Re: Passing optional parameters through a wrapper routine

[davidf(at)dfanning.com (David Fanning)]

Kenneth P. Bowman (bowman@null.tamu.edu) writes:

> (David Fanning) wrote:
> 
> > Hard to know what TVRD is doing
> 
> If a program with optional parameters did something like this:
> 
> PRO OPT_PARAM, p1, p2, p3
> 
> IF (N_PARAMS() EQ 2) OR (N_ELEMENTS(p3) EQ 0)) THEN 
>    ... do the default thing for p3
> ENDIF
> 
> then it would handle "wrapping", no?

I'm not sure I understand this question, Ken. In general,
with most built-in IDL routines, you will get an error
by passing an undefined variable. You could argue (quite
convincingly, I imagine) that built-in IDL routines
should handle this situation more gracefully, but
that is the situation we find ourselves with. You can
certainly make your own IDL routines more graceful when 
you write *them*, but there is little you can do about
what is already there.

Having said that, it absolutely behooves you to check
each and every variable you plan to use in your program
to make sure you have a defined variable at the time
you use it. This is normally done with the N_Elements
function, since this function returns a 0 if its argument
is undefined.

You would probably NOT use the odd syntax above for 
the simple reason that it would fail if the number
of parameters was 0 or 1. What you probably *would* do
if p1 and p2 are required parameters and p3 is optional,
is something like this:

   IF N_Params() LT 2 THEN Message, 'Whoops, two parameters required'
   IF N_Elements(p3) EQ 0 THEN p3 = p3DefaultValue

Cheers,

David

-- 
David Fanning, Ph.D.
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