R help - Nov 2001 - Selecting transitive couples

Dear list,

I'm trying to improve the following process:

Let pairs be a 2 col matrix of integers such as:

2 5
3 6
4 56
5 2
6 8
7 56
8 6
etc.

My goal is to select the transitive pairs, i.e., in
the above example:

2 5 (because there is 5 2 as well)
6 8 (because there is 8 6 as well)

The task is simple with a for loop, but
can anybody suggest a "genuine R" solution?
(That is, a vectorised operation).

Note that, in practice, matrix pairs
can have ~60000 rows.

Thanks

Agus

Dr. Agustin Lobo
Instituto de Ciencias de la Tierra (CSIC)
Lluis Sole Sabaris s/n
08028 Barcelona SPAIN
tel 34 93409 5410
fax 34 93411 0012
alobo at ija.csic.es


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On Tue, 13 Nov 2001 17:55:10 +0100 (MET), you wrote in message
<Pine.OSF.3.96.1011113175336.18472K-100000 at ija.csic.es>:
>My goal is to select the transitive pairs, i.e., in
>the above example:
>
>2 5 (because there is 5 2 as well)
>6 8 (because there is 8 6 as well)
>
>The task is simple with a for loop, but
>can anybody suggest a "genuine R" solution?
>(That is, a vectorised operation).
This isn't as fast as it could be (it sorts the data twice), and might
not give the results in the format you like, but it should provide a
starting point:

transitive <- function(pairs)
{  both <- rbind(cbind(0,pairs), 
                 cbind(1,pairs[,2],pairs[,1]))
   both <- paste(both[,1],both[,2],both[,3])
   both <- unique(both)
   both <- substring(both,first=3)
   both[duplicated(both)]
}

It works by sticking together the original data with a transposed
copy, then looking for duplicates. ( The 0's and 1's, the unique()
call in the middle, and the substring extraction are only necessary if
you haven't already assured that each pair is unique.)  It works with
strings, so if you've got real values that might be displayed
identically even when not identical, it won't work.

Duncan Murdoch
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