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Re: Efficient comparison of arrays [message #9780 is a reply to message #9707]

Mon, 11 August 1997 00:00

davidf
Messages: 2866
Registered: September 1996

Senior Member

Andy Loughe wrote the other day:

> Given vectors of the type...
>
> a = [1,2,3,4,5]
> b = [3,4,5,6,7]
>
> What is the most efficient way to determine which values that occur in
> a also occur in b (i.e., the values [3,4,5] occur in both a and b).

A friend (wishing anonymity) wrote to me with this solution.
I am not sure how general it is, but it worked with this
test case and several others I made up.

Given a and b:

a = [1,2,3,4,5]
b = [3,4,5,6,7]

Let,

array1 = BYTARR((MAX(a) > MAX(b)) - (MIN(a) < MIN(b)))
array2 = array1

Then, let,

ind1[a] = 1L
ind2[b] = 1L

Finally, let,

commonIndex = ind1 * ind2

The vector commonIndex now has 1s at the locations where there are
common values in the two sets. In other words,

Print, commonIndex
0 0 0 1 1 1

Something similar must be going on in the Venn diagram demo
I found recently in the IDL 5 demos, although a quick look
didn't find the relevant code snippet. Look at d_venn.pro
in the demo source directory.

Cheers,

David

-----------------------------------------------------
David Fanning, Ph.D.
Fanning Software Consulting
Customizable IDL Programming Courses
Phone: 970-221-0438 E-Mail: davidf@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com

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[Message index]

		Efficient comparison of arrays By: Andy Loughe on Fri, 08 August 1997 00:00
		Re: Efficient comparison of arrays By: J.D. Smith on Thu, 14 August 1997 00:00
		Re: Efficient comparison of arrays By: David Foster on Thu, 14 August 1997 00:00
		Re: Efficient comparison of arrays By: William Clodius on Thu, 14 August 1997 00:00
		Re: Efficient comparison of arrays By: William Clodius on Wed, 13 August 1997 00:00
		Re: Efficient comparison of arrays By: J.D. Smith on Wed, 13 August 1997 00:00
		Re: Efficient comparison of arrays By: David Foster on Wed, 13 August 1997 00:00
		Re: Efficient comparison of arrays By: J.D. Smith on Tue, 12 August 1997 00:00
		Re: Efficient comparison of arrays By: J.D. Smith on Mon, 11 August 1997 00:00
		Re: Efficient comparison of arrays By: davidf on Mon, 11 August 1997 00:00
		Re: Efficient comparison of arrays By: davidf on Mon, 11 August 1997 00:00
		Re: Efficient comparison of arrays By: John Votaw on Wed, 20 August 1997 00:00

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