\documentclass[a4paper]{article}

\title{Finer control of weaving and tangling}
\author{Vincent Goulet and R Core Team}

\begin{document}

\maketitle

The following code chunk contains a deliberate error, say for
didactic purposes. Using the option \texttt{eval=FALSE} avoids an
error with Sweave, but the chunk is commented out by Stangle.
<<main, eval=FALSE>>=
2 + does.not.exist
@

With \texttt{ignore.on.weave=TRUE} (or \texttt{weave=FALSE}), the
following chunk is neither parsed nor evaluated by Sweave (hence
missing from the PDF file). Therefore, the chunk may contain
unparsable R code. However, the code is extracted as usual by
Stangle, without being commented out.
<<main, ignore.on.weave=TRUE>>=
2 a
@

We remove the chunk separator before this next chunk. It will
appear immediately after the previous one in the tangled script.
<<main, chunk.sep=FALSE>>=
pi
@

Pairing \texttt{ignore.on.weave=TRUE} with the option
\texttt{extension} allows to keep, in a single source file, R
code as well as text or code in other programming languages that
should not be evaluated by Sweave.
<<hello, extension=sh, prefix=FALSE, ignore.on.weave=TRUE>>=
#!/bin/sh
echo "Hello, World!"
exit 0
@
<<README, extension=FALSE, prefix=FALSE, ignore.on.weave=TRUE>>=
Hello, World!
@

Finally, a chunk processed normally by Sweave, but ignored by
Stangle. This is convenient with \texttt{split = TRUE} to avoid
littering the output directory with unimportant scripts.
<<ignore.on.tangle=TRUE>>=
x <- rnorm(10)
@

\end{document}