Re: Functions and arrays

[Peter Mason <peterm(at)demsyd.syd.dem.csiro.au>]

On Sat, 7 Dec 1996, David R. Klassen wrote:
> See, right now my problem is just the opposite.  I reference a function
> that IDL is trying to interpret as an array even though the "array" has
> not been previously referenced.  I can't seem to understand why the
> program can't retrieve the function...

Here are some observations that might help you sort this out:

It appears that IDL always makes the distinction between variables and
functions at the time a routine is "compiled".   This distinction has to be
made if an identifier, say XXX, appears with brackets somewhere in the
routine - as in XXX(...).   (I must say that I was a little surprised to find
this out.   I've always considered IDL to be an interpreter with some sort of
"tokenising" pass - not a true compiler, and thought that the determination of
exactly what XXX is would usually be done at run-time.   Anyway...)

IDL appears to follow these rules to work out whether XXX is a variable or a
function:
1  If a function XXX has already been compiled, or declared in a
   FORWARD_FUNCTION statement somewhere, then it remains a function.
   (If a PROCEDURE XXX has been compiled, it is not noticed here.   IDL can
   live with both a function and a procedure named XXX!)
2  If XXX appears in a statement which could assign a value to XXX, BEFORE the
   first statement which contains XXX(, then IDL decides that XXX is a
   variable.   (IDL does appear to handle statements like SOME_ROUTINE,...,XXX
   as potential assignments, along with XXX=.)
3  If IDL first encounters XXX in the form XXX(, then it goes looking for a
   file XXX.pro (or XXX.sav, I guess).   If such a file is found, it doesn't
   compile it, but it decides that XXX is a function.   (IDL is expecting
   FUNCTION XXX to be the only or last routine defined in the file XXX.pro.
   It doesn't appear to check that this is actually the case, at this stage.)
   If it doesn't find such a file, then it decides that XXX is a variable.

The simplest thing for you to do would be to use a FORWARD_FUNCTION statement.
(You can use a FORWARD_FUNCTION statement anywhere, as long as IDL sees it
before it attempts to compile a routine which refers to the function in
question.   A handy place is in IDL's startup file.)


Peter Mason