[R] combinatorial programming problem
Spencer Graves
spencer.graves at pdf.com
Tue May 30 19:11:03 CEST 2006
Hi, Martin: Thanks much. I don't know if it solved Kjetil's problem,
but it helps educate me. Best Wishes, Spencer Graves
Martin Maechler wrote:
>>>>>> "SpG" == Spencer Graves <spencer.graves at 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 }
> >>
> >> ______________________________________________
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>
> SpG> ______________________________________________
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>
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