Apply a Function Over a Ragged Array

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.

iteration, category
tapply(X, INDEX, FUN = NULL, …, default = NA, simplify = TRUE)

an R object for which a split method exists. Typically vector-like, allowing subsetting with [.


a list of one or more factors, each of same length as X. The elements are coerced to factors by as.factor.


a function (or name of a function) to be applied, or NULL. In the case of functions like +, %*%, etc., the function name must be backquoted or 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: the Note section.


(only in the case of simplification to an array) the value with which the array is initialized as array(default, dim = ..). Before R 3.4.0, this was hard coded to array()'s default NA. If it is NA (the default), the missing value of the answer type, e.g. NA_real_, is chosen (as.raw(0) for "raw"). In a numerical case, it may be set, e.g., to FUN(integer(0)), e.g., in the case of FUN = sum to 0 or 0L.


logical; if FALSE, tapply always returns an array of mode "list"; in other words, a list with a dim attribute. If TRUE (the default), then if FUN always returns a scalar, tapply returns an array with the mode of the scalar.


If FUN is not NULL, it is passed to match.fun, and hence it can be a function or a symbol or character string naming a function.


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 such cell (e.g., functions mean or var) and when simplify is TRUE, tapply returns a multi-way array containing the values, and NA for the empty cells. 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 if the return value has a class (e.g., an object of class "Date") the class is discarded.

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.

When there is an array answer, its dimnames are named by the names of INDEX and are based on the levels of the grouping factors (possibly after coercion).

For a list result, the elements corresponding to empty cells are NULL.


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.


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.

  • tapply
library(base) # NOT RUN { require(stats) groups <- as.factor(rbinom(32, n = 5, prob = 0.4)) 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_len(1:3, n), levels = 1:5) table(fac) tapply(1:n, fac, sum) tapply(1:n, fac, sum, default = 0) # maybe more desirable tapply(1:n, fac, sum, simplify = FALSE) tapply(1:n, fac, range) tapply(1:n, fac, quantile) tapply(1:n, fac, length) ## NA's tapply(1:n, fac, length, default = 0) # == table(fac) # } # NOT RUN { ## 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) ## Some assertions (not held by all patch propsals): nq <- names(quantile(1:5)) stopifnot( identical(tapply(1:3, ind), c(1L, 2L, 4L)), identical(tapply(1:3, ind, sum), matrix(c(1L, 2L, NA, 3L), 2, dimnames = list(c("1", "2"), c("A", "B")))), identical(tapply(1:n, fac, quantile)[-1], array(list(`2` = structure(c(2, 5.75, 9.5, 13.25, 17), .Names = nq), `3` = structure(c(3, 6, 9, 12, 15), .Names = nq), `4` = NULL, `5` = NULL), dim=4, dimnames=list(as.character(2:5))))) # }
Documentation reproduced from package base, version 3.6.2, License: Part of R 3.6.2

Community examples

mjchen.gene@gmail.com at May 13, 2019 base v3.6.0

This example is originally given in [An Introduction to R](https://cran.r-project.org/doc/manuals/r-release/R-intro.html). ```{r} statef <- c("tas", "sa", "qld", "nsw", "nsw", "nt", "wa", "wa", "qld", "vic", "nsw", "vic", "qld", "qld", "sa", "tas", "sa", "nt", "wa", "vic", "qld", "nsw", "nsw", "wa", "sa", "act", "nsw", "vic", "vic", "act") incomes <- c(60, 49, 40, 61, 64, 60, 59, 54, 62, 69, 70, 42, 56, 61, 61, 61, 58, 51, 48, 65, 49, 49, 41, 48, 52, 46, 59, 46, 58, 43) (incmeans <- tapply(incomes, statef, mean)) ```