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

[marammagdysalem at gmail.com]

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 <wdunlap at 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 <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
>> 
>>         [[alternative HTML version deleted]]
>> 
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> 

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