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Re: Cyclic array interfaces [message #40198 is a reply to message #40193]

Tue, 13 July 2004 05:59

cedricl
Messages: 8
Registered: July 2004

Junior Member

Since you're aiming for optimization, I might as well point out an
obvious one:

> A=[[1,2,3],[4,5,6]]
> B=[7,8,9]
> A[*,1]=B

Use A[0, 1] = B instead, since the elements are contiguous in the
first dimension. You might also want to take a loot at "temporary" if
you haven't already.

> (e.g. Does IDL actually rewrite the entire array, regardless of how many
> elements are being changed,

No.

> or does it only change the particular elements
> and hence the first example should be about twice as fast as the second)?

It doesn't work that way, like the previous poster has shown. Your
performance mostly depends on the type of access that you're doing. If
you're accessing memory in a continuous fashion (the first dimension
varies), it's almost sure IMO that it's going to be faster.
Furthermore, consider the next example:

a = fltarr(5)
; Option 1: Create a new array
a = [3, 4, 5, 6, 7]
; Option 2: Rewrite the array
a[0] = [3, 4, 5, 6, 7]

Option 1 and 2 are equivalent, code-wise, but I would argue (and of
course, profile, were I not lazy) that option 2 is faster, because it
doesn't have to deallocate the array that was already contained in a,
and allocate a new one. Of course, there are many other optimizations
that IDL could do that would make 1 faster than 2, but considering
RSI's approach to compile-time optimizations, I'm confident in my
analysis.

Cedric

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[Message index]

		Re: Cyclic array interfaces By: Mark Hadfield on Tue, 13 July 2004 19:53
		Re: Cyclic array interfaces By: cedricl on Tue, 13 July 2004 18:48
		Re: Cyclic array interfaces By: Mark Hadfield on Tue, 13 July 2004 15:29
		Re: Cyclic array interfaces By: cedricl on Tue, 13 July 2004 05:59
		Re: Cyclic array interfaces By: David Fanning on Mon, 12 July 2004 17:50

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