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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 = formula(x), \dots)
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}
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\arguments{
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\item{x}{object to be stacked or unstacked}
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\item{select}{expression, indicating variables 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{formula(x)}
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in \code{unstack.data.frame}.}
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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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}
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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}
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