\documentclass[a4paper]{article}

\title{Using objects as chunk options}
\author{Vincent Goulet and R Core Team}

\begin{document}

\maketitle

The following code chunk checks if a package \textbf{foo} is
available on the system, and defines two dimensions.
<<>>=
hasfoo <- requireNamespace("foo", quietly = TRUE)
ht <- 5
wd <- (1 + sqrt(5))/2 * ht
@

Sweave will evaluate the following code chunk only if the package
is available (which should normally not be the case).
<<eval=hasfoo>>=
foo::foo(42)
@

We may also redo the boxplot of the \texttt{airquality} dataset
in proportions approximately equal to the golden ratio.
\begin{center}
<<echo=FALSE, fig=TRUE, width=wd, height=ht>>=
data(airquality, package="datasets")
library("graphics")
boxplot(Ozone ~ Month, data = airquality)
@
\end{center}

\end{document}