Doing enumerative combinatorics with rejection methods rarely works well. Try mapping your problem to the problem of choosing m-1 items from n-1. E.g., your code was
f0 <- function(n, m) { stopifnot(n > m) D<-matrix(0,nrow=n-m+1,ncol=m-1) for (i in 1:m-1){ D[,i]<-seq(0,n-m,1) } ED <- do.call(`expand.grid`,as.data.frame(D)) ED<-unname(as.matrix(ED)) lk<-which(rowSums(ED)<=(n-m)) ED[lk,] } and I think the following does the same thing in much less space by transforming the output of combn(). f1 <- function(n, m) { stopifnot(n > m) r0 <- t(diff(combn(n-1, m-1)) - 1L) r1 <- rep(seq(from=0, len=n-m+1), choose( seq(to=m-2, by=-1, len=n-m+1), m-2)) cbind(r0[, ncol(r0):1, drop=FALSE], r1, deparse.level=0) } The code for adding the last column is a bit clumsy and could probably be improved. Both f0 and f1 could also be cleaned up to work for m<=2. See Feller vol. 1 or Benjamin's "Proofs that (really) count" for more on this sort of thing. Bill Dunlap TIBCO Software wdunlap tibco.com On Thu, Oct 15, 2015 at 7:45 AM, Maram SAlem <marammagdysa...@gmail.com> wrote: > Dear All, > > I'm trying to do a simple task (which is in fact a tiny part of a larger > code). > > I want to create a matrix, D, each of its columns is a sequence from 0 to > (n-m), by 1. Then, using D, I want to create another matrix ED, whose rows > represent all the possible combinations of the elements of the columns of > D. Then from ED, I'll select only the rows whose sum is less than or equal > to (n-m), which will be called the matrix s. I used the following code: > > > n=5 > > m=3 > > D<-matrix(0,nrow=n-m+1,ncol=m-1) > > for (i in 1:m-1) > + { > + D[,i]<-seq(0,n-m,1) > + } > > ED <- do.call(`expand.grid`,as.data.frame(D)) > > ED<-as.matrix(ED) > > > lk<-which(rowSums(ED)<=(n-m)) > > > s<-ED[lk,] > > > This works perfectly well. But for rather larger values of n and m (which > are not so large actually), the number of all possible combinations of the > columns of D gets extremely large giving me this error (for n=25, m=15): > > > ED <- do.call(`expand.grid`,as.data.frame(D)) > Error in rep.int(rep.int(seq_len(nx), rep.int(rep.fac, nx)), orep) : > invalid 'times' value > In addition: Warning message: > In rep.int(rep.int(seq_len(nx), rep.int(rep.fac, nx)), orep) : > NAs introduced by coercion to integer range > > > Any help or suggestions will be greatly appreciated. > > Thanks, > > Maram Salem > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide > http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > [[alternative HTML version deleted]] ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.