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Re: FOR loops removal [message #62052 is a reply to message #61975]

Wed, 20 August 2008 05:36

loebasboy
Messages: 26
Registered: August 2008

Junior Member

On 20 aug, 13:50, Jeremy Bailin <astroco...@gmail.com> wrote:
> On Aug 19, 9:43 am, Wox <nom...@hotmail.com> wrote:
>
>
>
>
>
>> On Tue, 19 Aug 2008 05:38:50 -0700 (PDT), loebasboy
>
>> <stijn....@gmail.com> wrote:
>>> FOR l = 0, n*2 DO BEGIN
>>> temp = 0
>>> FOR i =0,max_y-1 DO BEGIN
>>> FOR j=0,max_x-1 DO BEGIN
>>> jtemp = j + l
>>> jtemp2 = j + n
>>> temp = temp + (arr[i,jtemp] * arr [i,jtemp2])
>>> ENDFOR
>>> ENDFOR
>>> output[l] = temp/(max_x*max_y)
>>> ENDFOR
>
>> The code below is a start. Does this processing have a name? It feels
>> familiar somehow. Btw, in IDL the first index of an array is the
>> column and the second is the row. So in your case y are the columns
>> and x are the rows. No problem with that off course, just check
>> whether this is how you intended it.
>
>> n = 8
>> max_x = 5
>> max_y = 5
>> output = fltarr(2*n+1)
>> arr = findgen(max_y, 2*n+max_x) +1
>
>> arr2=arr[0:max_y-1,n:max_x-1+n]
>> FOR l = 0, 2*n DO $
>> output[l] = total(arr[0:max_y-1,l:max_x-1+l]*arr2)
>> output/=max_x*max_y
>
> Following on that last version, I think we can *completely* get rid of
> the loop... though at the expense (as usual) of memory:
>
> n = 8
> max_x = 5
> max_y = 5
> arr = findgen(max_y, 2*n+max_x) +1
>
> max_area = max_x*max_y
> output = total( arr[rebin(lindgen(max_area),max_area,2*n+1) +
> max_y*rebin(reform(lindgen(2*n+1),1,2*n+1),max_area,2*n+1)] *
> rebin( (arr[*,n:max_x-1+n])[*], max_area,2*n+1), 1) / max_area
>
> Whether that's actually faster will depend on how big max_x, max_y and
> n are, of course... it ends up internally storing a couple of
> max_x*max_y*(2*n+1) arrays, so if that is going to take you into swap
> then you're best off sticking with Wox's version. If that stays in
> physical memory, though, I bet this will win.
>
> -Jeremy.- Tekst uit oorspronkelijk bericht niet weergeven -
>
> - Tekst uit oorspronkelijk bericht weergeven -

Great code, that is another hour of time profit again, and it works.
So vectorization comes down to instead of repeating an action per
element in matrix, putting all elements on the right spot in a matrix
and doing the action on the matrix, right? The only problem is then,
finding out when this is possible, or when it isn't ;).

thank you all, for all the great info allready!

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[Message index]

		FOR loops removal By: loebasboy on Tue, 19 August 2008 05:38
		Re: FOR loops removal By: loebasboy on Thu, 21 August 2008 07:27
		Re: FOR loops removal By: Jeremy Bailin on Thu, 21 August 2008 08:40
		Re: FOR loops removal By: Jeremy Bailin on Thu, 21 August 2008 06:39
		Re: FOR loops removal By: loebasboy on Thu, 21 August 2008 00:59
		Re: FOR loops removal By: Jeremy Bailin on Wed, 20 August 2008 07:07
		Re: FOR loops removal By: loebasboy on Wed, 20 August 2008 05:36
		Re: FOR loops removal By: David Fanning on Tue, 19 August 2008 13:14
		Re: FOR loops removal By: loebasboy on Mon, 25 August 2008 01:31
		Re: FOR loops removal By: Chris[6] on Fri, 22 August 2008 11:19
		Re: FOR loops removal By: ben.bighair on Fri, 22 August 2008 03:50
		Re: FOR loops removal By: loebasboy on Fri, 22 August 2008 02:08
		Re: FOR loops removal By: Chris[6] on Thu, 21 August 2008 11:34

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