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\name{function}
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\title{Function Definition}
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\usage{
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function( arglist ) expr
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return(value)
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}
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\alias{function}
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\alias{return}
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\description{
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  These functions provide the base mechanisms for defining
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  new functions in the \R language.
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}
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\arguments{
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    \item{arglist}{Empty or one or more name or name=expression terms.}
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    \item{value}{An expression.}
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}
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\details{
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  In \R (unlike S) the names in an argument list cannot be quoted
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  non-standard names.
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  If \code{value} is missing, \code{NULL} is returned.  If it is a
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  single expression, the value of the evaluated expression is returned.
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  If the end of a function is reached without calling \code{return}, the
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  value of the last evaluated expression is returned.
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}
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\section{Warning}{
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  Prior to \R 1.8.0, \code{value} could be a series of non-empty
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  expressions separated by commas.  In that case the value
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  returned is a list of the evaluated expressions, with names set to
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  the expressions where these are the names of \R objects. That is,
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  \code{a=foo()} names the list component \code{a} and gives it value
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  the result of evaluating \code{foo()}.
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  This has been deprecated (and a warning is given), as it was never
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  documented in S, and whether or not the list is named differs by S versions.
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}
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\seealso{
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  \code{\link{args}} and \code{\link{body}} for accessing the arguments
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  and body of a function.
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  \code{\link{debug}} for debugging; \code{\link{invisible}} for
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  \code{return(.)}ing \emph{invisibly}.
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}
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\references{
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  Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
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  \emph{The New S Language}.
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  Wadsworth \& Brooks/Cole.
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}
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\examples{
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norm <- function(x) sqrt(x\%*\%x)
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norm(1:4)
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## An anonymous function:
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(function(x,y){ z <- x^2 + y^2; x+y+z })(0:7, 1)
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}
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\keyword{programming}