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Re: counting bits [message #34214 is a reply to message #34080]

Thu, 20 February 2003 07:43

JD Smith
Messages: 850
Registered: December 1999

Senior Member

On Wed, 19 Feb 2003 08:47:31 -0700, Dick Jackson wrote:

> [sorry for the late entry, it seems my news server has been failing when
> I attach a file to a posting :-(]
>
> "JD Smith" <jdsmith@as.arizona.edu> wrote in message
> news:pan.2003.02.17.23.54.23.693563.14101@as.arizona.edu...
>
>> The method I found fastest
>> (among those I've tried), is a pretty silly and straightforward one,
>> namely table lookup:
>
> I don't think it's silly at all, but taking it one step further will
> really speed it up. If you have enough extra memory for it, just convert
> the whole ULong array to bytes, do one lookup and you're done. One more
> 'gotcha' came up when running this:
>

Good idea, and fast indeed.

> Data:
>
> rand_arr = ULIndgen(2048, 2048)
>
> JD's method:
>
> tot=ulong(total(bits[rand_arr AND 'FF'XUL] + $
> bits[ishft(rand_arr,-8) AND 'FF'XUL]+ $
> bits[ishft(rand_arr,-16) AND 'FF'XUL]+ $
> bits[ishft(rand_arr,-24) AND 'FF'XUL]))
>
> Dick's method:
>
> byte_rand_arr = Byte(rand_arr, 0, N_Elements(rand_arr)*4) tot =
> ULong(Total(bits[byte_rand_arr]))
>
> When I tried this, I got:
>
> 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

Indeed:

IDL> print,ulong(total(bits[rand_arr AND 'FF'XUL] + $
IDL> bits[ishft(rand_arr,-8) AND 'FF'XUL]+ $
IDL> bits[ishft(rand_arr,-16) AND 'FF'XUL]+ $
IDL> bits[ishft(rand_arr,-24) AND 'FF'XUL], /Double))
46137344
IDL> print,ulong(total(bits[rand_arr AND 'FF'XUL] + $
IDL> bits[ishft(rand_arr,-8) AND 'FF'XUL]+ $
IDL> bits[ishft(rand_arr,-16) AND 'FF'XUL]+ $
IDL> bits[ishft(rand_arr,-24) AND 'FF'XUL]))
46137344

I'm not sure what the difference could be (other than Win vs. Linux).

One thing I did notice when creating "random" arrays:

IDL> print,FORMAT='(F5.2,A)',total(ulong(randomu(sd,100)*2.^31) mod 2 eq 1),$
'% odd'

Try this a few times. That lowest bit just does not get set. Some
floating-point representation expert must have an explanation.

> My kingdom for a /Long flag on Total()!

I'll throw mine in also.

JD

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[Message index]

		Re: counting bits By: Craig Markwardt on Wed, 19 February 2003 21:31
		Re: counting bits By: Dick Jackson on Wed, 19 February 2003 07:47
		Re: counting bits By: Craig Markwardt on Tue, 18 February 2003 15:10
		Re: counting bits By: JD Smith on Tue, 18 February 2003 14:58
		Re: counting bits By: David Fanning on Tue, 18 February 2003 13:55
		Re: counting bits By: Rick Towler on Tue, 18 February 2003 13:27
		Re: counting bits By: JD Smith on Tue, 18 February 2003 11:56
		Re: counting bits By: eddie haskell on Tue, 18 February 2003 08:10
		Re: counting bits By: David Fanning on Mon, 17 February 2003 21:09
		Re: counting bits By: Craig Markwardt on Mon, 17 February 2003 20:45
		Re: counting bits By: JD Smith on Mon, 17 February 2003 15:54
		Re: counting bits By: David Fanning on Mon, 17 February 2003 12:41
		Re: counting bits By: David Fanning on Mon, 17 February 2003 12:38
		Re: counting bits By: JD Smith on Thu, 20 February 2003 07:43
		Re: counting bits By: JD Smith on Thu, 20 February 2003 07:24

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