Putting line number for R code with knitr

I wonder if there is any function to put line numbers with knitr in .Rnw. I found this discussion and some documents (now removed from the web) but could not find the way to put line numbers.

This solution uses the LaTeX listings package to create line numbers. I can only get them to work by accumulating across all code chunks, but I imagine there is a similar solution that will enumerate lines only within each chunk. Here's the .Rnw source:

\documentclass{article}
\usepackage{listings}
\begin{document}

<<setup, echo=FALSE>>=
knit_hooks$set(source = function(x, options) {
    paste("\\begin{lstlisting}[numbers=left, firstnumber=last]\n", x, 
        "\\end{lstlisting}\n", sep = "")
})
@

<<a, results='hold'>>=
1:2
3:4
5:6
@

<<b>>=
"test1"
"test2"
"test3"
@

\end{document}

The key parts of this are in the source hook, which is basically copied from here. The firstnumber=last tells listings to accumulate line numbers across listings. Without it, all lines are numbered 1 because knitr is putting each code line in its own listing.

And here's the result:

If you want each code block to start numbering from 1, add a hook to reset the counter:

knit_hooks$set(reset = function(before, options, envir){
if(before){
    return("\\setcounter{lstnumber}{1}")
}
})

and then use reset=TRUE to activate the hook in each chunk you want:

<<a, results='hold', reset=TRUE>>=
1:2
3:4
@