| Re: Curious Keywords [message #8390 is a reply to message #8356] |
Thu, 27 February 1997 00:00   |
davidf
Messages: 2866
Registered: September 1996
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Senior Member |
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JD Smith writes in a follow-up to this thread:
> The question is really about the mechanism of Keyword Checking for built-ins
> vs. non-built-ins. That is, if *I* wrote a routine that took the "THICK"
> keyword, a user could pass an undefined value through THICK and I would never
> know that he did. I could not use n_elements() or keyword_set() or any other
> mechanism I know of to discern in any way that he has used "THICK". And so,
> consequently, I would not be able to issue an error message in the case he
> passed an undefined variable. It is as if he never passed it at all! Not so
> for Plot and other built-in routines (for *some* of their keywords). They
> somehow "know" that I used e.g., "THICK", even when I pass them an undefined
> variable.
>
> My point was that this would be a useful feature to have (although it would
> possibly result in some subtle and harmful programmatic issues). For
> instance, if I pass a variable through a keyword into which I'd like to put
> the result of some calculation, I have to give that variable a value before
> passing it, in order to test whether to go through the bother of doing the
> calculation at all. There are, of course, other ways to do this (e.g.
> optional *parameters*), but I've found myself wishing for this particular
> mechanism on some occassions, when the other techniques had limitations.
JD is absolutely right about this. You cannot tell in an IDL procedure
if a keyword is *used* or not. You can only tell if the argument to the
keyword is *defined* or not.
(Most of the programs I look at, by the way, mistakenly think this
is what KEYWORD_SET is doing for them. Sigh...)
Since knowing if a keyword is *used* is often quite useful for
exactly the reasons JD mentions, I think it should be added to IDL.
Obviously IDL knows if the keyword is being used.
I think, if I remember correctly, that PV-Wave has a function
entitled KEYWORD_USED (or something like it) that does this very thing.
Cheers!
David
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David Fanning, Ph.D.
Fanning Software Consulting
2642 Bradbury Court, Fort Collins, CO 80521
Phone: 970-221-0438 Fax: 970-221-4762
E-Mail: davidf@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com
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