| Re: FOR loops removal [message #62023 is a reply to message #61975] |
Thu, 21 August 2008 06:39   |
Jeremy Bailin
Messages: 618
Registered: April 2008
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Senior Member |
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On Aug 21, 3:59 am, loebasboy <stijn....@gmail.com> wrote:
> So I tested the new finetuned program on the standard image and
> instead of a calculated 15 hour time profit it has become almost 20,5
> hour time profit. The program takes now 2.15 hours instead of 22.5
> hours. That is a major improvement (< 10x), so thanks for all the info
> allready. So I started out with even more improvements, I haven't
> found any vectorisation possibilities yet though. I tried to fasten
> the following code:
>
> n = 20
> size = 2*n+1
> array = randomn(seed, size)
> array[0] = 0
> array[5] = 0
> array[10] = 0
> array[20] = 0
> array[size-2] = 0
> array[size-1] = 0
>
> FOR x = 1, size-2 DO BEGIN
> IF (array[x] EQ 0) THEN BEGIN
> IF ((array[x-1] LE 2) AND (array[x+1] LE 2)) THEN
> BEGIN
> array[x] = 2
> ENDIF ELSE BEGIN
> IF ((array[x-1] GE 2) AND (array[x+1] GE 2)) THEN
> BEGIN
> array[x] = -2
> ENDIF
> ENDELSE
> ENDIF
> ENDFOR
>
> So I figured that if i use the WHERE function to find where the array
> equals 0, and then use a FOR loop that only goes trough the indices
> that the WHERE function has found. So If you consider the WHERE
> function to be much faster than the FOR loop, you could expect that
> the second FOR loop would be faster or equally fast than the first FOR
> loop. The code for the second FOR loop goes like this (some other
> extra IF functions are needed for special cases like a zero as a first
> element, last element or no zero at all):
>
> zeroindex = where (array EQ 0,m)
> IF (zeroindex[0] NE -1) THEN BEGIN
> IF (zeroindex[0] EQ 0) THEN k = 1 ELSE k = 0
> IF (zeroindex[m-1] EQ size-1) THEN l = 2 ELSE l = 1
> FOR i= k, m-l DO BEGIN
> IF ((array[zeroindex[i]-1] LE 2) AND (array[zeroindex[i]+1]
> LE 2)) THEN BEGIN
> array[zeroindex[i]] = 2
> ENDIF ELSE BEGIN
> IF ((array[zeroindex[i]-1] GE 2) AND (array[zeroindex[i]
> +1] GE 2)) THEN BEGIN
> array[zeroindex[i]] = -2
> ENDIF
> ENDELSE
> ENDFOR
>
> you could hear me coming from afar ofcourse ;) . The second FOR loop
> doesn't go faster, at all, with the variables set as above and the two
> loops repeated for 50000 times. The first loop takes 0.304 s and the
> second one 0.337 s. Only if the n-value is made larger than 25 the
> second loop starts to go faster. I checked out profiler to check if
> the WHERE function makes up for this slowing down this bit of
> programming and ofcourse it does, the difference in time is 0.033s
> while the WHERE function takes up 0.066s. So the second loop goes
> faster but the use of the WHERE function slows the whole program down.
> This is some nice checking out ofcourse but it doesn't help me getting
> any further. Is there a faster alternative of the WHERE function? Or
> did I reach the limit in finetuning here? :)
The solution, of course, is to also replace the inner FOR loops with
WHEREs. :-)=
zeroindex = where(array[1:size-2] eq 0, nzero)
if nzero gt 0 then begin
smallneighbours = where(array[zeroindex-1] le 2 and array[zeroindex
+1] le 2, nsmall)
if nsmall gt 0 then array[zeroindex[smallneighbours]] = 2
bigneighbours = where(array[zeroindex-1] le 2 and array[zeroindex+1]
le 2, nbig)
if nbig gt 0 then array[zeroindex[bigneighbours]] = -2
endif
-Jeremy.
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