comp.lang.idl-pvwave archive: archive » Re: Finding if a structure tag name is defined?

Home » Public Forums » archive » Re: Finding if a structure tag name is defined?

Show: Today's Messages :: Show Polls :: Message Navigator
E-mail to friend

Re: Finding if a structure tag name is defined? [message #36706 is a reply to message #36703]

Tue, 14 October 2003 07:32

David Fanning
Messages: 11724
Registered: August 2001

Senior Member

William writes:

> I want to know if a given keyword has been passed down via _extra, so
> I said "if (n_elements(e.c_linestyle) ne 0)" which of course doesn't
> work because if it isn't there you can't say e.linestyle (bring on
> perl...). I can use tag names to get all the tags, then search through
> for a match, but that seems silly and ugly. What is the correct way
> to find if a given structure has a certain tag in it? (And why
> couldn't I find the answer buried within DF's site ;-)?

_EXTRA is often a great convenience (and maybe even
a miracle when you are writing object programs), but
is should NOT be used to collect keywords you care
about (at least not in the way you appear to be using it
here). In fact, I'm adding a rule to Coyote's List of Rules
You Violate Only If You Wish Your Program to Break at the
Most Inopportune Moment:

Rule 3495: If you care about a keyword, be sure
*you* define it somewhere!

Consider the PLOT command. It has 50-60 different keywords
that can be used with it. But if you are writing some
kind of wrapper to the PLOT command that uses a nifty
new plot symbol, you could care less about 49-59 of those
keywords. But you care desperately about the PSYM keyword.
In that case, you MUST define a PSYM keyword for your
program.

It is not fair to write this program and then decide
later than you really care about the LINESTYLE keyword,
too, and decide to fish it out of the _EXTRA structure.
(And what a nightmare, since you will have to look for
*all* possible spellings of LINESTYLE!) If you care
about it, you define it. Period. :-)

All those other keywords can be collected easily and
simply with _EXTRA. It is a convenience. (And also,
occasionally, a pain in the neck. Maybe you really
want to use _STRICT_EXTRA instead. Think about it.)

_EXTRA gives you more time to go drink coffee. But it doesn't
take away the responsibility of being a conscientious
programmer. :-)

Cheers,

David

--
David W. Fanning, Ph.D.
Fanning Software Consulting, Inc.
Phone: 970-221-0438, E-mail: david@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155

Report message to a moderator

[Message index]

		Re: Finding if a structure tag name is defined? By: mvukovic on Wed, 15 October 2003 11:51
		Re: Finding if a structure tag name is defined? By: Mark Hadfield on Tue, 14 October 2003 19:35
		Re: Finding if a structure tag name is defined? By: David Fanning on Tue, 14 October 2003 15:19
		Re: Finding if a structure tag name is defined? By: wmconnolley on Tue, 14 October 2003 14:53
		Re: Finding if a structure tag name is defined? By: David Fanning on Tue, 14 October 2003 09:55
		Re: Finding if a structure tag name is defined? By: R.Bauer on Tue, 14 October 2003 09:34
		Re: Finding if a structure tag name is defined? By: David Fanning on Tue, 14 October 2003 07:32

Previous Topic:	Re: What does an optimal scientific programming language/environment need?
Next Topic:	efficient polynomial fitting

-=] Back to Top [=-

[ Syndicate this forum (XML) ] [

] [

]

Current Time: Wed Oct 08 17:04:08 PDT 2025

Total time taken to generate the page: 0.00393 seconds