| Re: vectorization challenge! (help!) [message #54907 is a reply to message #54844] |
Thu, 19 July 2007 04:53   |
mattf
Messages: 13
Registered: January 2007
|
Junior Member |
|
|
On Jul 17, 12:52 pm, Conor <cmanc...@gmail.com> wrote:
> 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...)
There are a number of vectorization strategies-- one is heavy use of
the WHERE() function. Given a condition C for an action on elements
some vector A, you can set A[WHERE(C)] to whatever you want.
Another trick is to treat a matrix as a (long) one dimensional vector
and then make use of modular arithmetic. But be careful when you go
back to matrix-land with the REFORM() function-- chances are you will
get rows and columns backwards.
A third vectorization trick is to exchange a FOR loop for N copies of
your variables all concatenated together, and then doing the N
calculations with matrix arithmetic. This is a last-ditch
vectorization strategy that depends on exchanging the processing time
in a FOR loop for a no-branching calculation that wastes a lot of
memory
|
|
|
|
comp.lang.idl-pvwave archive
Members
Search
Help
Login
Home
