| Re: FOR loops removal [message #62021 is a reply to message #61975] |
Thu, 21 August 2008 07:27   |
loebasboy
Messages: 26
Registered: August 2008
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Junior Member |
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On Aug 21, 3:39 pm, Jeremy Bailin <astroco...@gmail.com> wrote:
> On Aug 21, 3:59 am, loebasboy <stijn....@gmail.com> wrote:
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>
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>> 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.- Hide quoted text -
>
> - Show quoted text -
Good code, there was a +1 needed in the zeroindex declaration though.
It doesn't go any faster also, too bad, I guess that the use of the
WHERE function doesn't speed up. But thank you for the suggestion !
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