R / Sweave formatting numbers with \Sexpr{} in scientific notation

Question

I am just starting to write some documents with Sweave/R and I like the \sexpr{} command that lets one tow write numbers directly within text.

If I have a number like mus=0.0002433121, well I can say round it to a number of decimal places e.g.

\Sexpr{round(mus,7)}

How to write it in the scientific notation i.e. as LaTeX would be outputting

2.43 \times 10^{-4} 

and can we control the number of significant digits to be outputted like 3 in this example?

I note that a number like sigma = 2000000 is written automatically to 2e + 06 if I specify

\Sexpr{round(sigma,2)}. 

I would prefer that it would be written as

2 \times 10^6 

same as we would get in LaTeX notation and perhaps giving us the possibility to control the number of significant digits as well.

How to achieve this?

Answer 1

I think this function should work:

sn <- function(x,digits)
{
  if (x==0) return("0")
  ord <- floor(log(abs(x),10))
  x <- x / 10^ord
  if (!missing(digits)) x <- format(x,digits=digits)
  if (ord==0) return(as.character(x))
  return(paste(x,"\\\\times 10^{",ord,"}",sep=""))
}

Some tests:

> sn(2000000)
[1] "2\\\\times 10^{6}"
> sn(0.001)
[1] "1\\\\times 10^{-3}"
> sn(0.00005)
[1] "5\\\\times 10^{-5}"
> sn(10.1203)
[1] "1.01203\\\\times 10^{1}"
> sn(-0.00013)
[1] "-1.3\\\\times 10^{-4}"
> sn(0)
[1] "0"

If you want the result in math mode you could enter $ signs in the paste() call.

Edit:

Here is a Sweave example:

\documentclass{article}

\begin{document}
<<echo=FALSE>>= 
sn <- function(x,digits)
{
  if (x==0) return("0")
  ord <- floor(log(abs(x),10))
  x <- x / 10^ord
  if (!missing(digits)) x <- format(x,digits=digits)
  if (ord==0) return(as.character(x))
  return(paste(x,"\\\\times 10^{",ord,"}",sep=""))
}
@

Blablabla this is a pretty formatted number $\Sexpr{sn(0.00134,2)}$.

\end{document}