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

`apply(X, MARGIN, FUN, …)`

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`

.

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 `0`

, 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.

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 `…`

.

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

`lapply`

and there, `simplify2array`

;
`tapply`

, and convenience functions
`sweep`

and `aggregate`

.

# NOT RUN { ## 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) ## keeping named dimnames names(dimnames(x)) <- c("row", "col") x3 <- array(x, dim = c(dim(x),3), dimnames = c(dimnames(x), list(C = paste0("cop.",1:3)))) identical(x, apply( x, 2, identity)) identical(x3, apply(x3, 2:3, identity)) # } # NOT RUN { ##- 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 # }