| apply {base} | R Documentation |
Returns a vector or array or list of values obtained by applying a function to margins of an array.
apply(X, MARGIN, FUN, ...)
X |
the array to be used. |
MARGIN |
a vector giving the subscripts which the function will
be applied over.
1 indicates rows, 2 indicates columns,
c(1,2) indicates rows and columns. |
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 X is not an array but has a dimension attribute,
apply attempts to coerce it to an array via as.matrix
if it is two-dimensional (e.g., data frames) 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.
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.
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.
lapply, tapply, and convenience functions
sweep and aggregate.
## 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)) # not ok in R <= 0.63.2
## 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), nr = 2)
ma
apply(ma, 1, table) #--> a list of length 2
apply(ma, 1, quantile)# 5 x n matrix with rownames
stopifnot(dim(ma) == dim(apply(ma, 1:2, sum)))## wasn't ok before R 0.63.1
## Example with different lengths for each call
z <- array(1:24, dim=2:4)
zseq <- apply(z, 1:2, function(x) seq(length=max(x)))
zseq ## a 2 x 3 matrix
typeof(zseq) ## list
dim(zseq) ## 2 3
zseq[1,]
apply(z, 3, function(x) seq(length=max(x))) # a list without a dim attribute