| apply {base} | R Documentation |
Apply Functions Over Array Margins
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, ..., simplify = TRUE)
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 |
FUN |
the function to be applied: see ‘Details’.
In the case of functions like |
... |
optional arguments to |
simplify |
a logical indicating whether results should be simplified if possible. |
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,
and simplify is TRUE,
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,
or if simplify is FALSE,
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
## 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))
##- 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