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.

Usage

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

Arguments

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. In the case of functions like +, %*%, etc., the function name must be quoted.
...	optional arguments to FUN.

Details

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.

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 the calls to FUN return vectors of different lengths, apply returns a list of length dim(X)[MARGIN].

See Also

lapply, 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)) # 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

---