# Re: [R] combinatorial programming problem

From: Spencer Graves <spencer.graves_at_pdf.com>
Date: Mon 29 May 2006 - 09:21:53 EST

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

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

```	  Hope this helps,
Spencer Graves

```

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