Writing your own functions.

IDL uses files with the extension '.pro' for storing listings of routines. When an unknown function is called, whilst using an interactive IDL session, an attempt is made to find a file. This search is for a file of the same name as the routine and it occurs in the directories specified by IDL_PATH. If the file is found it is compiled.

JETDSP automatically searches the AddIns directory for these files, which makes it a useful place to put them.

Previously it was necessary to create a '.sav' file but these do not need to be created now.

As an example a function to normalise a signal will be written. The code for this is given below, copy it into an editor an save to a file named 'my_normalise.pro' in your AddIns directory. Note that due to a bug in the Linux release of IDL the filename should all be in lowercase.

function my_normalise,data
	min_data=min(data,max=max_data)
	scale=1.0/(max_data-min_data)
	return,(data-min_data)*scale
end

These functions can take other variables as well, but one function is required for each set of data (signal/time/x) required. For example to perform FFT's two calls are required, the first has the form

function my_fft,data,time
	;put the data onto a uniform time axis using interpolate commands.
	;fft the data
	return, data_fft
end

function my_fft_time,time
	; calculate the 'time' points required.
	return, time_fft
end

This is far from ideal in more complicated cases where the same processing must be done three times to return all the data. To get round this problem a new way of using signal processing function has been introduced. This uses only one function call and must return a specific structure. This is used to create the new signal object and is shown pictorially below.

This is illustrated in the my_truncate function. An example of this is the fixgrid function previously discussed. For further information on using these structures please contact Data and Coding, whose address is linked at the bottom of the page.