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Re: undefined keyword variables

Craig Markwardt wrote:
> davidf@dfanning.com (David Fanning) writes:
> >
> > Mark Fardal (fardal@weka.astro.umass.edu) writes:
> >
> > > A question: should you always be able to pass undefined variables as
> > > keywords to IDL routines?
> >
> > Oh, this is absolutely normal behavior. (At least under the
> > usual standards by which such things are judged in IDL.)
> > Is it *correct* behavior? Don't know. But I would doubt it.
> > Seems to me *any* optional input keyword should be capable of
> > accepting an undefined variable as an argument. I would run
> > it by RSI for confirmation.
> >
> > > In general I don't know why you should be able to safely feed
> > > undefined variables to routines and expect them to work.
> >
> > Well, because you expect decent programmers to test any
> > variable they expect to receive and define default values
> > if one is not passed in. (As well as testing for data type
> > and structure, but who among us does this except under
> > exceptional conditions?)
> I am never sure any more how undefined keywords are passed.  It seems
> to make a difference whether it's a built-in routine, or an IDL
> routine.  It seems to make a difference whether it's IDL 4 or 5.  It
> seems to make a difference if you refer to the variable by name before
> calling the procedure (not necessarily setting its value).  All these
> factors make it hard to handle pass-through keywords consistently.  It
> would be nice (no, crucial!) to have this more carefully documented by
> RSI.

It's really not a difference between built-in and compiled routines,
just well-written and poorly written routines.  Back when I first
noticed this phenomenon of built-in routines recognizing undefined
variables, I immediately knew that RSI programmers had access to some
argument functionality we in compiled-land did not.  Thus was
arg_present() born.  I can now write a compiled routine which can:

1) Discern if a keyword is passed at all.
2) Discern if a keyword is passed with a value.
3) Discern if a keyword is passed which has scope in the passing level
(by reference).  

Both 2 & 3 can be simultaneously true.  So, since the introduction of
arg_present, we can make programs which handle undefined submitted
keywords gracefully, in whatever way necessary.  This doesn't mean we
*will*.  Here is an example which demonstrates the various
possibilities.  Note that keyword_set is a really a subset of
n_elements, and so isn't explicity included, though it can be useful.

pro testkey,KEY1=k1
   case arg_present(k1)+2L*(n_elements(k1) ne 0) of
      0: print,'Nothing was passed through the keyword.'
      1: print,'An undefined variable was passed.'
      2: print,'A value without scope in the passing level was passed.'
      3: print,'A defined and valued variable was passed.'

IDL> testkey
Nothing was passed through the keyword.
IDL> testkey,KEY1=1
A value without scope in the passing level was passed.
IDL> testkey,KEY1=undef_var
An undefined variable was passed.
IDL> undef_var=[1,2,3]
IDL> testkey,KEY1=undef_var
A defined and valued variable was passed.

RSI programmers have similar (and perhaps more) functionality for
writing built-in programs.  This doesn't mean they'll use it
consistently or correctly.  

I can easily produce a routine which fails on some keywords and not on
others when passed undefined variables.  So can RSI.  The problem is
there isn't always a correct thing to do... maybe an error is actually
appropriate in some cases, but consistency should be policy.


 J.D. Smith                             |*|      WORK: (607) 255-5842    
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