\Header{expression}{Unevaluated Expressions}
\alias{as.expression}{expression}
\methalias{as.expression.default}{expression}
\alias{is.expression}{expression}
\keyword{programming}{expression}
\keyword{dplot}{expression}
\begin{Description}\relax
Creates or tests for objects of mode \code{"expression"}.
\end{Description}
\begin{Usage}
\begin{verbatim}
expression(...)

is.expression(x)
as.expression(x, ...)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{...}] valid \R{} expressions.
\item[\code{x}] an arbitrary \R{} object.
\end{ldescription}
\end{Arguments}
\begin{Value}
\code{expression} returns a vector of mode \code{"expression"}
containing its arguments as unevaluated \dQuote{\link{call}s}.

\code{is.expression} returns \code{TRUE} if \code{expr} is an
expression object and \code{FALSE} otherwise.

\code{as.expression} attempts to coerce its argument into an
expression object.
\end{Value}
\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{SeeAlso}\relax
\code{\Link{call}},
\code{\Link{eval}},
\code{\Link{function}}.
Further,
\code{\Link{text}} and
\code{\Link{legend}}
for plotting math expressions.
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
length(ex1 <- expression(1+ 0:9))# 1
ex1
eval(ex1)# 1:10

length(ex3 <- expression(u,v, 1+ 0:9))# 3
mode(ex3 [3]) # expression
mode(ex3[[3]])# call
rm(ex3)
\end{ExampleCode}
\end{Examples}

