R: Apply a Function to a Data Frame Split by Factors

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, ..., simplify = TRUE)

Arguments

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

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 turn.

Object data will be coerced to a data frame by the default method, but if this results in a 1-column data frame, the objects passed to FUN are dropped to a subsets of that column.

Value

An object of class "by", giving the results for each subset. This is always a list if simplify is false, otherwise a list or array (see tapply).

Examples

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

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

[Package base version 2.15.1 Index]