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Re: vector of bin indices using histogram? [message #50777 is a reply to message #50775]

Wed, 18 October 2006 08:33

Foldy Lajos
Messages: 268
Registered: October 2001

Senior Member

On Wed, 18 Oct 2006, David Fanning wrote:

> Paolo Grigis writes:
>
>> You might also want to change the computation of b with
>> a division into a multiplication by the reciprocal (see
>> example below).
>>
>> x=fltarr(5d7)
>>
>> t=systime(/seconds)
>> y=x/2.
>> print,systime(/seconds)-t
>> 0.82191920
>>
>> t=systime(/seconds)
>> y=x*(1./2)
>> print,systime(/seconds)-t
>> 0.33465910
>
> Well, that's interesting. Do you have a theory about this? :-)
>

CPU latency. Eg. for Pentium 4, the latency is 7 clock cycles for
FMUL, and 43 for FDIV (this is worst case, depends on the data, and
assumes that the data is in the L1 cache). Decent compilers (including
FL :-) replace division by float const by multiplication.

regards,
lajos

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[Message index]

		Re: vector of bin indices using histogram? By: Paolo Grigis on Thu, 19 October 2006 00:45
		Re: vector of bin indices using histogram? By: greg michael on Wed, 18 October 2006 14:42
		Re: vector of bin indices using histogram? By: greg michael on Wed, 18 October 2006 14:07
		Re: vector of bin indices using histogram? By: Foldy Lajos on Wed, 18 October 2006 11:12
		Re: vector of bin indices using histogram? By: C. E. Ordonez on Wed, 18 October 2006 10:57
		Re: vector of bin indices using histogram? By: Jean H. on Wed, 18 October 2006 10:09
		Re: vector of bin indices using histogram? By: Paolo Grigis on Wed, 18 October 2006 09:56
		Re: vector of bin indices using histogram? By: Paolo Grigis on Wed, 18 October 2006 09:27
		Re: vector of bin indices using histogram? By: Foldy Lajos on Wed, 18 October 2006 09:25
		Re: vector of bin indices using histogram? By: David Fanning on Wed, 18 October 2006 09:06
		Re: vector of bin indices using histogram? By: Foldy Lajos on Wed, 18 October 2006 08:53
		Re: vector of bin indices using histogram? By: greg michael on Wed, 18 October 2006 08:38
		Re: vector of bin indices using histogram? By: Foldy Lajos on Wed, 18 October 2006 08:33
		Re: vector of bin indices using histogram? By: David Fanning on Wed, 18 October 2006 08:08
		Re: vector of bin indices using histogram? By: Paolo Grigis on Wed, 18 October 2006 07:47
		Re: vector of bin indices using histogram? By: greg michael on Wed, 18 October 2006 07:00
		Re: vector of bin indices using histogram? By: btt on Wed, 18 October 2006 06:19

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