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vectorization challenge! (help!) [message #54844]

Tue, 17 July 2007 09:52

Conor
Messages: 138
Registered: February 2007

Senior Member

I'm 'vectorizing' a piece of code to speed it up. It's part of a
larger program. One of the sections is turning out to be very
difficult to vectorize (am I using that word right? What I mean is
I'm trying to get rid of for loops). Anyway, maybe someone has some
thoughts on how to vectorize it. Maybe it's just not worth it for
this section. Here's the basic idea, filled in with dummy data:

n = 24
npeeps = 50
gn1 = findgen(n,npeeps)
gn2 = findgen(n,npeeps)
cutoff = .01

for i=0,npeeps-1 do begin

; make a random value to determine if we do anything with this
row
if randomu(seed,1) lt cutoff then begin

; this row has been selected. Swap the last (random number)
of digits in gn1[*,i] with gn2[*,i]
randindex = long(randomu(seed,1)*n*nd)
temp = gn1[randindex:*,i]
gn1[randindex:*,i] = gn2[randindex:*,i]
gn2[randindex:*,i] = temp

endif

endfor

That's it. For randomly selected rows, swap a random number of
elements at the end of the row with another array. It is surpisingly
difficult to get rid of that for loop. Maybe I'm just a bit out of it
today though. I thought of generating a list of indexes to be
swapped, but I can't quite figure it out. Oh, if only IDL allowed the
syntax: arr[st:ed] where st and ed are arrays themselves! Then this
would be really easy (something like this would do it):

st = long(randomu(seed,npeeps)*n*nd)
ed = make_array(npeeps,/integer,value=n)
indlist = indgen(n)
inds = indlist[st:ed] + indgen(npeeps)*n
temp = gn1[inds]
gn1[inds] = gn2[inds]
gn2[inds] = temp

Alas, indlist[st:ed] isn't allowed! (Also, indgen(npeeps)*n has the
wrong dimensions anyway...)

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[Message index]

		vectorization challenge! (help!) By: Conor on Tue, 17 July 2007 09:52
		Re: vectorization challenge! (help!) By: MarioIncandenza on Thu, 19 July 2007 10:56
		Re: vectorization challenge! (help!) By: alvin[1] on Thu, 19 July 2007 09:40
		Re: vectorization challenge! (help!) By: Brian Larsen on Thu, 19 July 2007 06:33
		Re: vectorization challenge! (help!) By: mattf on Thu, 19 July 2007 04:53
		Re: vectorization challenge! (help!) By: Maarten[1] on Thu, 19 July 2007 01:35
		Re: vectorization challenge! (help!) By: alvin[1] on Thu, 19 July 2007 01:01
		Re: vectorization challenge! (help!) By: Brian Larsen on Wed, 18 July 2007 13:37

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