\Header{apply}{Apply Functions Over Array Margins}
\keyword{iteration}{apply}
\keyword{array}{apply}
\begin{Description}\relax
Returns a vector or array or list of values obtained by applying a
function to margins of an array.\end{Description}
\begin{Usage}
\begin{verbatim}
apply(X, MARGIN, FUN, ...)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{X}] the array to be used.
\item[\code{MARGIN}] a vector giving the subscripts which the function will
be applied over.
\code{1} indicates rows, \code{2} indicates columns,
\code{c(1,2)} indicates rows and columns.
\item[\code{FUN}] the function to be applied.
In the case of functions like \code{+}, \code{\%*\%}, etc., the
function name must be quoted.
\item[\code{...}] optional arguments to \code{FUN}.
\end{ldescription}
\end{Arguments}
\begin{Value}
If each call to \code{FUN} returns a vector of length \code{n}, then
\code{apply} returns an array of dimension \code{c(n, dim(X)[MARGIN])}
if \code{n > 1}.  If \code{n} equals \code{1}, \code{apply} returns a
vector if \code{MARGIN} has length 1 and an array of dimension
\code{dim(X)[MARGIN]} otherwise.

If the calls to \code{FUN} return vectors of different lengths,
\code{apply} returns a list of length \code{dim(X)[MARGIN]}.\end{Value}
\begin{SeeAlso}\relax
\code{\Link{lapply}}, \code{\Link{tapply}}, and convenience functions
\code{\Link{sweep}} and \code{\Link{aggregate}}.\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
## 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


\end{ExampleCode}
\end{Examples}

