Sweave processes \Sexpr in commented LaTeX source (2.3.1patched and 2.4.0)

On Wed, 20 Sep 2006 10:07:26 -0700,
Seth Falcon (SF) wrote:

  > "Antonio, Fabio Di Narzo" <antonio.fabio at gmail.com> writes:

  >> 2006/9/20, Seth Falcon <sfalcon at fhcrc.org>:

  >>> Peter Dalgaard <p.dalgaard at biostat.ku.dk> writes:

  >>> > ..not to mention TeX comments inside Sexpr (e.g. %*%...). Skipping
  >>> > lines with % as the first character might be a viable compromise
  >>> > though.

  >>> 
  >>> +1.  You could probably ignore any lines where the first
  >>> non-whitespace char is '%'.  But if that seems to risky, then only
  >>> recognizing first-char-is-% seems a worthwhile heuristic.
  >>> 
  >>> Another place where this has bitten people is when they do:
  >>> 
  >>> %\usepackage{Sweave}
  >>> 
  >>> Sweave picks that up and doesn't insert the usepackage line itself.

  >> 
  >> I've found that extremely useful for sweaving Stex files which aren't
  >> "master" files (i.e., only files to include in a main latex file).
  >> Inserting '\usepackage{Sweave}' in each would result in a latex error.
  >> So commenting it out is a useful workaround.

  > Commenting it out _and_ having Sweave see it?

Yes, I actually do that quite often, e.g., when I have my own
definitions of the Sinput/Soutput/... environments in the document
preamble and don't want to use any Sweave style file.

Note that we recently had a thread on the \usepackage{Sweave} path
insertion problems in windows and as a result I will stop being
special about it at all, i.e., users will have to put a
\usepackage{Sweave} into their documents, and take care that latex
finds a version of it.

The thread was end of August and I didn't do it for 2.4 because I
didn't want to break all vignettes that close to a release. But I will
do for the 2.5 series once 2.4.0 is released.

Ad evaluation of \Sexpr{}: That can be considered a bug, hence I could
try a fix even in feature freeze. But as I will be offline for a week
starting tomorrow this will also have to wait until the 2.5 series (or
2.4.1, depending on the change).

My favorite would not be a separate option eval.Sexpr but eveluate
only when the global option for eval is true. Then an

	\SweaveOpts{eval=false}

at any place in a document would stop all evalualtions, be it code
chunks (which do not override the default) or Sexpr. If somebody has
good useage for a separate option I could be convinced to have it,
otherwise I'll go for \Sexpr{} listening to the eval option.