| Re: array multiplying (for a change) [message #38025 is a reply to message #38022] |
Tue, 17 February 2004 08:21   |
Craig Markwardt
Messages: 1869
Registered: November 1996
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Senior Member |
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"Christopher Lee" <cl@127.0.0.1> writes:
> What I want is result = a * b'
> where b' = rebin(reform(b, [1,20,1]), 10,20,30)
> , which (clearly :) I know how to do in principle.
...
> Are there any functions, buit-in or otherwise, that I can use? I found
> CMAPPLY, which I can beat into a form which works. (I use a similar
> function now but it's very _VERY_ bad code).
>
> A quick test using loops versus rebin/reform of the shows loops to be
> slower (for a matrix 72,36,31,200) which I'm not really surprised by. Is
> this a case where a DLM would be faster?
My philosophy is that DLMs are almost always bad, unless you are
developing an embedded system. They tie you to a particular version
of IDL and a particular OS and architecture. They are rather difficult
to debug, and making changes is rather laborious. DLMs = bleccchhh.
With that out of my system, I think that a slab-oriented multiply
would probably do okay. By "slab oriented" I mean to expand B in a
few but not all dimensions, so essentially this will be a hybrid
between REBIN/REFORM and FOR-loop.
Example:
bp = rebin(reform(b, [1,20,1]), 10,20,1)
result = a
for i = 0, 29 do result(*,*,i) = a(*,*,i) * bp
Since that last dimension of 20 is not a part of BP, it doesn't take
up nearly as much memory, but the number of loop iterations is also
rather small. This notation is compact enough that I typically do not
make a wrapper procedure.
You can save even more execution time by using the IDL "trick" of
specifying only the start index for the destination array:
for i = 0, 29 do result(0,0,i) = a(*,*,i) * bp
Good luck!
Craig
--
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Craig B. Markwardt, Ph.D. EMAIL: craigmnet@REMOVEcow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
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