combn Generate All Combinations of n Elements, Taken m at a Time

Description

Generate all combinations of the elements of x taken m at a time. If x is a positive integer, returns all combinations of the elements of seq(x) taken m at a time. If argument FUN is not NULL, applies a function given by the argument to each point. If simplify is FALSE, returns a list; otherwise returns an array, typically a matrix. ... are passed unchanged to the FUN function, if specified.

Usage

combn(x, m, FUN = NULL, simplify = TRUE, ...)

Arguments

x	vector source for combinations, or integer n for x <- seq_len(n).
m	number of elements to choose.
FUN	function to be applied to each combination; default NULL means the identity, i.e., to return the combination (vector of length m).
simplify	logical indicating if the result should be simplified to an array (typically a matrix); if FALSE, the function returns a list. Note that when simplify = TRUE as by default, the dimension of the result is simply determined from FUN(1st combination) (for efficiency reasons). This will badly fail if FUN(u) is not of constant length.
...	optionally, further arguments to FUN.

Details

Factors x are accepted.

Value

A list or array, see the simplify argument above. In the latter case, the identity dim(combn(n, m)) == c(m, choose(n, m)) holds.

Author(s)

Scott Chasalow wrote the original in 1994 for S; R package combinat and documentation by Vince Carey [email protected]; small changes by the R core team, notably to return an array in all cases of simplify = TRUE, e.g., for combn(5,5).

References

Nijenhuis, A. and Wilf, H.S. (1978) Combinatorial Algorithms for Computers and Calculators; Academic Press, NY.

See Also

choose for fast computation of the number of combinations. expand.grid for creating a data frame from all combinations of factors or vectors.

Examples

combn(letters[1:4], 2)
(m <- combn(10, 5, min))   # minimum value in each combination
mm <- combn(15, 6, function(x) matrix(x, 2, 3))
stopifnot(round(choose(10, 5)) == length(m), is.array(m), # 1-dimensional
          c(2,3, round(choose(15, 6))) == dim(mm))

## Different way of encoding points:
combn(c(1,1,1,1,2,2,2,3,3,4), 3, tabulate, nbins = 4)

## Compute support points and (scaled) probabilities for a
## Multivariate-Hypergeometric(n = 3, N = c(4,3,2,1)) p.f.:
# table.mat(t(combn(c(1,1,1,1,2,2,2,3,3,4), 3, tabulate, nbins = 4)))

## Assuring the identity
for(n in 1:7)
 for(m in 0:n) stopifnot(is.array(cc <- combn(n, m)),
                         dim(cc) == c(m, choose(n, m)),
                         identical(cc, combn(n, m, identity)) || m == 1)