# [Rd] Sweave processes \Sexpr in commented LaTeX source (2.3.1patched and 2.4.0)

Friedrich Leisch friedrich.leisch at stat.uni-muenchen.de
Wed Sep 20 08:43:31 CEST 2006

>>>>> On Tue, 19 Sep 2006 19:14:39 -0500,
>>>>> Marc Schwartz (MS) wrote:

> Hi all,
> On FC5, using:

>   Version 2.3.1 Patched (2006-08-06 r38829)

> and today's

>   R version 2.4.0 alpha (2006-09-19 r39397)

> with the following .Rnw file:

> \documentclass[10pt]{article}
> \begin{document}

>    This line should print '2': \Sexpr{1 + 1}
> %% This line should NOT print '2': \Sexpr{1 + 1}

> \end{document}

> The \Sexpr in the second line is processed even though the line is
> commented. This results in the following .tex file content (in the case
> of R 2.4.0):

> \documentclass[10pt]{article}
> \usepackage{/home/marcs/R.Files/SourceCode/R-alpha/share/texmf/Sweave}
> \begin{document}

>    This line should print '2': 2
> %% This line should NOT print '2': 2

> \end{document}

> Shouldn't Sweave just generally ignore commented LaTeX code? In
> reviewing Sweave.R I did not see a check for this, so perhaps there are
> circumstances where one wants a \Sexpr in commented LaTeX code
> processed. An example escapes me at the moment however.

Sweave does not parse the LaTeX part of the document at all (which
makes it a lot easier), all it does is looking for its own "magic
strings" and replacing those, wherever they might occur.

If we would start to respect comments, we would need a some kind of
parser for LaTeX, which we currently don't need ... and starting to
parse LaTeX would also open a can of worms of possible requests (what
to do with Sexpr inside verbatim, etc, etc).

Best,
Fritz