How Do you export only R code from Rnw file

Question

I am working with knitr, polymode, ess and I would like to export just the R code from my .Rnw into a .R file. A basic file might look something like this:

\documentclass[a4paper]{article}

My Rnw file 

<<>>=
##This is some R code 
1+1 
@

\end{document}

In the end, I would like an R file with just

##This is some R code
1+1

Answer 1

In R, tangling is implemented via knitr::purl and utils::Stangle.

ESS provides an interface to these commands. See the variable ess-swv-processor and the ess-swv-weave function (bound to M-n s in Rnw-mode).