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Re: fast array comparison [message #33161 is a reply to message #33160]

Sun, 08 December 2002 09:45

David Fanning
Messages: 11724
Registered: August 2001

Senior Member

Sean Raffuse (sean@me.wustl.edu) writes:

> Let's say I have two arrays.
>
> requested_array = [5,6,7,8,9,10]
> available_array = [3,7,8,9,12,13,16]
>
> What is the absolute fastest way to determine the indices of available_array
> that contain values in requested_array? The indices need not match. i.e.,
> if the two arrays above were used, I would like to return index=[1,2,3]
> because the requested values 7, 8 and 9 are in the available array.

The absolute fastest way MUST involve histograms, so
I maintain with a great deal of confidence (say, in the 40-50
percent range) that this is the fastest possible algorithm:

************************************************************ ************
FUNCTION SetIntersection, a, b, Indices=indices
minab = Min(a, Max=maxa) > Min(b, Max=maxb) ;Only need intersection of
ranges
maxab = maxa < maxb

; If either set is empty, or ranges don't intersect: result = NULL.

IF maxab LT minab OR maxab LT 0 THEN RETURN, -1
r = Where((Histogram(a, Min=minab, Max=maxab) NE 0) AND $
(Histogram(b, Min=minab, Max=maxab) NE 0), count)
IF Arg_Present(indices) THEN $
indices = Where((Histogram(a, Min=minab, Max=maxab) NE 0))
IF count EQ 0 THEN RETURN, -1 ELSE RETURN, r + minab
END
************************************************************ ************

Use it like this:

IDL> request_array = [5,6,7,8,9,10]
IDL> avail_array = [3,7,8,9,12,13,16]
IDL> int = setintersection(avail_array, request_array, Indices=i)
IDL> print, int, i
7 8 9
2 3 4

Cheers,

David

--
David W. Fanning, Ph.D.
Fanning Software Consulting, Inc.
Phone: 970-221-0438, E-mail: david@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155

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[Message index]

		Re: fast array comparison By: JD Smith on Mon, 09 December 2002 09:59
		Re: fast array comparison By: David Fanning on Mon, 09 December 2002 08:07
		Re: fast array comparison By: David Fanning on Mon, 09 December 2002 08:00
		Re: fast array comparison By: Sean Raffuse on Mon, 09 December 2002 07:48
		Re: fast array comparison By: JD Smith on Mon, 09 December 2002 07:25
		Re: fast array comparison By: Sean Raffuse on Mon, 09 December 2002 07:24
		Re: fast array comparison By: Craig Markwardt on Sun, 08 December 2002 17:39
		Re: fast array comparison By: David Fanning on Sun, 08 December 2002 12:47
		Re: fast array comparison By: David Fanning on Sun, 08 December 2002 12:40
		Re: fast array comparison By: Sean Raffuse on Sun, 08 December 2002 12:35
		Re: fast array comparison By: Sean Raffuse on Sun, 08 December 2002 12:32
		Re: fast array comparison By: David Fanning on Sun, 08 December 2002 09:45

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