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FOR loops removal [message #61975]

Tue, 19 August 2008 05:38

loebasboy
Messages: 26
Registered: August 2008

Junior Member

Dear all,

Some weeks ago I've noticed that IDL is rather slow with FOR loops the
hard way...after reading about it and not believing it, offcourse.
Last week I started removing some FOR loops in my code, apart from
some embarasing ones like:

sum = 0
FOR i=0,max_y-1 DO BEGIN
FOR j=0,max-1 DO BEGIN
sum_int = sum_int + data[i,j]
ENDFOR
ENDFOR

which has a straightforward solution. However I have some other FOR
loops which aren't that obvious at all. Like for instance this one:

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

which I could alter, not that straigthforwardly into:

z = size(arr)
arr = reform(in_arr, z[1]*z[2], /overwrite)
endt = (max_y*max_x)-1
FOR l = 0, n*2 DO BEGIN
temp = 0
FOR i=0,endt DO temp = temp + (arr[i+l*max_y] * in_arr [i
+n*max_y])
output[l] = temp/(max_x*max_y)
ENDFOR
in_arr = reform(in_arr, z[1],z[2], /overwrite)

where 1 FOR loop is removed. However there is hardly any time profit
at all. It is even so that the following code is faster than both,
which is a very straightforward alteration of the first loop:

FOR l = 0, n*2 DO BEGIN
temp = 0
FOR i =0,max_y-1 DO FOR j=0,max_x-1 DO temp = temp + (arr[i,j
j + l] * arr [i, j + n])
output[l] = temp/(max_x*max_y)
ENDFOR.

With the following variables set and the for loops repeated with
another FOR loop of i= 0,10000 (to see a time difference, and in the
full program it is repeated about that many times too, but with even
larger arrays):

n = 8
max_x = 5
max_y = 5
output = fltarr(2*n+1)
arr = findgen(interval_y, region) +1

I have for the first for loop : 1.6279998 s
the second : 1.6060002 s
the third : 1.2749999 s

I measured the times with SYSTIME, /SECONDS command.

(the full program takes 22,5 h for a standard image, with the
alterations I have allready done, I've came up with 18.1 h, which is
still 17 h to go to make it workable, i've used the last loop in the
above example so far...)

Can anybody tell me why removing one loop doesn't help in this case or
what i'm doing wrong?

thnx
Stijn Van der Linden

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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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