# apply

##### Apply Functions Over Array Margins

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 `…`

.

##### Value

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.

##### References

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

##### See Also

`lapply`

and there, `simplify2array`

;
`tapply`

, and convenience functions
`sweep`

and `aggregate`

.

##### Examples

`library(base)`

```
# 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
# }
```

*Documentation reproduced from package base, version 3.6.1, License: Part of R 3.6.1*