Is it possible to include a Sexpr before the expression has been evaluated in Sweave / R ?

Question

Hello,

I'm writing a Sweave document, and I want to include a small section that details the R and package versions, platofrms and how long ti took to evalute the doucment, however, I want to put this in the middle of the document !

I was using a \Sexpr{elapsed} to do this (which didn't work), but thought if I put the code printing elapsed in a chunk that evaluates at the end, I could then include the chunk half way through, which also fails.

My document looks something like this

% 
\documentclass[a4paper]{article}
\usepackage[OT1]{fontenc}
\usepackage{longtable}
\usepackage{geometry}
\usepackage{Sweave}
\geometry{left=1.25in, right=1.25in, top=1in, bottom=1in}
\begin{document}

<<label=start, echo=FALSE, include=FALSE>>=
startt<-proc.time()[3]
@ 
Text and Sweave Code in here
% 
This document was created on \today, with \Sexpr{print(version$version.string)} running
 on a \Sexpr{print(version$platform)} platform. It took approx sec to process.
<<>>=
    <<elapsed>>
@ 
More text and Sweave code in here
<<label=bye, include=FALSE, echo=FALSE>>= 
odbcCloseAll()
endt<-proc.time()[3]
elapsedtime<-as.numeric(endt-startt)
@ 
<<label=elapsed, include=FALSE, echo=FALSE>>=
print(elapsedtime)
@ 
\end{document}

But this doesn't seem to work (amazingly !)

Does anyone know how I could do this ?

Thanks

Paul.

Answer 1

This works just fine for me:

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

<<label=start, echo=FALSE, include=FALSE>>=
startt<-proc.time()[3]
@

Text and Sweave Code in here

This document was created on \today, with
\Sexpr{print(version$version.string)}.

<<results=hide,echo=FALSE>>=
Sys.sleep(2)  # instead of real work
@

More text and Sweave code in here

<<label=bye, include=FALSE, echo=FALSE>>=
endt<-proc.time()[3]
elapsedtime<-as.numeric(endt-startt)
@

It took approx \Sexpr{elapsedtime} seconds to process.

\end{document}

I had to remove the version string inside the \Sexp{} as I get an underscore with via x86_64 which then upsets LaTeX. Otherwise just fine, and you now get the elapsed time of just over the slept amount.

You could use either R to cache the elapsed time in a temporary file for the next run, or pass it to LaTeX as some sort of variable -- but you will not be able to use 'forward references' as the R chunks gets evaluated in turn.

Answer 2

btw you don't usually need print to evaluate variables R

\Sexpr{version$version.string}

works fine as well

Answer 3

Dirk's answer is almost perfect, but still doesn't let you put the answer half way through the document. I got quite frustrated thinking it should work, but realised that the code I had was opening the time file at the start of each run (and emptying it) and writing the empty result into my document, then putting the answer in the time file at the end !

I eventually did something similar but using R to only open and write the file at the end, which worked great !;

\documentclass[a4paper]{article} 
\usepackage[OT1]{fontenc} 
\usepackage{longtable} 
\usepackage{geometry} 
\usepackage{Sweave} 
\geometry{left=1.25in, right=1.25in, top=1in, bottom=1in} 
\begin{document} 

<<label=start, echo=FALSE, include=FALSE>>= 
startt<-proc.time()[3] 
@  
Text and Sweave Code in here 
%  
This document was created on \today, with \Sexpr{print(version$version.string)} running 
 on a \Sexpr{print(version$platform)} platform. It took approx \input{time}
 sec to process. 

More text and Sweave code in here 
<<label=bye, include=FALSE, echo=FALSE>>=  
odbcCloseAll() 
endt<-proc.time()[3] 
elapsedtime<-as.numeric(endt-startt) 
@  
<<label=elapsed, include=FALSE, echo=FALSE>>=
fileConn<-file("time.tex", "wt") 
writeLines(as.character(elapsedtime), fileConn) 
close(fileConn) 
@ 
\end{document}