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Re: Intersection of 2 sets--Beginner IDL question [message #27297 is a reply to message #27294]

Tue, 16 October 2001 22:00

John-David T. Smith is currently offline

John-David T. Smith
Messages: 384
Registered: January 2000

Senior Member

David Fanning wrote:
>
> Ted Cary (tedcary@yahoo.com) writes:
>
>> Is there any programming technique for finding the intersection of two sets
>> (arrays) of numbers without using WHERE in a loop to search the larger array
>> for every element in the smaller array? It seems like a very clumsy way to
>> find values shared by both arrays, especially with integer sets/arrays.
>
> How about the very tiny program, SetIntersection,
> which uses--what else--a Histogram! :-)
>
> http://www.dfanning.com/tips/set_operations.html

It's amazing how much recycled information flows through the newsgroup, if you
watch it long enough. I remember just like it was 4 years ago the detailed
discussions with which we whiled away our days, concerning value-based
intersection vs index-based intersection, order N vs. unknown order operations,
etc.

I do, however, still cringe when I read on that page of yours:

"These routines are faster than previously published functions, e.g. Contain..."

since I whipped up contain() to prevent the problem of an operation scaling out
of control on sparse data sets (as set_intersection does -- a feature they warn
you of). You can read all about a variety of intersection methods if you like:

http://groups.google.com/groups?selm=38CBF8B6.5BF0AB50%40ast ro.cornell.edu

In a classic example posed by Mark Fardal, you are matching up social security
numbers in two lists containing age and income. The set_intersection style
solution fails miserably here, and to a lesser degree for any arrays which are
somewhat sparse (where *somewhat* seems to be about 1 in 10, depending on lots
of factors).

Ah, the glory days.

JD

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[Message index]

		Re: Intersection of 2 sets--Beginner IDL question By: John-David T. Smith on Wed, 17 October 2001 09:51
		Re: Intersection of 2 sets--Beginner IDL question By: Craig Markwardt on Wed, 17 October 2001 08:05
		Re: Intersection of 2 sets--Beginner IDL question By: David Fanning on Wed, 17 October 2001 06:58
		Re: Intersection of 2 sets--Beginner IDL question By: R.Bauer on Tue, 16 October 2001 23:58
		Re: Intersection of 2 sets--Beginner IDL question By: John-David T. Smith on Tue, 16 October 2001 22:00
		Re: Intersection of 2 sets--Beginner IDL question By: David Fanning on Tue, 16 October 2001 20:52
		Re: Intersection of 2 sets--Beginner IDL question By: Mark Hadfield on Tue, 16 October 2001 20:37
		Re: Intersection of 2 sets--Beginner IDL question By: David Fanning on Tue, 16 October 2001 20:01
		Re: Intersection of 2 sets--Beginner IDL question By: Craig Markwardt on Wed, 17 October 2001 14:49

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