how can i create a combinations of element with at most x repetitions for each one

user_10021885 0
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I would be generate a combinations of a vector x of 5 element with at most 2 repetitions for each element. 

x = c(1,2,3,4,5,6,7,8,9,10)

I'm using combn(x,5) but return the combination of 5 element without repetition.

In output I would be like : ((1,2,3,4,5), (1,2,3,4,6), ..., (1,1,3,4,5),(1,2,3,4,4), (1,1,2,2,3), ...)

Most important thing is that each element is represented at most twice. I would like to know if there is a function that can do this.

combinations permutation r
2 Answers

You are after combinations of multisets. There are a couple of libraries capable of producing the desired output without producing any duplicates combinations: arrangements and RcppAlgos, which I authored. Here is how you would use it:

arrangements::combinations(5, 5, freq = rep(2, 5))

RcppAlgos::comboGeneral(5, 5, freqs = rep(2, 5))

Note, the output from both libraries is in lexicographical order. Also, these libraries are written in compiled languages, so they extend well to larger cases.

user_44085380

You could just double the elements in x using rep. Designing your own vector like so gives you total control over the number of repetitions.

combn(x,5)
x <- rep(c(1:5),2)
> head(t(combn(x,5)))
     [,1] [,2] [,3] [,4] [,5]
[1,]    1    2    3    4    5
[2,]    1    2    3    4    1
[3,]    1    2    3    4    2
[4,]    1    2    3    4    3
[5,]    1    2    3    4    4
[6,]    1    2    3    4    5

EDIT: as commented, this will generate duplicates, if this is not what you want, you could use unique(t(combn...))) to remove these duplicates.

user_52242360
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