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Re: Optional parameters [message #38194 is a reply to message #38192]

Tue, 24 February 2004 09:59

Pavel Romashkin
Messages: 166
Registered: April 1999

Senior Member

I agree. I never had a problem with this because I never put unneeded
variables in a call to a procedure. I fail to see the logic in putting

myfunction, var1=uselessvar

as opposed to just

myfunction

and then worrying about uselessvar getting defined all of a sudden. The
only time this might happen in real life I suppose is if you use
copy-paste to insert function calls that address all of the provided keywords.
Pavel

David Fanning wrote:
>
> David Fanning writes:
>
>> Here is the same program with an optional keyword parameter
>> named "junk":
>>
>> PRO TEST, JUNK=junk
>> On_Error, 1
>> IF N_Elements(junk) EQ 0 THEN junk = 3
>> Print, junk
>> END
>
> An alert reader, who wishes to remain anonymous, but who
> is well-known in the IDL programming community, points out
> that there is one possible gotcha in this formulation.
> Namely, that an input variable that was previously undefined
> is now defined by my example. In other words:
>
> IDL> Help, junk
> JUNK UNDEFINED = <Undefined>
> IDL> Test, JUNK=junk
> IDL> Help, junk
> JUNK INT = 3
>
> If the user were counting on the variable junk to remain
> undefined for some reason, there would be a nasty surprise
> in store here.
>
> This is, of course, a consequence of parameters being input
> or output (or both) according to usage. I guess strictly
> speaking, you would need two different variables to represent
> parameters that are strictly input variables. The reader suggests
> this:
>
> PRO TEST, JUNK=junk_local
> On_Error, 1
> junk = n_elements(junk_local) eq 0 ? 3 : junk_local
> Print, junk
> END
>
> While I absolutely agree with the logic, I find this to be a royal
> pain in the neck in practice. Do you read Whinnie the Pooh? I am
> one of those "bears of little brain" and having two names for the
> same variable sends me into fits of confusion every time. Lord knows
> I have enough problems just keeping up with defining the same keywords
> over and over again for INIT, SetProperty, and GetProperty methods.
> Double names sounds like double work to me. I guess I'd rather track
> down the very rare "defined when I didn't want it to be" error than
> be frustrated with all the damn typos! :-)
>
> Cheers,
>
> David
>
> P.S. Let's just say I admire the good folks at RSI who are going
> to all the trouble to make input keywords strictly input. :-)
>
> --
> David Fanning, Ph.D.
> Fanning Software Consulting
> Coyote's Guide to IDL Programming: http://www.dfanning.com/

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[Message index]

		Re: Optional parameters By: Pavel Romashkin on Wed, 25 February 2004 10:21
		Re: Optional parameters By: David Fanning on Wed, 25 February 2004 09:26
		Re: Optional parameters By: Craig Markwardt on Wed, 25 February 2004 08:48
		Re: Optional parameters By: David Fanning on Wed, 25 February 2004 07:08
		Re: Optional parameters By: Paul Van Delst[1] on Wed, 25 February 2004 06:49
		Re: Optional parameters By: Craig Markwardt on Tue, 24 February 2004 21:50
		Re: Optional parameters By: JD Smith on Tue, 24 February 2004 14:03
		Re: Optional parameters By: R.G. Stockwell on Tue, 24 February 2004 14:02
		Re: Optional parameters By: Pavel Romashkin on Tue, 24 February 2004 13:45
		Re: Optional parameters By: JD Smith on Tue, 24 February 2004 12:13
		Re: Optional parameters By: Pavel Romashkin on Tue, 24 February 2004 09:59
		Re: Optional parameters By: David Fanning on Tue, 24 February 2004 09:08
		Re: Optional parameters By: David Fanning on Tue, 24 February 2004 08:31
		Re: Optional parameters By: JD Smith on Tue, 24 February 2004 08:10
		Re: Optional parameters By: David Fanning on Mon, 23 February 2004 17:39
		Re: Optional parameters By: David Fanning on Mon, 23 February 2004 17:05

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