The R Project SVN R

\name{combn}
\alias{combn}
\title{Generate All Combinations of n Elements, Taken m at a Time }
\description{
  Generate all combinations of the elements of \code{x} taken \code{m}
  at a time.  If \code{x} is a positive integer, returns all
  combinations of the elements of \code{seq(x)} taken \code{m} at a
  time.  If argument \code{FUN} is not \code{NULL}, applies a function given
  by the argument to each point.  If simplify is FALSE,  returns
  a list; else returns a vector or an array.  \code{...} are passed
  unchanged to the \code{FUN} function, if specified.
}
\usage{
combn(x, m, FUN = NULL, simplify = TRUE, \dots)
}
\arguments{
  \item{x}{vector source for combinations, or integer \code{n} for
    \code{x <- 1:n}.}
  \item{m}{number of elements to choose.}
  \item{FUN}{function to be applied to each combination; default
    \code{NULL} means the identity, i.e., to return the combination
    (vector of length \code{m}).}
  \item{simplify}{logical indicating if the result should be simplified
    to a vector or \code{\link{array}}; if FALSE, the function returns a
    \code{\link{list}}. Note that when \code{simplify=TRUE} as by
    default, the dimension of the result is simply determined from
    \code{FUN(\emph{<1st combination>})}, for efficiency reasons.  This
    will badly fail if \code{FUN(u)} is not of constant length.}
  \item{\dots}{optionally, further arguments to \code{FUN}.}
}
\value{
  a list, vector or array, see the \code{simplify} argument above.
}
\references{
  Nijenhuis, A. and Wilf, H.S. (1978)
  \emph{Combinatorial Algorithms for Computers and Calculators};
  Academic Press, NY.
}
\author{Scott Chasalow wrote the original in 1994 for S;
  R package \pkg{combinat} and documentation by Vince Carey
  \email{stvjc@channing.harvard.edu};
  small changes by the R core team.
}
\seealso{
  \code{\link{choose}} for fast computation of the \emph{number} of
  combinations.
}
\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),
          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)))
}
\keyword{utilities}
%\keyword{ combinatorics }% substitute:
\keyword{iteration}