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Re: counting bits [message #34103 is a reply to message #34102]

Mon, 17 February 2003 20:45

Craig Markwardt
Messages: 1869
Registered: November 1996

Senior Member

JD Smith <jdsmith@as.arizona.edu> writes:
>
> For a 2048x2048 array of random long integers, this is 4-6 times as
> fast as a more traditional shift method, like the one David suggests.
> And here's an ultra-bizzarre one, which requires no look-up table, but
> holds its own against the LUT method (note the original array is
> destroyed):
>
> arr = ishft(arr AND 'AAAAAAAA'XUL,-1) + (arr AND '55555555'XUL)
> arr = ishft(arr AND 'CCCCCCCC'XUL,-2) + (arr AND '33333333'XUL)
> arr = ishft(arr AND 'F0F0F0F0'XUL,-4) + (arr AND '0F0F0F0F'XUL)
> arr = ishft(arr AND 'FF00FF00'XUL,-8) + (arr AND '00FF00FF'XUL)
> arr = ishft(arr AND 'FFFF0000'XUL,-16) + (arr AND '0000FFFF'XUL)
> tot=ulong(total(arr))
>
> See if you can figure that one out ;).

Very wicked!

Okay, here's a problem I've always solved by the brute force method:

* what is the lowest / highest bit position set in an integer?

For example, a the lowest bit position in the binary number 11010100
is 2 (bit positions are labeled starting with 0 of course). The brute
force method involves testing each bit in succession using something
like (VALUE AND 2L^I) for each I, until a set bit is found.

Craig

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Craig B. Markwardt, Ph.D. EMAIL: craigmnet@cow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
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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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