% File src/library/base/man/sort.Rd
% Part of the R package, http://www.R-project.org
% Copyright 1995-2009 R Core Development Team
% Distributed under GPL 2 or later

\name{sort}
\alias{sort}
\alias{sort.default}
\alias{sort.POSIXlt}
\alias{sort.int}
\title{Sorting or Ordering Vectors}
\description{
  Sort (or \emph{order}) a vector or factor (partially) into
  ascending or descending order.  For ordering along more than one
  variable, e.g., for sorting data frames, see \code{\link{order}}.
}
\usage{
sort(x, decreasing = FALSE, \dots)

\method{sort}{default}(x, decreasing = FALSE, na.last = NA, \dots)

sort.int(x, partial = NULL, na.last = NA, decreasing = FALSE,
         method = c("shell", "quick"), index.return = FALSE)
}
\arguments{
  \item{x}{for \code{sort} an \R object with a class or a numeric,
    complex, character or logical vector.  For \code{sort.int}, a
    numeric, complex, character or logical vector, or a factor.}
  \item{decreasing}{logical.  Should the sort be increasing or decreasing?
     Not available for partial sorting.}
  \item{\dots}{arguments to be passed to or from methods or (for the
    default methods and objects without a class) to \code{sort.int}.}
  \item{na.last}{for controlling the treatment of \code{NA}s.
    If \code{TRUE}, missing values in the data are put last; if
    \code{FALSE}, they are put first; if \code{NA}, they are removed.}
  \item{partial}{\code{NULL} or an integer vector of indices for
    partial sorting.}
  \item{method}{character string specifying the algorithm used.}
  \item{index.return}{logical indicating if the ordering index vector should
    be returned as well; this is only available for a few cases, the default
    \code{na.last = NA} and full sorting of non-factors.}
}
\details{
  \code{sort} is a generic function for which methods can be written,
  and \code{sort.int} is the internal method which is compatible
  with S if only the first three arguments are used.
  
  The default \code{sort} method makes use of \code{\link{order}} for
  classed objects, which in turn makes use of the generic function
  \code{\link{xtfrm}} (and can be slow unless a \code{xtfrm} method has
  been defined unless \code{\link{is.numeric}(x)} is true).

  If \code{partial} is not \code{NULL}, it is taken to contain indices
  of elements of the result which are to be placed in their correct
  positions in the sorted array by partial sorting.  For each of the
  result values in a specified position, any values smaller than that
  one are guaranteed to have a smaller index in the sorted array and any
  values which are greater are guaranteed to have a bigger index in the
  sorted array.  (This is included for efficiency, and many of the
  options are not available for partial sorting.  It is only
  substantially more efficient if \code{partial} has a handful of
  elements, and a full sort is done if there are more than 10.)  Names
  are discarded for partial sorting.

  Complex values are sorted first by the real part, then the imaginary
  part.

  The sort order for character vectors will depend on the collating
  sequence of the locale in use: see \code{\link{Comparison}}.
  The sort order for factors is the order of their levels (which is
  particularly appropriate for ordered factors).

  Method \code{"shell"} uses Shellsort (an \eqn{O(n^{4/3})} variant
  from Sedgewick (1996)).  If \code{x} has names a stable sort is used,
  so ties are not reordered.  (This only matters if names are present.)

  Method \code{"quick"} uses Singleton's Quicksort implementation and is
  only available when \code{x} is numeric (double or integer) and
  \code{partial} is \code{NULL}.  (For other types of \code{x} Shellsort
  is used, silently.)  It is normally somewhat faster than Shellsort
  (perhaps twice as fast on vectors of length a million) but has poor
  performance in the rare worst case. (Peto's modification using a
  pseudo-random midpoint is used to make the worst case rarer.)  This is
  not a stable sort, and ties may be reordered.
}
\value{
  For \code{sort}, the result depends on the S3 method which is
  dispatched.  If \code{x} does not have a class the rest of this section
  applies.  For classed objects which do not have a specific method the
  default method will be used and is equivalent to \code{x[order(x,
    ...)]}: this depends on the class having a suitable method for
  \code{[} (and also that \code{\link{order}} will work, which is not
  the case for a class based on a list).

  For \code{sort.int} the value is the sorted vector unless
  \code{index.return} is true, when the result is a list with components
  named \code{x} and \code{ix} containing the sorted numbers and the
  ordering index vector.  In the latter case, if \code{method ==
    "quick"} ties may be reversed in the ordering, unlike
  \code{sort.list}, as quicksort is not stable.

  All attributes are removed from the return value (see Becker \emph{et
    al}, 1988, p.146) except names, which are sorted.  (If
  \code{partial} is specified even the names are removed.)   Note that
  this means that the returned value has no class, except for factors
  and ordered factors (which are treated specially and whose result is
  transformed back to the original class).
}
\references{
  Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
  \emph{The New S Language}.
  Wadsworth & Brooks/Cole.

  Sedgewick, R. (1986)
  A new upper bound for Shell sort.
  \emph{J. Algorithms} \bold{7}, 159--173.

  Singleton, R. C. (1969) An efficient algorithm for sorting with
  minimal storage: Algorithm 347.
  \emph{Communications of the ACM} \bold{12}, 185--187.
}
\seealso{
  \sQuote{\link{Comparison}} for how character strings are collated.
  
  \code{\link{order}} for sorting on or reordering multiple variables.

  \code{\link{is.unsorted}}. \code{\link{rank}}.
}
\examples{
require(stats)

x <- swiss$Education[1:25]
x; sort(x); sort(x, partial = c(10, 15))
median.default # shows you another example for 'partial'

## illustrate 'stable' sorting (of ties):
sort(c(10:3,2:12), method = "sh", index.return=TRUE) # is stable
## $x : 2  3  3  4  4  5  5  6  6  7  7  8  8  9  9 10 10 11 12
## $ix: 9  8 10  7 11  6 12  5 13  4 14  3 15  2 16  1 17 18 19
sort(c(10:3,2:12), method = "qu", index.return=TRUE) # is not
## $x : 2  3  3  4  4  5  5  6  6  7  7  8  8  9  9 10 10 11 12
## $ix: 9 10  8  7 11  6 12  5 13  4 14  3 15 16  2 17  1 18 19
##        ^^^^^

x <- c(1:3, 3:5, 10)
is.unsorted(x)                #-> FALSE: is sorted
is.unsorted(x, strictly=TRUE) #-> TRUE : is not (and cannot be) sorted strictly

\dontrun{
## Small speed comparison simulation:
N <- 2000
Sim <- 20
rep <- 1000 # << adjust to your CPU
c1 <- c2 <- numeric(Sim)
for(is in 1:Sim){
  x <- rnorm(N)
  c1[is] <- system.time(for(i in 1:rep) sort(x, method = "shell"))[1]
  c2[is] <- system.time(for(i in 1:rep) sort(x, method = "quick"))[1]
  stopifnot(sort(x, method = "s") == sort(x, method = "q"))
}
rbind(ShellSort = c1, QuickSort = c2)
cat("Speedup factor of quick sort():\n")
summary({qq <- c1 / c2; qq[is.finite(qq)]})

## A larger test
x <- rnorm(1e7)
system.time(x1 <- sort(x, method = "shell"))
system.time(x2 <- sort(x, method = "quick"))
stopifnot(identical(x1, x2))
}}
\keyword{univar}
\keyword{manip}
\keyword{arith}
