functions exist in files whose
name ends in a ".m", and are therefore called m-files.
Most of the functions that are used in matlab are actually just m-files.
Therefore, you can see how they work and modify them to suit your needs.
To list a m-file, just
>> type fname
where fname is the function you want to look at. As an example look at,
>> type flipud
(Note: you can also see this by typing
more /mit/matlab/current/distrib/toolbox/matlab/elmat/flipud.m
at the athena% prompt.)
----------------------------------------
function y = flipud(x)
%FLIPUD Flip matrix in the up/down direction.
% FLIPUD(X) returns X with columns preserved and rows flipped
% in the up/down direction. For example,
%
% X = 1 4 becomes 3 6
% 2 5 2 5
% 3 6 1 4
%
% See also FLIPLR.
[m,n] = size(x);
y = x(m:-1:1,:);
----------------------------------------
Let us examine the structure of this file:
1. The first line begins with the word "function"
This tells matlab that this is function. If you leave this
out, matlab will treat the file as a "script" (see below).
2. Following the function declaration there is a list of output
arguments (in this case 1), a statement of the function name, and
a list of input arguments.
Output arguments are listed in the format, [V1,V2,V3... etc] (if
you have only 1 you don't need the brackets).
The name you use must be the same as the filename (minus the .m).
The input arguments are listed after the function name in the
format (x1,x2,x3,...etc) These variables are assigned values when
the function is called. By default all variables in m-files are
local, so you may call them whatever you want.
3. After the function declaration, there are a series of lines that
start with "%" (these are comments).
The first contiguous group of lines that begin with "%" are
treated as the help blurb for the function. You can see this by,
>> help flipud
When you write your own files, it is useful to use this "help"
facility to remind you of its syntax and purpose.
Here is an example of a simple m-file that will scramble the elements in a
vector, and output both the scrambled vector and a vector containing
indices used to descramble the vector.
----------------------------------------
******************************************
scram.m
******************************************
function [Xs,Ir]=scram(X)
% SCRAM Scramble the elements in a vector.
% SCRAM(X) returns a scrambled vector and a vector
% containing the indices used to descramble it.
N=max(size(X));
I=-ones(1,N);
for j=1:N
Q=-1;
while Q~=0
tmp=round(rand*N+.5);
Q=sum(tmp==I);
end
I(j)=tmp;
end
Xs=X(I);
Ir(I)=1:N;
*******************************************
>> help scram
SCRAM Scramble the elements in a vector.
SCRAM(X) returns a scrambled vector and a vector
containing the indices used to descramble it.
>> Y=1:2:20
Y =
1 3 5 7 9 11 13 15 17 19
>> [Ys,I]=scram(Y)
Ys =
9 7 17 19 13 5 11 15 3 1
I =
5 4 9 10 7 3 6 8 2 1
>> Ys(I)
ans =
1 3 5 7 9 11 13 15 17 19
SCRIPTS
=======
If you leave out the "function" line, then a mfile is considered to be a
"script". Calling a script is equivalent to entering each of the lines in
the script at the ">>" prompt. All variables defined in the parent
workspace are accessible to the script and can be modified by the script.
In this sense, all variables are considered global in a script. This
feature is both handy and dangerous. You don't have to explicitly pass
arguments to a script, but if you assign a value to a variable in a script
that exists in the parent workspace, its value is overwritten.
PROGRAMMING HINTS
=================
1. Vectorize wherever possible to save immense amounts of computation time.
>> for i=1:500, % This took about 2 minutes
for j=1:500, % to complete.
A(i,j)=i*j;
end
end
>> A=(1:500)'*(1:500); % Takes less than 1 second
% to complete.
Wherever possible, use matrix operations rather than loops.
2. Functions can take variable numbers of arguments. "nargin" and
"nargout" are permanent variables that contain the number of input and
output arguments.
3. You can do almost anything in a mfile that you can do from the matlab
prompt including; Loading and saving, issuing system commands ("!command",
see ">> type print"), writing to files (fprintf), call other mfiles
(including itself), plot, etc..
4. An easy way to call programs written in other languages is:
o Save variables in a file
o Run external program which reads the file and writes output to
another file.
o Load the data back in.
For example:
function y = garfield(a,b,q,r)
save gardata a b q r
!gareqn
load gardata
5. Editing a mfile causes matlab to recompile it the next time it is
called in matlab. You can, therefore, run a function, modify the mfile,
and then run the new version. Note: Modifying a mfile during the execution
of the function does not affect the current process.
6. Use the Matlab debugging commands to help track down programming
errors. For more information, consult the "Debugging MATLAB .m files"
answer in the stock answers browser.
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