comp.lang.idl-pvwave archive: archive

Home » Public Forums » archive » Re: counting bits

Show: Today's Messages :: Show Polls :: Message Navigator
E-mail to friend

Re: counting bits [message #34069]

Wed, 19 February 2003 21:31

Craig Markwardt
Messages: 1869
Registered: November 1996

Senior Member

JD Smith <jdsmith@as.arizona.edu> writes:
>
> Here's a reasonably fast implementation of your proposed method for
> arrays of unsigned longs, to count the highest bit (or rather, the
> number of leading 0 bits).
>
> function leading_zeroes_reg,num
> num=[num]
> zeroes=make_array(/BYTE,DIMENSION=size(num,/DIMENSIONS),VALU E=255b)
> for i=0,31 do begin
> shft=ishft(num,-(31-i)) AND 1
> zeroes=(zeroes ne 255b)*zeroes+(zeroes eq 255b)* $
> ((shft eq 1)*i+(shft ne 1)*255b)
> endfor
> return, (zeroes eq 255b)*32+(zeroes ne 255b)*zeroes
> end

Thanks. Thinking about it further, one could probably do a pretty fast look up
table on a byte-by-byte basis, similar to the counting-bits lookup
that you presented initially.

Craig

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

Report message to a moderator

[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

Previous Topic:	running out of memory! can all memory be restored in idl?
Next Topic:	Re: howto bind shortcut key to button

-=] Back to Top [=-

[ Syndicate this forum (XML) ] [

] [

]

Current Time: Fri Oct 10 14:26:19 PDT 2025

Total time taken to generate the page: 1.82290 seconds