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Re: Intersection of 2 sets--Beginner IDL question [message #27281 is a reply to message #27276]

Wed, 17 October 2001 08:05

Craig Markwardt
Messages: 1869
Registered: November 1996

Senior Member

John-David Smith <jdsmith@astro.cornell.edu> writes:

> 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.
>
...
> 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).

Hi JD--

Thanks for beating me to the punch. The HISTOGRAM method is indeed
very cool for a new learner, but it definitely starts to suck air (and
memory) when the data sets become sparse.

Long ago (1 year?) I tried to collect all the various algorithms that
were being discussed, and some that weren't yet, to do set operations.
CMSET_OP has the dreaded "CM" prefix, but it also knows how to do
intersections, unions, and exclusive or's. It can do X AND NOT Y type
intersections as well, in one self contained function.

The syntax is:

x_and_y = cmset_op(X, 'AND', y)

It can return by value or index.

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: 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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