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Writing your own functions

If you put any valid sequence of Matlab statements into a file and give the file a name ending with the suffix .m, then Matlab will execute those statements if you type the name of the file (without the suffix). You can use any text editor to create the file. Matlab's edit command will try to use Matlab's own editor, which has a built-in debugger.

For example, if you have a file called dothis.m which contains the text

	back = []
	for k=10:-1:1, back=[back,k], end

then just typing dothis will cause the for loop to be executed. Such files are called script files.

If the first line of such a file has the form

	function outputs = somename(inputs)

then it becomes a function. In this case inputs and outputs are formal parameters, and variables used inside the file will be local to that file, which means that they will not clash with other variables you may have used having the same names (unless these are explicitly declared to be global). (Matlab functions correspond to subroutines in Fortran or functions in C++.)

In an earlier section we wrote a Matlab statement for finding the centre of gravity of a set of weights, given their locations along a straight line. If you wanted to calculate centres of gravity repeatedly, then you could write a function which would perform the required calculation, and store it for future use. All you need to do is to create a file called cofg.m in your directory (using an editor) which contains the following text:

	function cg = cofg(weights, locations)
	% Comments are introduced by percent signs, as here.
	% These initial comments will be written to the terminal
	% if you type 'help cofg'. It is useful to remind the
	% user how to call the function by putting in the line:
	%        cg = cofg(weights, locations)
	cg = weights * locations' / sum(weights);

and that's all there is to it! When this file exists in your directory, you can type the following in Matlab:

cg = cofg([3 1 7],[-2 0 5])

Of course it may be advisable to make your function more sophisticated, for example to check whether both input arguments are row vectors, or to make sure that it will work with either row or column vectors. (Note that this function as written here will in fact work in two or three dimensions. Can you see why the following works?

	xlocations = [-2 0 5]
	ylocations = [1 2 3]
	xylocations = [xlocations ; ylocations]
	cg2 = cofg([3 1 7], xylocations)

This gives the x and y coordinates of the centre of gravity.)

To see the whole of your function from within Matlab, type type cofg. Try this with some other functions, such as type angle or type gradient.

It is also possible to pick up subroutines which have been written in other languages and pre-compiled. See the Interface Guide for details.