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Re: Vectorizing Code [message #29535 is a reply to message #29531]

Wed, 27 February 2002 16:00

Jeff Guerber
Messages: 41
Registered: July 2000

Member

On Wed, 27 Feb 2002, Steve Jones wrote:

> Is it possible to vectorize a simple double for-loop?
>
> for i=0,nstate-1 do begin
> for j=0,nvec-1 do begin
> sa(i,j)=10.^2*exp(-abs(i-j)*dz/h)
> endfor
> endfor
>
> I tend to write a large number of such loops and my indexes have been
> steadily increasing of late... Is there a faster alternative? Thanks
> in advance

I'm not sure about completely vectorizing this particular case, but two
things that should help:

1) Exchange your i and j loops, since in IDL as in Fortran the leftmost
index varies fastest. (See the current thread titled "IDL2MATLAB".)
2) Move the invariant terms outside the loops:

d=lonarr(nstate,nvec)
for j=0,nvec-1 do begin
for i=0,nstate-1 do begin
d(i,j) = i - j
endfor
endfor
sa = 10.^2*exp(-abs(d)*dz/h)

(Hmmm. You may be able to create (using indgen and replicate) two
appropriate nstate-by-nvec arrays, one for i and one for j, then subtract
those... In this case, I'm not sure that would be faster, though.)

By the way, ^ has higher precedence than *, so I think you may want to
say 10.^(2*exp(...)) [note the extra set of parentheses] instead. What
you wrote is equivalent to 100.*exp(...).

Another tip: If your indices might exceed 32767, be sure to write your
loops "for i=0L, ...", otherwise the index will be an int instead of a
long. That one's bitten me, and it's painful!

Jeff Guerber

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[Message index]

		Re: Vectorizing Code By: marc schellens[1] on Wed, 27 February 2002 22:17
		Re: Vectorizing Code By: Pavel Romashkin on Wed, 27 February 2002 21:43
		Re: Vectorizing Code By: Kenneth P. Bowman on Wed, 27 February 2002 18:56
		Re: Vectorizing Code By: Jeff Guerber on Wed, 27 February 2002 16:00
		Re: Vectorizing Code By: K. Bowman on Thu, 28 February 2002 07:43
		Re: Vectorizing Code By: James Tappin on Thu, 28 February 2002 08:35

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