\HeaderA{missing}{Does a Formal Argument have a Value?}{missing}
\keyword{programming}{missing}
\begin{Description}\relax
\code{missing} can be used to test whether a value was specified
as an argument to a function.
\end{Description}
\begin{Usage}
\begin{verbatim}
missing(x)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{x}] a formal argument.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
\code{missing(x)} is only reliable if \code{x} has not been altered
since entering the function: in particular it will \emph{always}
be false after \code{x <- match.arg(x)}. 

The example shows how a plotting function
can be written to work with either a pair of vectors
giving x and y coordinates of points to be plotted
or a single vector giving y values to be plotted
against their indexes.

Currently \code{missing} can only be used in the immediate body of
the function that defines the argument, not in the body of a nested
function or a \code{local} call.  This may change in the future.
\end{Details}
\begin{References}\relax
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
\emph{The New S Language}.
Wadsworth \& Brooks/Cole.

Chambers, J. M. (1998)
\emph{Programming with Data. A Guide to the S Language}.
Springer.
\end{References}
\begin{SeeAlso}\relax
\code{\LinkA{substitute}{substitute}} for argument expression;
\code{\LinkA{NA}{NA}} for missing values in data.
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
myplot <- function(x,y) {
                if(missing(y)) {
                        y <- x
                        x <- 1:length(y)
                }
                plot(x,y)
        }
\end{ExampleCode}
\end{Examples}

