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Re: matching lists [message #19277]

Mon, 13 March 2000 00:00

John-David T. Smith is currently offline

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

Senior Member

Mark Fardal wrote:
>
> Hi,
>
> Somehow I knew that if I mentioned HISTOGRAM, that would stir up some
> real IDL programmers. :->
>
> J.D.'s sort method seems like the winner. The modest speed advantage
> of the histogram method in certain cases is not important. If you are
> in a situation where just matching up the elements is the limitation,
> you are probably going to be in trouble doing any analysis with them
> (let alone reading them in).
>
> The problem of repeated elements, which is the only advantage of
> WHERE_ARRAY, is not of any concern, at least to me. The point of
> the key variables a and b is that they are supposed to be unique
> identifiers. I would just like the routine not to break completely
> in case the same element was copied into the arrays twice. The
> sort method does fine in that respect (finds the last of the
> duplicate elements in a and the first in b).

By the way, I found an implementation I had mentioned a while back on the news
group but had forgotten about from the nasa lib called "match" which does pretty
much the same thing. It's probably less efficient, since it uses an auxiliary
list of indices in addition to the flag vector, instead of just using the sort()
results directly, and performs a few more "where" tests as a result. But a
similar idea, written first in 1986! Match() is also more immune to changes in
sort() than my routine, as a result of carrying around these additional index
arrays.

Take a look.

JD

--
J.D. Smith |*| WORK: (607) 255-5842
Cornell University Dept. of Astronomy |*| (607) 255-6263
304 Space Sciences Bldg. |*| FAX: (607) 255-5875
Ithaca, NY 14853 |*|

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[Message index]

		Re: matching lists By: John-David T. Smith on Mon, 13 March 2000 00:00
		Re: matching lists By: Mark Fardal on Mon, 13 March 2000 00:00
		Re: matching lists By: John-David T. Smith on Sun, 12 March 2000 00:00
		Re: matching lists By: Craig Markwardt on Fri, 10 March 2000 00:00
		Re: matching lists By: Andy Loughe on Fri, 10 March 2000 00:00
		Re: matching lists By: John-David T. Smith on Fri, 10 March 2000 00:00
		Re: matching lists By: Craig Markwardt on Mon, 13 March 2000 00:00

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