by {base}R Documentation

Apply a Function to a Data Frame split by Factors

Description

Function by is an object-oriented wrapper for tapply applied to data frames.

Usage

by(data, INDICES, FUN, ...)

Arguments

data an R object, normally a data frame, possibly a matrix.
INDICES a factor or a list of factors, each of length nrow(x).
FUN a function to be applied to data frame subsets of x.
... further arguments to FUN.

Details

A data frame is split by row into data frames subsetted by the values of one or more factors, and function FUN is applied to each subset in term.

Object data will be coerced to a data frame by default.

Value

A list of class "by", giving the results for each subset.

See Also

tapply

Examples

require(stats)
data(warpbreaks)
attach(warpbreaks)
by(warpbreaks[, 1:2], tension, summary)
by(warpbreaks[, 1], list(wool=wool, tension=tension), summary)
by(warpbreaks, tension, function(x) lm(breaks ~ wool, data=x))

## now suppose we want to extract the coefficients by group
tmp <- by(warpbreaks, tension, function(x) lm(breaks ~ wool, data=x))
sapply(tmp, coef)

detach("warpbreaks")
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