The R Project SVN R

\name{sample}
\alias{sample}
\title{Random Samples and Permutations}
\description{
  \code{sample} takes a sample of the specified size from the elements
  of \code{x} using either with or without replacement.
}
\usage{
sample(x, size, replace = FALSE, prob = NULL)
}
\arguments{
  \item{x}{Either a (numeric, complex, character or logical) vector of
    more than one element from which to choose, or a positive integer.}
  \item{size}{non-negative integer giving the number of items to choose.}
  \item{replace}{Should sampling be with replacement?}
  \item{prob}{A vector of probability weights for obtaining the elements
    of the vector being sampled.}
}
\details{
  If \code{x} has length 1, sampling takes place from
  \code{1:x}.  \emph{Note} that this convenience feature may lead to
  undesired behaviour when \code{x} is of varying length
  \code{sample(x)}.  See the \code{resample()} example below.

  By default \code{size} is equal to \code{length(x)}
  so that \code{sample(x)} generates a random permutation
  of the elements of \code{x} (or \code{1:x}).

  The optional \code{prob} argument can be used to give a vector
  of weights for obtaining the elements of the vector being
  sampled. They need not sum to one, but they should be nonnegative
  and not all zero.
  If \code{replace} is false, these probabilities are applied
  sequentially, that is the probability of choosing the next item is
  proportional to the probabilities amongst the remaining items. The number
  of nonzero weights must be at least \code{size} in this case.
}
\references{
  Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
  \emph{The New S Language}.
  Wadsworth \& Brooks/Cole.
}
\examples{
x <- 1:12
# a random permutation
sample(x)
# bootstrap sampling -- only if length(x) > 1 !
sample(x,replace=TRUE)

# 100 Bernoulli trials
sample(c(0,1), 100, replace = TRUE)

## More careful bootstrapping --  Consider this when using sample()
## programmatically (i.e., in your function or simulation)!

# sample()'s surprise -- example
x <- 1:10
    sample(x[x >  8]) # length 2
    sample(x[x >  9]) # oops -- length 10!
try(sample(x[x > 10]))# error!

## This is safer:
resample <- function(x, size, ...)
  if(length(x) <= 1) { if(!missing(size) && size == 0) x[FALSE] else x
  } else sample(x, size, ...)

resample(x[x >  8])# length 2
resample(x[x >  9])# length 1
resample(x[x > 10])# length 0
}
\keyword{distribution}