From: Martin Maechler <maechler_at_stat.math.ethz.ch>

Date: Mon 29 May 2006 - 19:02:44 EST

SpG> http://www.R-project.org/posting-guide.html

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https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Mon May 29 19:17:35 2006

Date: Mon 29 May 2006 - 19:02:44 EST

>>>>> "SpG" == Spencer Graves <spencer.graves@pdf.com> >>>>> on Sun, 28 May 2006 16:21:53 -0700 writes:

SpG> I'm not sure I understand your question, but SpG> are you asking how to index choose(k, r) objects? SpG> Almost 3 years ago, I asked a question like this. Andy SpG> Liaw referred me to nchoosek(vsn) SpG> (http://finzi.psych.upenn.edu/R/Rhelp02a/archive/12518.html). SpG> This produces a matrix of dimension (r, choose(k, r)). SpG> With this matrix, you could convert an integer between SpG> 1 and choose(k, r) into an r-vector by table look-up. SpG> Reading the code for nchoosek might help you further if SpG> this does not seem appropriate for you.

Note that *if* the above is the answer,

I'd rather recommend to use combn() from package "combinat",
since a (slightly improved) version of combn() has been part of R-devel
(to become R 2.4.0 in October) for a while. Also, combn() from
"combinat" precedes nchoosek() historically and is also faster.

Martin Maechler, ETH Zurich

SpG> I found this just now using 'RSiteSearch("all SpG> subsets of a size")', which produced 102 hits. Another SpG> one that looked like it might help you is SpG> "http://finzi.psych.upenn.edu/R/Rhelp02a/archive/1717.html". SpG> Hope this helps, Spencer Graves

SpG> Kjetil Brinchmann Halvorsen wrote:

>> Hola!

* >>
** >> I am programming a class (S3) "symarray" for storing the
** >> results of functions symmetric in its k
** >> arguments. Intended use is for association indices for
** >> more than two variables, for instance coresistivity
** >> against antibiotics.
** >>
** >> There is one programming problem I haven't solved, making
** >> an inverse of the index function indx() --- se code
** >> below. It could for instance return the original k
** >> indexes in strictly increasing order, to make indx()
** >> formally invertible.
** >>
** >> Any ideas?
** >>
** >> Kjetil
** >>
** >>
** >> Code:
** >>
** >>
** >> # Implementing an S3 class for symarrays with array rank
** >> r for dimension # [k, k, ..., k] with k>=r repeated r
** >> times. We do not store the diagonal.
** >>
** >> # Storage requirement is given by {r, k}= choose(k, r) #
** >> where r=array rank, k=maximum index
** >>
** >> symarray <- function(data=NA, dims=c(1,1)){ r <- dims[1]
** >> k <- dims[2] if(r > k) stop("symarray needs dimension
** >> larger than array rank") len <- choose(k, r) out <-
** >> data[1:len] attr(out, "dims") <- dims class(out) <-
** >> "symarray" out }
** >>
** >> # Index calculation:
** >>
** >> indx <- function(inds, k){ r <- length(inds) if(r==1)
** >> return(inds) else { if(inds[1]==1) { return(
** >> indx(inds[-1]-1, k-1 ) ) } else { return(
** >> indx(c(inds[1]-1, seq(from=k-r+2, by=1, to=k)), k) +
** >> indx( inds[-1]-inds[1], k-inds[1] )) } } } # end indx
** >>
** >> # Methods for assignment and indexing:
** >>
** >> "[.symarray" <- function(x, inds, drop=FALSE){ dims <-
** >> attr(x, "dims") k <- dims[2] inds <- indx(inds, k) res <-
** >> NextMethod("[", x) res }
** >>
** >> "[<-.symarray" <- function(x, inds, value){ dims <-
** >> attr(x, "dims") k <- dims[2] inds <- indx(inds, k) res <-
** >> NextMethod("[<-", x) res }
** >>
** >> ______________________________________________
** >> R-help@stat.math.ethz.ch mailing list
** >> https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do
** >> read the posting guide!
** >> http://www.R-project.org/posting-guide.html
*

SpG> ______________________________________________ SpG> R-help@stat.math.ethz.ch mailing list SpG> https://stat.ethz.ch/mailman/listinfo/r-help PLEASE doSpG> read the posting guide!

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R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Mon May 29 19:17:35 2006

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