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\name{combs}
\alias{combs}
\title{All Combinations of k Elements from Vector v}
\description{Finds all unordered combinations of \code{k} elements from vector
\code{v}.}
\usage{combs(v,k)}
\arguments{
\item{v}{Any numeric vector}
\item{k}{Number of elements to choose from vector \code{v}. Integer smaller
or equal than length of \code{v}. }
}
\value{
\code{combs(v,k)} (where \code{v} has length \code{n}) creates a matrix with
\eqn{\frac{n!}{(n-k)! k!}}{n!/((n-k)! k!)} (\code{n} choose \code{k}) rows
and \code{k} columns containing all possible combinations of \code{n} elements
taken \code{k} at a time.
}
\author{Jarek Tuszynski (SAIC) \email{jaroslaw.w.tuszynski@saic.com}}
\seealso{
I discovered recently that R packages already have two functions with
similar capabilities:
\code{\link[gtools]{combinations}} from \pkg{gTools} package and
\code{\link[vsn]{nchoosek}} from \pkg{vsn} package.
Also similar to Matlab's \code{nchoosek} function (\url{
http://www.mathworks.com/access/helpdesk/help/techdoc/ref/nchoosek.html})
}
\examples{
combs(2:5, 3) # display examples
combs(c("cats", "dogs", "mice"), 2)
a = combs(1:4, 2)
b = matrix( c(1,1,1,2,2,3,2,3,4,3,4,4), 6, 2)
stopifnot(a==b)
}
\keyword{models}
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