[R] Unordered combinations with repetition

[William Dunlap]

That combnWithRepetition (based on combn) can use much
less memory (and time) than the algorithm in prob:::urnsamples.default
with replace=TRUE, ordered=FALSE.  Perhaps urnsamples()
could be updated to use combn instead of unique(as.matrix(expand.grid())).

See the urn chapter in Feller vol. 1.

Bill Dunlap
TIBCO Software
wdunlap tibco.com

On Tue, Jun 9, 2015 at 8:56 AM, William Dunlap <wdunlap at tibco.com> wrote:

> > combnWithRepetition <- function(n, k) combn(n+k-1, k) - seq(from=0,
> len=k)
> > combnWithRepetition(2, 2)
>      [,1] [,2] [,3]
> [1,]    1    1    2
> [2,]    1    2    2
> > combnWithRepetition(3, 2)
>      [,1] [,2] [,3] [,4] [,5] [,6]
> [1,]    1    1    1    2    2    3
> [2,]    1    2    3    2    3    3
>
>
>
> Bill Dunlap
> TIBCO Software
> wdunlap tibco.com
>
> On Tue, Jun 9, 2015 at 2:11 AM, Thomas Chesney <
> Thomas.Chesney at nottingham.ac.uk> wrote:
>
>> Does anyone know of a function that will return all unordered
>> combinations of n elements from a list with repetition?
>>
>> The combs function in caTools will do this without repetition:
>>
>> combs(1:2, 2)
>>
>>      [,1] [,2]
>> [1,]    1    2
>>
>> What I'd like is:
>>
>> 1 1
>> 1 2
>> 2 2
>>
>> Thank you,
>>
>> Thomas Chesney
>>
>>
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