# apply {base}

### Description

Returns a vector or array or list of values obtained by applying a function to margins of an array or matrix.

### Usage

apply(X, MARGIN, FUN, ...)

### Arguments

- X
- an array, including a matrix.
- MARGIN
- a vector giving the subscripts which the function will be applied over. E.g., for a matrix
`1`

indicates rows,`2`

indicates columns,`c(1, 2)`

indicates rows and columns. Where`X`

has named dimnames, it can be a character vector selecting dimension names. - FUN
- the function to be applied: see ‘Details’. In the case of functions like
`+`

,`%*%`

, etc., the function name must be backquoted or quoted. - ...
- optional arguments to
`FUN`

.

### Details

If `X`

is not an array but an object of a class with a non-null `dim`

value (such as a data frame), `apply`

attempts to coerce it to an array via `as.matrix`

if it is two-dimensional (e.g., a data frame) or via `as.array`

.

`FUN`

is found by a call to `match.fun`

and typically is either a function or a symbol (e.g. a backquoted name) or a character string specifying a function to be searched for from the environment of the call to `apply`

.

Arguments in `...`

cannot have the same name as any of the other arguments, and care may be needed to avoid partial matching to `MARGIN`

or `FUN`

. In general-purpose code it is good practice to name the first three arguments if `...`

is passed through: this both avoids partial matching to `MARGIN`

or `FUN`

and ensures that a sensible error message is given if arguments named `X`

, `MARGIN`

or `FUN`

are passed through `...`

.

### Values

If each call to `FUN`

returns a vector of length `n`

, then `apply`

returns an array of dimension `c(n, dim(X)[MARGIN])`

if `n > 1`

. If `n`

equals `1`

, `apply`

returns a vector if `MARGIN`

has length 1 and an array of dimension `dim(X)[MARGIN]`

otherwise. If `n`

is ` `

, the result has length 0 but not necessarily the ‘correct’ dimension.

If the calls to `FUN`

return vectors of different lengths, `apply`

returns a list of length `prod(dim(X)[MARGIN])`

with `dim`

set to `MARGIN`

if this has length greater than one.

In all cases the result is coerced by `as.vector`

to one of the basic vector types before the dimensions are set, so that (for example) factor results will be coerced to a character array.

### References

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

### Examples

## Compute row and column sums for a matrix: x <- cbind(x1 = 3, x2 = c(4:1, 2:5)) dimnames(x)[[1]] <- letters[1:8] apply(x, 2, mean, trim = .2) col.sums <- apply(x, 2, sum) row.sums <- apply(x, 1, sum) rbind(cbind(x, Rtot = row.sums), Ctot = c(col.sums, sum(col.sums))) stopifnot( apply(x, 2, is.vector)) ## Sort the columns of a matrix apply(x, 2, sort) ##- function with extra args: cave <- function(x, c1, c2) c(mean(x[c1]), mean(x[c2])) apply(x, 1, cave, c1 = "x1", c2 = c("x1","x2")) ma <- matrix(c(1:4, 1, 6:8), nrow = 2) ma apply(ma, 1, table) #--> a list of length 2 apply(ma, 1, stats::quantile) # 5 x n matrix with rownames stopifnot(dim(ma) == dim(apply(ma, 1:2, sum))) ## Example with different lengths for each call z <- array(1:24, dim = 2:4) zseq <- apply(z, 1:2, function(x) seq_len(max(x))) zseq ## a 2 x 3 matrix typeof(zseq) ## list dim(zseq) ## 2 3 zseq[1,] apply(z, 3, function(x) seq_len(max(x))) # a list without a dim attribute

Documentation reproduced from R 3.0.2. License: GPL-2.