On Friday, March 27, 1998 at 11:31:22 am CET,
<suobs@verinet.com> wrote:
>Is there a simple way to obtain the products of all possible pairs of
>values that are in a data frame column?
>
>Steve Bousquin
Steve,
Yes, there is a more efficient way; one which also avoids the dreaded
"expressions nested beyond limit" error:
> z <- 1:10
> which <- combn2(n = length(z))
> z[which[,1]] * z[which[,2]]
[1] 2 3 4 5 6 7 8 9 10 6 8 10 12 14 16 18 20 12 15 18 21 24 27 30
20
[26] 24 28 32 36 40 30 35 40 45 50 42 48 54 60 56 63 70 72 80 90
This uses a general strategy that, as often heard on this list, often is
preferable in S-PLUS to the use of loops or recursion: first expand the
vector by subscripting and then form all the products "at once" using
vectorized computation.
Function combn2 is in my combinat module.
A Windows zip archive of this module is available at:
http://www.spg.wau.nl/pv/pub/chasalow/S/win/combinat .
A gzipped tar archive for UNIX is available at
http://www.spg.wau.nl/pv/pub/chasalow/S/unix/combinat .
Cheers,
Scott
*==================================*
Scott.Chasalow@users.pv.wau.nl
Wageningen Agricultural University
Laboratory of Plant Breeding
P.O. Box 386
6700 AJ Wageningen
THE NETHERLANDS
*==================================*
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