[R] combinatorial programming problem

[Spencer Graves]

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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