comp.lang.idl-pvwave archive: archive » Re: Is it really more efficient to work with arrays than FOR loops?

Home » Public Forums » archive » Re: Is it really more efficient to work with arrays than FOR loops?

Show: Today's Messages :: Show Polls :: Message Navigator
E-mail to friend

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

Senior Member

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.
>

Report message to a moderator

[Message index]

		Re: Is it really more efficient to work with arrays than FOR loops? By: Alvin on Fri, 24 November 2006 02:34
		Re: Is it really more efficient to work with arrays than FOR loops? By: David Fanning on Thu, 23 November 2006 09:13
		Re: Is it really more efficient to work with arrays than FOR loops? By: Jean H. on Thu, 23 November 2006 08:40
		Re: Is it really more efficient to work with arrays than FOR loops? By: Paolo Grigis on Thu, 23 November 2006 07:21

Previous Topic:	Thanks!
Next Topic:	Julian time problem

-=] Back to Top [=-

[ Syndicate this forum (XML) ] [

] [

]

Current Time: Thu Oct 09 23:56:14 PDT 2025

Total time taken to generate the page: 0.08131 seconds