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 (usually 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.

For the default method, an object with dimensions (e.g., a matrix) is coerced to a data frame and the data frame method applied. Other objects are also coerced to a data frame, but `FUN` is applied separately to (subsets of) each column of the data frame.

### 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`).

### See Also

`tapply`, `simplify2array`. `ave` also applies a function block-wise.

### 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 3.1.2 Index]