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 <[EMAIL PROTECTED]> >>>>>> 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 } > >> > >> ______________________________________________ > >> [email protected] 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> [email protected] mailing list > SpG> https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do > SpG> read the posting guide! > SpG> http://www.R-project.org/posting-guide.html > > ______________________________________________ > [email protected] mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html > ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
