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Re: Arrays of Structures [message #52447 is a reply to message #20472]

Thu, 08 February 2007 12:42

JD Smith
Messages: 850
Registered: December 1999

Senior Member

On Thu, 08 Feb 2007 14:02:08 -0500, Paul van Delst wrote:

> Mick Brooks wrote:

> You know, I'm confused now too. Check out the precedence in the IDL help:

> So, you see that the the structure field dereference operator, ".", and the array
> subscript operator, "[]" , have the same precedence. Operators with equal precedence are
> evaluated from left to right.
>
> So,
>
> structs.a[23]
>
> means FIRST dereference the structure field (structs.a), THEN index the array ([23]). The
> way I see it,
>
> structs.a[23]
> and
> (structs.a)[23]
>
> should be equivalent.

The problem is that structs.a is not an array until the ".a" part has been
applied. What if structs had been;

structs={a:indgen(25)}

what should structs.a[23] do then, and how should IDL know the difference.
Until IDL knows what shape "structs.a" will have, it cannot make any
informed decision about how to index it. You might object, saying that
IDL should just evaluate that to begin with always, but think how
expensive that is. Creating "structs.a" would cause a large temporary
array to be created, only to finally index a single element. Compare the
memory usage of the following:

IDL> struct=replicate({a:lindgen(100,100,100)},100)
IDL> print,(m=memory(/HIGHWATER))/1024/1024.,"MB"
382.472MB
IDL> val=struct[10].a[4]
IDL> print,(memory(/HIGHWATER)-m)/1024/1024.,"MB extra"
0.00000MB extra
IDL> val2=(struct.a)[10,4]
IDL> print,(memory(/HIGHWATER)-m)/1024/1024.,"MB extra"
381.470MB extra

So the latter method first creates a temporary variable of size
100,100,100,100, and then pulls a single element out of it. Not
exactly good form.

JD

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[Message index]

		Arrays of Structures By: Ben Tupper on Wed, 21 June 2000 00:00
		Re: Arrays of Structures By: news.qwest.net on Thu, 08 February 2007 12:52
		Re: Arrays of Structures By: JD Smith on Thu, 08 February 2007 12:42
		Re: Arrays of Structures By: Paul Van Delst[1] on Fri, 09 February 2007 08:01
		Re: Arrays of Structures By: Mick Brooks on Fri, 09 February 2007 02:13
		Re: Arrays of Structures By: Michael Galloy on Thu, 08 February 2007 12:21
		Re: Arrays of Structures By: news.qwest.net on Thu, 08 February 2007 11:44
		Re: Arrays of Structures By: Paul Van Delst[1] on Thu, 08 February 2007 11:32
		Re: Arrays of Structures By: Mick Brooks on Thu, 08 February 2007 11:27
		Re: Arrays of Structures By: Brian Larsen on Thu, 08 February 2007 11:11
		Re: Arrays of Structures By: news.qwest.net on Thu, 08 February 2007 11:09
		Re: Arrays of Structures By: Paul Van Delst[1] on Thu, 08 February 2007 11:02
		Re: Arrays of Structures By: Brian Larsen on Thu, 08 February 2007 10:59
		Re: Arrays of Structures By: Mick Brooks on Thu, 08 February 2007 10:04
		Re: Arrays of Structures By: Brian Larsen on Thu, 08 February 2007 09:41
		Re: Arrays of Structures By: JD Smith on Fri, 09 February 2007 12:51

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