Get Emacs to ignore contents of \Sexpr{} command in Sweave document to prevent incorrect $-based syntax highlighting

Question

When editing an Sweave document in LaTeX (using the Noweb mode), Emacs knows to "ignore" code that is in <<>>= blocks. However, for interstitial \Sexpr{} blocks, this isn't the case. Given that R references by columns via '$' and LaTeX uses $ to set-off equations, these \Sexpr{} blocks often break the syntax highlighting, like so:

I have a very rudimentary understanding the elisp & Emacs syntax highlighting, but my hope is that it might be possible to add something to .emacs that will disable any parsing/$ detection within \Sexpr{}'s.

Answer 1

I thought emacs with ESS has correct syntax highlighting for Sweave?

Anyway, the easiest "fix" is to just not use the $ operator but [[ instead. For example:

foo$p.value
foo[['p.value']]

Should give the same result. I think foo$p.value is just short for foo[["p.value",exact=FALSE]]

Answer 2

I don't have a fix either, but I'll pass along my workaround, which is to never (well, rarely) do any processing in \Sexpr chunks but instead to store things I want to use in \Sexpr in variables, and to do so in the same chunk I do the main calculations in.

<<echo=FALSE, results=hide>>=
t1 <- chisq.test(someVar)
p1 <- formatC(t1$p.value, format="%f", digits=2)
@

\dots with a $p$-value of \Sexpr{p1}.

While there are some downsides to this, I find it helps me to better keep track of what I want to present, and how I want to present it.

As an aside, consider using formatC instead of round as it can keep significant zeros (ie, 0.10 instead of 0.1).

Answer 3

I have no good answer for you as I am not an Emacs hacker, so I usually do one of two things:

• Either add a simple % $ comment at the of the line to "close" the math expression from $ to $,

• Or rewrite the expression to not use $-based subsetting:
  round(as.numeric(chisq.test(someVar)["p.value"]), 2).