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% File src/library/utils/man/stack.Rd
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% Part of the R package, http://www.R-project.org
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% Copyright 1995-2011 R Core Team
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% Distributed under GPL 2 or later
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\name{stack}
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\alias{stack}
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\alias{stack.default}
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\alias{stack.data.frame}
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\alias{unstack}
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\alias{unstack.default}
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\alias{unstack.data.frame}
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\title{Stack or Unstack Vectors from a Data Frame or List}
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\description{
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  Stacking vectors concatenates multiple vectors into a single vector
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  along with a factor indicating where each observation originated.
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  Unstacking reverses this operation.
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}
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\usage{
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stack(x, \dots)
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\method{stack}{default}(x, \dots)
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\method{stack}{data.frame}(x, select, \dots)
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unstack(x, \dots)
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\method{unstack}{default}(x, form, \dots)
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\method{unstack}{data.frame}(x, form, \dots)
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}
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\arguments{
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  \item{x}{a list or data frame to be stacked or unstacked.}
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  \item{select}{an expression, indicating which variable(s) to select from a
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    data frame.}
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  \item{form}{a two-sided formula whose left side evaluates to the
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    vector to be unstacked and whose right side evaluates to the
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    indicator of the groups to create.  Defaults to \code{\link{formula}(x)}
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    in the data frame method for \code{unstack}.}
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  \item{\dots}{further arguments passed to or from other methods.}
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}
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\details{
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  The \code{stack} function is used to transform data available as
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  separate columns in a data frame or list into a single column that can
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  be used in an analysis of variance model or other linear model.  The
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  \code{unstack} function reverses this operation.
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  Note that \code{stack} applies to \emph{vectors} (as determined by
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  \code{\link{is.vector}}): non-vector columns (e.g., factors) will be
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  ignored (with a warning as from \R 2.15.0).  Where vectors of
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  different types are selected they are concatenated by
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  \code{\link{unlist}} whose help page explains how the type of the
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  result is chosen.
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  These functions are generic: the supplied methods handle data frames
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  and objects coercible to lists by \code{\link{as.list}}.
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}
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\value{
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  \code{unstack} produces a list of columns according to the formula
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  \code{form}.  If all the columns have the same length, the resulting
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  list is coerced to a data frame.
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  \code{stack} produces a data frame with two columns:
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  \item{values}{the result of concatenating the selected vectors in
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    \code{x}.}
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  \item{ind}{a factor indicating from which vector in \code{x} the
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    observation originated.}
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}
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\author{Douglas Bates}
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\seealso{
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  \code{\link{lm}}, \code{\link{reshape}}
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}
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\examples{
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require(stats)
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formula(PlantGrowth)         # check the default formula
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pg <- unstack(PlantGrowth)   # unstack according to this formula
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pg
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stack(pg)                    # now put it back together
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stack(pg, select = -ctrl)    # omitting one vector
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}
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\keyword{manip}