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Re: Optional parameters [message #38196 is a reply to message #38194]

Tue, 24 February 2004 09:08

David Fanning
Messages: 11724
Registered: August 2001

Senior Member

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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