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Re: counting bits [message #34191 is a reply to message #34107]

Thu, 20 February 2003 14:53

Dick Jackson
Messages: 347
Registered: August 1998

Senior Member

"JD Smith" <jdsmith@as.arizona.edu> wrote in message
news:pan.2003.02.20.15.43.26.137656.2731@as.arizona.edu...
> On Wed, 19 Feb 2003 08:47:31 -0700, Dick Jackson wrote:
>
>> IDL> CountingBits
>> IShft-AND-lookup method: 2.063 seconds. tot = 46137328
>> Byte-lookup method: 0.691 seconds. tot = 46137292
>>
>> Uh-oh... I set the Total calls to have /Double and then we both get
the
>> same (I hope correct) answer:
>>
>> IDL> CountingBits
>> IShft-AND-lookup method: 2.063 seconds. tot = 46137344
>> Byte-lookup method: 0.671 seconds. tot = 46137344
>
> That's very strange. Here's what I get for my four independent
> methods without any /DOUBLE:
>
> 3.4187140
> 46137344
> 6.9928349
> 46137344
> 1.2564960
> 46137344
> 1.2767580
> 46137344

In case anyone's still following this exercise, we found that if 'bits'
is a byte array (not integer) my floating-point problem goes away, and
makes it faster! Adding this after 'bits' is defined will do it:

bits = Byte(bits)

... and gives another nice speedup:

IDL> CountingBits
IShft-AND-lookup method: 1.883 seconds.
tot = 46137344
Byte-lookup method: 0.541 seconds.
tot = 46137344

Cheers,
--
-Dick

Dick Jackson / dick@d-jackson.com
D-Jackson Software Consulting / http://www.d-jackson.com
Calgary, Alberta, Canada / +1-403-242-7398 / Fax: 241-7392

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[Message index]

		counting bits By: Joe Foose on Mon, 17 February 2003 11:50
		Re: counting bits By: thompson on Wed, 26 February 2003 12:15
		Re: counting bits By: JD Smith on Wed, 26 February 2003 10:29
		Re: counting bits By: condor on Tue, 25 February 2003 15:17
		Re: counting bits By: Dick Jackson on Thu, 20 February 2003 14:53

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