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% File src/library/base/man/tapply.Rd
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% File src/library/base/man/tapply.Rd
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% Part of the R package, https://www.R-project.org
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% Part of the R package, https://www.R-project.org
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% Copyright 1995-2017 R Core Team
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% Copyright 1995-2019 R Core Team
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% Distributed under GPL 2 or later
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% Distributed under GPL 2 or later
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\name{tapply}
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\name{tapply}
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\alias{tapply}
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\alias{tapply}
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\title{Apply a Function Over a Ragged Array}
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\title{Apply a Function Over a Ragged Array}
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    exists.  Typically vector-like, allowing subsetting with
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    exists.  Typically vector-like, allowing subsetting with
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    \code{\link{[}}.}
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    \code{\link{[}}.}
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  \item{INDEX}{a \code{\link{list}} of one or more \code{\link{factor}}s,
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  \item{INDEX}{a \code{\link{list}} of one or more \code{\link{factor}}s,
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    each of same length as \code{X}.  The elements are coerced to
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    each of same length as \code{X}.  The elements are coerced to
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    factors by \code{\link{as.factor}}.}
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    factors by \code{\link{as.factor}}.}
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  \item{FUN}{the function to be applied, or \code{NULL}.  In the case of
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  \item{FUN}{a function (or name of a function) to be applied, or \code{NULL}.
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    functions like \code{+}, \code{\%*\%}, etc., the function name must
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    In the case of functions like \code{+}, \code{\%*\%}, etc.,
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    be backquoted or quoted.  If \code{FUN} is \code{NULL}, tapply
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    the function name must be backquoted or quoted.  If \code{FUN} is
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    returns a vector which can be used to subscript the multi-way array
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    \code{NULL}, tapply returns a vector which can be used to subscript
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    \code{tapply} normally produces.}
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    the multi-way array \code{tapply} normally produces.}
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  \item{\dots}{optional arguments to \code{FUN}: the Note section.}
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  \item{\dots}{optional arguments to \code{FUN}: the Note section.}
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  \item{default}{(only in the case of simplification to an array) the
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  \item{default}{(only in the case of simplification to an array) the
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    value with which the array is initialized as
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    value with which the array is initialized as
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    \code{\link{array}(default, dim = ..)}.  Before \R 3.4.0, this
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    \code{\link{array}(default, dim = ..)}.  Before \R 3.4.0, this
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    was hard coded to \code{\link{array}()}'s default \code{NA}.  If it
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    was hard coded to \code{\link{array}()}'s default \code{NA}.  If it
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    an array of mode \code{"list"}; in other words, a \code{\link{list}}
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    an array of mode \code{"list"}; in other words, a \code{\link{list}}
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    with a \code{\link{dim}} attribute.  If \code{TRUE} (the default), then if
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    with a \code{\link{dim}} attribute.  If \code{TRUE} (the default), then if
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    \code{FUN} always returns a scalar, \code{tapply} returns an array
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    \code{FUN} always returns a scalar, \code{tapply} returns an array
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    with the mode of the scalar.}
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    with the mode of the scalar.}
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}
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}
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\value{
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\details{
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  If \code{FUN} is not \code{NULL}, it is passed to
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  If \code{FUN} is not \code{NULL}, it is passed to
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  \code{\link{match.fun}}, and hence it can be a function or a symbol or
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  \code{\link{match.fun}}, and hence it can be a function or a symbol or
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  character string naming a function.
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  character string naming a function.
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}
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\value{
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  When \code{FUN} is present, \code{tapply} calls \code{FUN} for each
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  When \code{FUN} is present, \code{tapply} calls \code{FUN} for each
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  cell that has any data in it.  If \code{FUN} returns a single atomic
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  cell that has any data in it.  If \code{FUN} returns a single atomic
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  value for each such cell (e.g., functions \code{mean} or \code{var})
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  value for each such cell (e.g., functions \code{mean} or \code{var})
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  and when \code{simplify} is \code{TRUE}, \code{tapply} returns a
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  and when \code{simplify} is \code{TRUE}, \code{tapply} returns a
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  multi-way \link{array} containing the values, and \code{NA} for the
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  multi-way \link{array} containing the values, and \code{NA} for the
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  \code{INDEX} has components; the number of levels in a dimension is
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  \code{INDEX} has components; the number of levels in a dimension is
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  the number of levels (\code{nlevels()}) in the corresponding component
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  the number of levels (\code{nlevels()}) in the corresponding component
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  of \code{INDEX}.  Note that if the return value has a class (e.g., an
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  of \code{INDEX}.  Note that if the return value has a class (e.g., an
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  object of class \code{"\link{Date}"}) the class is discarded.
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  object of class \code{"\link{Date}"}) the class is discarded.
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  Note that contrary to S, \code{simplify = TRUE} always returns an
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  array, possibly 1-dimensional.
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  \code{simplify = TRUE} always returns an array, possibly 1-dimensional.
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  If \code{FUN} does not return a single atomic value, \code{tapply}
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  If \code{FUN} does not return a single atomic value, \code{tapply}
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  returns an array of mode \code{\link{list}} whose components are the
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  returns an array of mode \code{\link{list}} whose components are the
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  values of the individual calls to \code{FUN}, i.e., the result is a
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  values of the individual calls to \code{FUN}, i.e., the result is a
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  list with a \code{\link{dim}} attribute.
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  list with a \code{\link{dim}} attribute.