How to format output of results using output hooks in knitr?

Question

I have a Rnw file a.Rnw with the following contents:

\documentclass{beamer}
\begin{document}
\begin{frame}[fragile]
<<>>=
1+1
@
\end{frame}
\end{document}

I produce a pdf by

Rscript -e 'knitr::knit("a.Rnw")'
pdflatex a.tex

The output in a.pdf looks like this:

How do I get the output to look like this:

R> 1+1
## [1] 2

That is, how do I put R> in front of the R code and remove the blank line between code and output?

Answer 1

To show R> in front of all R commands, I set the R prompt using options and tell knitr to show the prompt using opts_chunk (code at bottom of answer).

Getting rid of the new line is a bit trickier because the R code and R output in the generated tex file looks like this:

\begin{alltt}
\hlstd{R> }\hlnum{1}\hlopt{+}\hlnum{1}
\end{alltt}
\begin{verbatim}
## [1] 2
\end{verbatim}

So the newline between R code and its output is not generated explicitly by knitr, but is due to a new paragraph being started between \end{alltt} and \begin{verbatim}. The verbatim environment adds above and below it the current value of \topsep (see here). So I patch the knitrout environment such that this variable is locally set to 0pt. Here is the new version of a.Rnw:

\documentclass{beamer}

% reduce whitespace between R code and R output
\let\oldknitrout\knitrout
\renewenvironment{knitrout}{
  \begin{oldknitrout}
  \topsep=0pt
}{
  \end{oldknitrout}
}

% show R> prompt before R commands
<<r setup, echo=FALSE>>=
options(prompt='R> ')
knitr::opts_chunk$set(prompt=TRUE) 
@

\begin{document}
\begin{frame}[fragile]
<<>>=
1+1
@
\end{frame}
\end{document}

and the output looks like this: