\HeaderA{NULL}{The Null Object}{NULL}
\aliasA{as.null}{NULL}{as.null}
\methaliasA{as.null.default}{NULL}{as.null.default}
\aliasA{is.null}{NULL}{is.null}
\keyword{attribute}{NULL}
\keyword{manip}{NULL}
\keyword{list}{NULL}
\keyword{sysdata}{NULL}
\begin{Description}\relax
\code{NULL} represents the null object in R.
NULL is used mainly to represent the lists with zero length,
and is often returned by expressions and functions whose
value is undefined.

\code{as.null} ignores its argument and returns the value \code{NULL}.

\code{is.null} returns \code{TRUE} if its argument
is \code{NULL} and \code{FALSE} otherwise.
\end{Description}
\begin{Usage}
\begin{verbatim}
NULL
as.null(x, ...)
is.null(x)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{x}] an object to be tested or coerced.
\item[\code{...}] ignored.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
\code{is.null} is generic: you can write methods to handle
specific classes of objects, see \LinkA{InternalMethods}{InternalMethods}.
\end{Details}
\begin{References}\relax
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
\emph{The New S Language}.
Wadsworth \& Brooks/Cole.
\end{References}
\begin{Examples}
\begin{ExampleCode}
is.null(list())    # FALSE (on purpose!)
is.null(integer(0))# F
is.null(logical(0))# F
as.null(list(a=1,b='c'))
\end{ExampleCode}
\end{Examples}

