[R] combinations
Warnes, Gregory R
gregory_r_warnes at groton.pfizer.com
Thu Nov 7 22:22:25 CET 2002
The "gregmisc" package provides the 'combinations' function, which can be
used as:
> combinations(n=4, r=3, c('a','b','c','d') )
[,1] [,2] [,3]
[1,] "a" "b" "c"
[2,] "a" "b" "d"
[3,] "a" "c" "d"
[4,] "b" "c" "d"
where each row contains one possible combination.
The 'combinat' package provides the 'combn' function which can be used as:
> combn( c('a','b','c','d'), 3 )
[,1] [,2] [,3] [,4]
[1,] "a" "a" "a" "b"
[2,] "b" "b" "c" "c"
[3,] "c" "d" "d" "d"
where each column contains one possible combination.
-Greg
> -----Original Message-----
> From: Michaell Taylor [mailto:pols1oh at bestweb.net]
> Sent: Thursday, November 07, 2002 2:52 PM
> To: r-help at stat.math.ethz.ch
> Subject: [R] combinations
>
>
>
> I need to construct all possible combinations of an vector of
> length N taken
> X at a time for simulation purposes. Taking a a small vector
> as an example:
>
>
> >input <- c('a','b','c','d')
> >somefunction(input)
> a,b,c
> a,b,d
> a,c,d
> b,c,d
>
> my only solution thus far is:
>
> somefunction <- function(x){
> ...a series of grotesque and horribly inefficient loops
> deleted to save face.
> }
>
> For my purposes, N=20 and X=5, meaning there are about 15k
> combinations.
>
> I assumed this might be a common problem, but I am coming up
> empty on my searches
> of the usual suspect web sites.
>
> Thanks in advance for any pointers anyone may have.
>
> Michaell
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