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.

See Also

the convenience function aggregate (using tapply); apply, lapply with its version sapply.

Examples

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

data(warpbreaks)
## 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)

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

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