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Re: Efficient comparison of arrays [message #9684] Mon, 11 August 1997 00:00 Go to previous message
wonko is currently offline  wonko
Messages: 22
Registered: March 1997
Junior Member
afl@cdc.noaa.gov (Andy Loughe) wrote:

> 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).
>
> Presumably this needs to be done without loops (to be efficient), but
> an obvious solution escapes me.

What type of data would this be? If you are dealing with integers, what
about this:

max_value = max( [ a, b ], min_value=min_value )
mask = indgen( max_value-min_value+1 )
mask(a-min_value) = 1
mask(b-min_value) = mask(b-min_value) + 1
values = where( mask eq 2, count ) + min_value

Voila, no loops, I would give this a try. Bad luck if you have floating
point data.

Someone already came up with this question five months ago, but I don't
remember the responses. Maybe you can find the thread via dejanews.

Alex
--
Alex Schuster Wonko@weird.cologne.de PGP Key available
alex@pet.mpin-koeln.mpg.de

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