Thanks a lot for helping William. Will check the reference as well.
Sent from my iPhone
> On Oct 15, 2015, at 5:52 PM, William Dunlap <wdun...@tibco.com> wrote:
>
> 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.
