tapply {base}R Documentation

Apply a Function Over a “Ragged” Array

Description

Apply a function to each cell of a ragged array, that is to each (non-empty) group of values given by a unique combination of the levels of certain factors.

Usage

tapply(X, INDEX, FUN = NULL, ..., simplify = TRUE)

Arguments

X an atomic object, typically a vector.
INDEX list of factors, each of same length as X.
FUN the function to be applied. In the case of functions like +, %*%, etc., the function name must be quoted. If FUN is NULL, tapply returns a vector which can be used to subscript the multi-way array tapply normally produces.
... optional arguments to FUN.
simplify If FALSE, tapply always returns an array of mode "list". If TRUE (the default), then if FUN always returns a scalar, tapply returns an array with the mode of the scalar.

Value

When FUN is present, tapply calls FUN for each cell that has any data in it. If FUN returns a single atomic value for each cell (e.g., functions mean or var) and when simplify is TRUE, tapply returns a multi-way array containing the values. The array has the same number of dimensions as INDEX has components; the number of levels in a dimension is the number of levels (nlevels()) in the corresponding component of INDEX.
Note that contrary to S, simplify = TRUE always returns an array, possibly 1-dimensional.
If FUN does not return a single atomic value, tapply returns an array of mode list whose components are the values of the individual calls to FUN, i.e., the result is a list with a dim attribute.
Note that optional arguments to FUN supplied by the ... argument are not divided into cells. It is therefore inappropriate for FUN to expect additional arguments with the same length as X.

References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

See Also

the convenience functions by and aggregate (using tapply); apply, lapply with its versions sapply and mapply.

Examples

require(stats)
groups <- as.factor(rbinom(32, n = 5, p = .4))
tapply(groups, groups, length) #- is almost the same as
table(groups)

## contingency table from data.frame : array with named dimnames
tapply(warpbreaks$breaks, warpbreaks[,-1], sum)
tapply(warpbreaks$breaks, warpbreaks[, 3, drop = FALSE], sum)

n <- 17; fac <- factor(rep(1:3, len = n), levels = 1:5)
table(fac)
tapply(1:n, fac, sum)
tapply(1:n, fac, sum, simplify = FALSE)
tapply(1:n, fac, range)
tapply(1:n, fac, quantile)

## example of ... argument: find quarterly means
tapply(presidents, cycle(presidents), mean, na.rm = TRUE)

ind <- list(c(1, 2, 2), c("A", "A", "B"))
table(ind)
tapply(1:3, ind) #-> the split vector
tapply(1:3, ind, sum)

[Package base version 2.1.0 Index]