[R] combinatorics again

[Uwe Ligges]

Robin Hankin wrote:

> Thank you Jacques
> 
> but your solution  misses (eg) c(1,1,2) which I need.

See ?combinations which should point you to

combinations(5,3, repeats.allowed=TRUE)

Best,
Uwe



> best wishes
> 
> Robin
> 
> 
> 
> On 6 Mar 2006, at 09:17, Jacques VESLOT wrote:
> 
> 
>>>library(gtools)
>>>combinations(5,3)
>>
>>     [,1] [,2] [,3]
>>[1,]    1    2    3
>>[2,]    1    2    4
>>[3,]    1    2    5
>>[4,]    1    3    4
>>[5,]    1    3    5
>>[6,]    1    4    5
>>[7,]    2    3    4
>>[8,]    2    3    5
>>[9,]    2    4    5
>>[10,]    3    4    5
>>
>>
>>Robin Hankin a écrit :
>>
>>
>>>Hi
>>>
>>>I want to enumerate all vectors of length "J", whose elements are
>>>integers in the range 1 to S, without regard to ordering.
>>>
>>>With J=S=3, the combinations are as follows:
>>>
>>>
>>>       [,1] [,2] [,3]
>>> [1,]    1    1    1
>>> [2,]    1    1    2
>>> [3,]    1    1    3
>>> [4,]    1    2    2
>>> [5,]    1    2    3
>>> [6,]    1    3    3
>>> [7,]    2    2    2
>>> [8,]    2    2    3
>>> [9,]    2    3    3
>>>[10,]    3    3    3
>>>
>>>
>>>Note that (eg) c(1,2,1) is not on the list because we already have
>>>c(1,1,2) which would be equivalent [because the problem is to
>>>enumerate the cases without regard to ordering] and I do not want
>>>repeats.
>>>
>>>The best I can do is to create all S^J possibilities and weed out the
>>>repeats, using unique() ; code below.
>>>
>>>Why is this no good?  Well, even for the tiny case of J=S=10, this
>>>would require a matrix of 10^10 rows, and my little linux machine
>>>refuses to cooperate, complaining about allocating a vector of
>>>length 1410065408.  For these values of J and S, I happen to know
>>>that the are 6360 distinct combinations, which is eminently  
>>>handleable.
>>>
>>>
>>>Anyone got any better ideas?
>>>
>>>
>>>
>>>
>>>
>>>
>>>allcomb <- function(J,S){
>>>
>>>  f <- function(...) {
>>>    1:S
>>>  }
>>>  out <- as.matrix(do.call("expand.grid", lapply(1:J, FUN = f)))
>>>  out <- t(apply(out,1,sort))
>>>  unique(out)
>>>}
>>>
>>>
>>>--
>>>Robin Hankin
>>>Uncertainty Analyst
>>>National Oceanography Centre, Southampton
>>>European Way, Southampton SO14 3ZH, UK
>>> tel  023-8059-7743
>>>
>>>______________________________________________
>>>R-help at stat.math.ethz.ch mailing list
>>>https://stat.ethz.ch/mailman/listinfo/r-help
>>>PLEASE do read the posting guide! http://www.R-project.org/posting- 
>>>guide.html
>>>
>>>
> 
> 
> --
> Robin Hankin
> Uncertainty Analyst
> National Oceanography Centre, Southampton
> European Way, Southampton SO14 3ZH, UK
>   tel  023-8059-7743
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html