[R] (S|odf)weave : how to intersperse (\LaTeX{}|odf) comments in source code ? Delayed R evaluation ?

Emmanuel Charpentier emm.charpentier at free.fr
Sat Dec 11 23:58:10 CET 2010

Dear list,

Inspired by the original Knuth tools, and for paedaogical reasons, I wish 
to produce a document presenting some source code with interspersed 
comments in the source (see Knuth's books rendering TeX and metafont 
sources to see what I mean).

I seemed to remember that a code chunk could be defined piecewise, like in


<<Chunk1, eval=FALSE, echo=TRUE>>=

Some other comments...

<<Chunk2, eval=FALSE, echo=TRUE>>=

And finally,

<<Chunk3, eval=TRUE, echo=TRUE>>=

That works ... as long as SomeCode, MoreCode and EndOfTheCode are self-
standing pieces of R code, but *not* code fragments. You can *not* 
intersperse comments in, say, a function body, or  local() environment 
this way : when Sweaving, *R* complains of an incomplete source (makes 
noise about an unexpected end of input at the end of Chunk1, IIRC, and 
never sees Chunk2).

I hoped that Sweave's "alternative" syntax could offer a way out : no 
such luck.

There seems to be no way to delay R evaluation of a R chunk passed by 
Sweave ; at least, the "eval=FALSE" option of chunk declaration is not 
sufficient for that.

Am I missing something in the Sweave nd odfWeve documentations (that I 
read till I grew green and moldy) ? Or does this require a fundamental 
change in the relevant Sweave drivers ?

Can you suggest alternative ways of doing what I mean to do ? The only 
workaround I found is to paste a second copy of my code in a \verbatim 
environment (or, in the case of odfWeave, in the "text" part), and spice 
it with \end{verbatim} comments.. \begin{verbatim} chunks. This way, I 
lose any guarantee of consistency between commented text and effective 

Any other idea ?

					Emmanuel Charpentier

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