Apply a Function to a Data Frame Split by Factors

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

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

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.


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.

  • by
  • by.default
library(base) 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)
Documentation reproduced from package base, version 3.0.3, License: Part of R 3.0.3

Community examples at Dec 24, 2018 base v3.5.2

1. This example uses `mtcars` dataframe. 2. Following r snippet first displays the structure of the `mtcars` dataframe and the uses `by` to group the rows of the dataframe by `cyl`(cylinders), and compute sum of `mpg` (miles per galon) for each group. ``` str(mtcars) by(data=mtcars$mpg, INDICES=mtcars$cyl, FUN=sum, na.rm=TRUE) ``` The output will be as follows: ``` mtcars$cyl: 4 [1] 293.3 ------------------------------------------------------------ mtcars$cyl: 6 [1] 138.2 ------------------------------------------------------------ mtcars$cyl: 8 [1] 211.4 ``` In addition to bulit in functions like `sum`, `by` also works for user defined functions e.g. in the following snippet `range_diff` a user defined function first calculates the `range` and then the `diff` of the argument. ``` range_diff <- function(x){diff(range(x))} by(data=mtcars$mpg, INDICES=mtcars$cyl, FUN=range_diff) ``` Calling `range_diff` in `by` returns the following output. ``` mtcars$cyl: 4 [1] 12.5 ------------------------------------------------------------ mtcars$cyl: 6 [1] 3.6 ------------------------------------------------------------ mtcars$cyl: 8 [1] 8.8 ```