Q:

Sweave,R,Beamer : How to convert the LaTex text in an Rnw file to R comments?

Say I have a .Rnw file containing the usual LaTex mixed in with R code chunks. (I'm especially interested in converting a .Rnw slides document, but this question applies to any .Rnw document). Now I want to convert this to a file which contains all of the R code, plus all of the text that would normally be generated by LaTex, as R comments. In other words, the functionality I want is similar to what Stangle() does, but I also want all the text part of the LaTex converted to plain text that's commented out in the resulting .R file. 
This would be a very convenient way to automatically generate a commented R file that's easy to look at in your favorite syntax-highlighting editor (e.g. emacs). This might not sound like a great idea for an Sweave document that's a long article with only a bit of R code, but it starts to look appealing when the .Rnw document is actually a slide presentation (e.g. using beamer) -- then the text portion of the slides would make perfect comments for the R code.  
Anyone have any ideas on how to do this? Thanks in advance.

A:

Here is one approach using regex. There are still some issues that remain, and I will maintain a list which will be updated with resolutions.
# READ LINES FROM RNW FILE
lines <- readLines('http://users.stat.umn.edu/~charlie/Sweave/foo.Rnw')

# DETECT CODE LINES USING SWEAVE CHUNK DEFINITIONS
start_chunk <- grep("^<<.*=$", lines)
end_chunk   <- grep("^@" , lines)
r_lines     <- unlist(mapply(seq, start_chunk + 1, end_chunk - 1))

# COMMENT OUT NON CODE LINES AND WRITE TO FILE
lines[-r_lines] <- paste("##", lines[-r_lines])
writeLines(lines, con='codefile.R')

ISSUES REMAINING:

Does not deal well with chunks called inside other chunks using <<chunk_name>>

