% File src/library/base/man/unique.Rd
% Part of the R package, http://www.R-project.org
% Copyright 1995-2007 R Core Development Team
% Distributed under GPL 2 or later

\name{unique}
\alias{unique}
\alias{unique.default}
\alias{unique.data.frame}
\alias{unique.matrix}
\alias{unique.array}
\title{Extract Unique Elements}
\description{
  \code{unique} returns a vector, data frame or array like \code{x}
  but with duplicate elements removed.
}
\usage{
unique(x, incomparables = FALSE, \dots)

\method{unique}{default}(x, incomparables = FALSE, fromLast = FALSE, \dots)

\method{unique}{matrix}(x, incomparables = FALSE, MARGIN = 1, fromLast = FALSE, \dots)

\method{unique}{array}(x, incomparables = FALSE, MARGIN = 1, fromLast = FALSE, \dots)
}
\arguments{
  \item{x}{a vector or a data frame or an array or \code{NULL}.}
  \item{incomparables}{a vector of values that cannot be compared.
    Currently, \code{FALSE} is the only possible value, meaning that all
    values can be compared.}
  \item{fromLast}{logical indicating if duplication should be considered
    from the last, i.e., the last (or rightmost) of identical elements will
    be kept.  This only matters for \code{\link{names}} or
    \code{\link{dimnames}}.}
  \item{\dots}{arguments for particular methods.}
  \item{MARGIN}{the array margin to be held fixed: a single integer.}
}
\details{
  This is a generic function with methods for vectors, data frames and
  arrays (including matrices).

  The array method calculates for each element of the dimension
  specified by \code{MARGIN} if the remaining dimensions are identical
  to those for an earlier element (in row-major order).  This would most
  commonly be used for matrices to find unique rows (the default) or columns
  (with \code{MARGIN = 2}).
}
\value{
  For a vector, an object of the same type of \code{x}, but with only
  one copy of each duplicated element.  No attributes are copied (so
  the result has no names).

  For a data frame, a data frame is returned with the same columns but
  possibly fewer rows (and with row names from the first occurrences of
  the unique rows).

  A matrix or array is subsetted by \code{[, drop = FALSE]}, so
  dimensions and dimnames are copied appropriately, and the result
  always has the same number of dimensions as \code{x}.
}
\section{Warning}{
  Using this for lists is potentially slow, especially if the elements
  are not atomic vectors (see \code{\link{vector}}) or differ only
  in their attributes.  In the worst case it is \eqn{O(n^2)}.
}
\references{
  Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
  \emph{The New S Language}.
  Wadsworth \& Brooks/Cole.
}
\seealso{
  \code{\link{duplicated}} which gives the indices of duplicated
  elements.
}
\examples{
x <- c(3:5, 11:8, 8 + 0:5)
(ux <- unique(x))
(u2 <- unique(x, fromLast = TRUE)) # different order
stopifnot(identical(sort(ux), sort(u2)))

length(unique(sample(100, 100, replace=TRUE)))
## approximately 100(1 - 1/e) = 63.21

unique(iris)
}
\keyword{manip}
\keyword{logic}