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| Re: Is it really more efficient to work with arrays than FOR loops? [message #51513 is a reply to message #51493] |
Thu, 23 November 2006 09:13  |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Alvin writes:
> I was wondering if it is really that more efficient to work with arrays
> (large ones that is). For example I have the following simple code,
> which takes about 30 min to run:
>
> FOR z=0L, 400 DO BEGIN
> FOR y=0L, 400 DO BEGIN
> FOR x=0L, 400 DO BEGIN
> fn=f(z) ;a function of z
> gn=f(z) ;another function of z
> IF ( f(z) * x + g(z) * y GE f(z) * g(z) ) THEN BEGIN
> blah
> blah
> blah
> ENDIF
> ENDFOR
> ENDFOR
> ENDFOR
>
> Now if I tried to vectorize the above, would it do me any good in
> saving time and possibly memory? If I say something like f(z) # x +
> g(z) # y, where these are all vectors, I have a feeling that I am not
> covering all the possible combinations as the FOR loop above. Does
> anyone have any ideas, or suggestions?
http://www.dfanning.com/code_tips/slowloops.html
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Sepore ma de ni thui. ("Perhaps thou speakest truth.")
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| Re: Is it really more efficient to work with arrays than FOR loops? [message #51514 is a reply to message #51513] |
Thu, 23 November 2006 08:40 |
Jean H.
Messages: 472
Registered: July 2006
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Senior Member |
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Alvin wrote:
> Hey all,
> I was wondering if it is really that more efficient to work with arrays
> (large ones that is). For example I have the following simple code,
> which takes about 30 min to run:
>
> FOR z=0L, 400 DO BEGIN
> FOR y=0L, 400 DO BEGIN
> FOR x=0L, 400 DO BEGIN
> fn=f(z) ;a function of z
> gn=f(z) ;another function of z
> IF ( f(z) * x + g(z) * y GE f(z) * g(z) ) THEN BEGIN
> blah
> blah
> blah
> ENDIF
> ENDFOR
> ENDFOR
> ENDFOR
>
> Now if I tried to vectorize the above, would it do me any good in
> saving time and possibly memory?
first, in you example, compute gz and fz only once, in the Z loop (not
in the Y nor X loop!). THen use gz and fz instead of g(z) and f(z) AGAIN
in the X loop... compute fz*gz in the Z loop, compute gz*y in the Y
loop... there is no reason for computing this more than once!
Now YES, it is usually much faster to vectorize your code... especially
with small arrays. I have some functions that are several hundreds time
faster once vectorized with *small* arrays (few hundred thousands) but
the same function becoms dozen of time slower with large arrays (few
dozen of millions).
Memory can become an issue too... with arrays of size 400 it should not
be a problem... apply Paolo's suggestion. However, again, if you have
several millions entries, it might be just too much to do the
multiplication of the arrays..
x=findgen(n)#replicate(1,n)
ind=where( x*a+y*b GE c)
Jean
If I say something like f(z) # x +
> g(z) # y, where these are all vectors, I have a feeling that I am not
> covering all the possible combinations as the FOR loop above. Does
> anyone have any ideas, or suggestions?
> Thanks,
> Alvin.
>
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| Re: Is it really more efficient to work with arrays than FOR loops? [message #51518 is a reply to message #51514] |
Thu, 23 November 2006 07:21 |
Paolo Grigis
Messages: 171
Registered: December 2003
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Senior Member |
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Well, you might want to express x and y as 2-dimensional arrays,
something like
n=400
x=findgen(n)#replicate(1,n)
y=transpose(x)
then you can write condition in the form
ind=where( x*a+y*b GE c)
and compute whatever you want to compute for
x[ind] and y[ind]...
Ciao,
Paolo
Alvin wrote:
> Hey all,
> I was wondering if it is really that more efficient to work with arrays
> (large ones that is). For example I have the following simple code,
> which takes about 30 min to run:
>
> FOR z=0L, 400 DO BEGIN
> FOR y=0L, 400 DO BEGIN
> FOR x=0L, 400 DO BEGIN
> fn=f(z) ;a function of z
> gn=f(z) ;another function of z
> IF ( f(z) * x + g(z) * y GE f(z) * g(z) ) THEN BEGIN
> blah
> blah
> blah
> ENDIF
> ENDFOR
> ENDFOR
> ENDFOR
>
> Now if I tried to vectorize the above, would it do me any good in
> saving time and possibly memory? If I say something like f(z) # x +
> g(z) # y, where these are all vectors, I have a feeling that I am not
> covering all the possible combinations as the FOR loop above. Does
> anyone have any ideas, or suggestions?
> Thanks,
> Alvin.
>
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