| 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.
In the case of functions like +, %*%, etc., the
function name must be 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.
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.
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