| Re: strange behaviour of bytscl by large arrays [message #80046 is a reply to message #79957] |
Mon, 23 April 2012 13:22   |
chris_torrence@NOSPAM
Messages: 528
Registered: March 2007
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Senior Member |
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On Monday, April 23, 2012 10:14:21 AM UTC-6, fawltyl...@gmail.com wrote:
>
> I think IDL's FINDGEN() implementation is wrong: it uses a float counter instead of an integer one. The following test shows the difference:
>
> pro test
> cpu, tpool_nthreads=1
> n=10l^8
> nn=n-1
> a1=findgen(n) ; real FINDGEN()
> a2=fltarr(n)
> count=0.0
> for j=0l, nn do a2[j]=count++ ; IDL's implementation
> a3=fltarr(n)
> count=0ll
> for j=0l, nn do a3[j]=count++ ; better implementation
> print, a1[nn], a2[nn], a3[nn], format='(3F15.3)'
> end
>
> (Multithreading must be disabled because the starting values for the threads are calculated as an integer. So the result of FINDGEN() depends on the number of your CPU cores, too :-)
>
> regards,
> Lajos
Well, wrong is perhaps too strong of a word. The real word is "fast". I just did a test where I changed the internal implementation of FINDGEN to use an integer counter. The "float" counter is 4 times faster than using an integer counter and converting it to floats.
However, perhaps we could look at the size of the input array, and switch to using the slower integer counter if it was absolutely necessary. I'll give it a thought.
Thanks for reporting this!
Cheers,
Chris
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