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% File src/library/base/man/by.Rd
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% File src/library/base/man/by.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 1995-2014 R Core Team
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% Copyright 1995-2025 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{by}
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\name{by}
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\alias{by}
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\alias{by}
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\alias{by.default}
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\alias{by.default}
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 \item{data}{an \R object, normally a data frame, possibly a matrix.}
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 \item{data}{an \R object, normally a data frame, possibly a matrix.}
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 \item{INDICES}{a factor or a list of factors, each of length
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 \item{INDICES}{a factor or a list of factors, each of length
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   \code{nrow(data)}. For the data frame method, \code{INDICES} can also
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   \code{nrow(data)}. For the data frame method, \code{INDICES} can also
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   be a formula as in the \code{f} argument of the \code{\link{split}}
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   be a formula as in the \code{f} argument of the \code{\link{split}}
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   method for data frames.}
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   method for data frames.}
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 \item{FUN}{a function to be applied to (usually data-frame) subsets of
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 \item{FUN}{a \code{\link{function}} to be applied to (usually data-frame)
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   \code{data}.}
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   subsets of \code{data}.}
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 \item{\dots}{further arguments to \code{FUN}.}
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 \item{\dots}{further arguments to \code{FUN}.}
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 \item{simplify}{logical: see \code{\link{tapply}}.}
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 \item{simplify}{logical: see \code{\link{tapply}}.}
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}
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}
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\details{
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\details{
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  A data frame is split by row into data frames
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  A data frame is split (calling \code{\link{split}()} by row into data frames
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  subsetted by the values of one or more factors, and function
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  subsetted by the values of one or more factors, and function
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  \code{FUN} is applied to each subset in turn.
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  \code{FUN} is applied to each subset in turn.
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  For the default method, an object with dimensions (e.g., a matrix) is
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  For the default method, an object with dimensions (e.g., a matrix) is
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  coerced to a data frame and the data frame method applied.  Other
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  coerced to a data frame and the data frame method applied.  Other
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            by(warpbreaks, tension,
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            by(warpbreaks, tension,
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               function(x) lm(breaks ~ wool, data = x)))
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               function(x) lm(breaks ~ wool, data = x)))
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sapply(tmp1, coef)
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sapply(tmp1, coef)
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## another way
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## another way
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tmp2 <- by(warpbreaks, ~ tension,
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by(warpbreaks, ~ tension,
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           with, coef(lm(breaks ~ wool)))
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   with, coef(lm(breaks ~ wool))) |> array2DF(simplify = TRUE)
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array2DF(tmp2, simplify = TRUE)
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}
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}
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\keyword{iteration}
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\keyword{iteration}
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\keyword{category}
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\keyword{category}