[R] Error in rep.int() invalid 'times' value

[William Dunlap]

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 <marammagdysalem at 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
>
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>
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