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Re: Finding Common Elements in Two Arrays [message #2112 is a reply to message #2111]

Wed, 01 June 1994 23:26

stl
Messages: 70
Registered: February 1994

Member

In article <1JUN199416454238@stars.gsfc.nasa.gov> kucera@stars.gsfc.nasa.gov (Terry Kucera) writes:
> I'm looking for a quick way to compare two arrays in IDL, A and B,
> and determine which elements of B are also in A,
> so if:
> A=[2,1,3,5,3,8,2,5]
> B=[3,4,2,8,7,8]
> I would get [0,2,3,5], because 3, 2, and 8 are in A as well as B.
>
> I can do this with loops, but that takes too long for big arrays. Does anyone
> have a way to do this using array functions or perhaps an external routine?

I just discovered (a few weeks ago) that this is a very usefull
function! I was forced to use one loop (not sure if used one or two
because you mention above 'loops'). If there is an array function I
would LOVE to see it cause I need to do this LOTS of times, with arrays
of up to 10,000 elements!

Here is what I did:
index = 0
for i = 0,n_elements(b)-1 do begin
set = where(a eq b(i))
if set(0) ne -1 then index = [index,set]
endfor
;get ride of the first 0!
if n_elements(index) gt 1 then index = index[1:*] $
else index = -1

for huge array, this is a bummer to have to do over and over! I am
interested in an discussion and better solutions to this seamingly
trivial but painfull little problem.

-stephen
--
Stephen C Strebel / SKI TO DIE
stl@maz.sma.ch / and
Swiss Meteorological Institute, Zuerich / LIVE TO TELL ABOUT IT
01 256 93 85 / (and pray for snow)

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[Message index]

		Finding Common Elements in Two Arrays By: kucera on Wed, 01 June 1994 13:45
		Re: Finding Common Elements in Two Arrays By: stl on Wed, 01 June 1994 23:26
		Re: Finding Common Elements in Two Arrays By: stl on Mon, 06 June 1994 01:09
		Re: Finding Common Elements in Two Arrays By: landers on Fri, 03 June 1994 06:10
		Re: Finding Common Elements in Two Arrays By: landers on Fri, 03 June 1994 06:02
		Re: Finding Common Elements in Two Arrays By: dan on Wed, 01 June 1994 16:23
		Re: Finding common elements in two arrays By: andy on Tue, 26 July 1994 09:39

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