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Re: Optional parameters [message #38215 is a reply to message #38214] Mon, 23 February 2004 17:05 Go to previous message
David Fanning is currently offline  David Fanning
Messages: 11724
Registered: August 2001
Senior Member
Michael Wallace writes:

> The learning of IDL continues.... today's question is about optional
> parameters. Specifically, how do you make a parameter optional? I can
> find stuff in the IDL documentation regarding position parameters and
> keywords, but I haven't found anything yet about optional parameters.
> So, how do you create optional parameters and what are common gotchas
> with using them?

An optional parameter can be either a positional parameter or a keyword
parameter, although the general rule of thumb is that positional
parameters are always required and keyword parameters are always
optional. The rule is broken frequently, especially with positional
parameters and less so with keyword parameters (having to suffer
the wrath of Coyote if you violate the "keyword is optional" dictum
probably accounts for the decreased frequency of the latter).

What makes a parameter "required", oddly enough, does not depend
on how it is defined or used (e.g., is it an input or output
parameter). Rather, what makes a parameter "required" is how
you respond to its absence or presence. It is required if you
notice it is not there when you need it and issue an error message.
It is optional if you notice it is not there when you need it and
you arbitrarily define a default value for it.

Here is a simple program with a required positional parameter
named "junk":

PRO TEST, junk
On_Error, 1
IF N_Elements(junk) EQ 0 THEN Message, 'JUNK is required.'
Print, junk
END

Here is the same program with an optional positional parameter
named "junk":

PRO TEST, junk
On_Error, 1
IF N_Elements(junk) EQ 0 THEN junk = 3
Print, junk
END

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

I'm not going to show you a program with a required keyword
parameter because no one in his right mind (with the possible
exception of the good folks at RSI) would write one. :-)

Note that if the parameter has a *binary* quality (that is
it is 0 or 1, yes or no, on or off, etc.) then you can check
the parameter with KEYWORD_SET. All other parameters should
be checked with N_ELEMENTS. This is a subtle point that only
bites you when your equipment is on it way to Mars. :-)

Good programming!

Cheers,

David

P.S. Let's just say the only gotchas with optional parameters is
forgetting to check whether they are defined or not, and using
the wrong function to check whether they are defined or not.
Remember, you can *never* go wrong with N_ELEMENTS. :-)

--
David Fanning, Ph.D.
Fanning Software Consulting
Coyote's Guide to IDL Programming: http://www.dfanning.com/

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