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Re: Structures [message #42916 is a reply to message #23873]

Tue, 01 March 2005 16:40

Michael Wallace
Messages: 409
Registered: December 2003

Senior Member

Marshall Perrin wrote:
> Michael Wallace <mwallace.no.spam@no.spam.swri.edu.invalid> wrote:
>
>> I learned all this the hard way by trying to figure out why I kept
>> getting errors when I'd attempt to use a different array size for some
>> of my structure variables. I finally ran across this in the IDL
>> documentation, but there wasn't any actual reason given for such
>> draconian policy, especially when compared to the rest of the language.
>
>
> I *think* it's something along these lines: We want to be able to
> create arrays of structs, which (from a language designer's
> viewpoint!) is much, much easier when all the structs are the same
> size. If you let array sizes inside of structs vary, then you might
> have foo1.somearray with 10 elements, and foo2.somearray with 100.
> Then when the user does bar = [foo1,foo2], suddenly you're dealing
> with an array of heterogenous data types and memory access becomes a
> whole lot uglier and more inefficient. At that point, it's probably
> easiest just to switch to Perl. ;-)
>

I can understand their decision if it had to do with memory access.
With static sizes you can easily index into an array of structures
rather that having to dynamically determine where the data is in memory.
Actually it makes sense considering that IDL is array based.

> You can also create named structures containing pointers to arrays, too.
> This very problem is what finally pushed me over the potential barrier
> into learning how to do pointers in IDL!

Seems like I'm going down the same road you went. That little thought
about needing to use pointers in my structures has been trundling
through my mind the last couple days. :-)

-Mike

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[Message index]

		Structures: By: Chris Bull on Wed, 21 February 2001 11:15
		Re: Structures By: marc schellens[1] on Wed, 02 March 2005 19:14
		Re: Structures By: David Fanning on Wed, 02 March 2005 09:36
		Re: Structures By: Benjamin Hornberger on Wed, 02 March 2005 09:33
		Re: Structures By: Paul Van Delst[1] on Wed, 02 March 2005 09:31
		Re: Structures By: Michael Wallace on Wed, 02 March 2005 09:04
		Re: Structures By: Michael Wallace on Wed, 02 March 2005 08:59
		Re: Structures By: Paul Van Delst[1] on Wed, 02 March 2005 07:33
		Re: Structures By: marc schellens[1] on Tue, 01 March 2005 19:43
		Re: Structures By: David Fanning on Tue, 01 March 2005 16:49
		Re: Structures By: Michael Wallace on Tue, 01 March 2005 16:40
		Re: Structures By: mperrin+news on Tue, 01 March 2005 16:21
		Re: Structures By: Paul Van Delst[1] on Tue, 01 March 2005 14:39
		Re: Structures By: Michael Wallace on Tue, 01 March 2005 14:38
		Re: Structures By: David Fanning on Tue, 01 March 2005 13:52

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