Changing the Color of negative numbers to Red in a table generated with xtable()?

Question

I\'m writing a report in R with knitr. I\'m using xtable() to generate tables in the report. One of my tables includes both negative and positive numbers. I\'d like to chang

Answer 1

You can use capture.output() to capture the lines printed by the (implicit) call to print.xtable(). Then apply gsub() to the output, using a pattern and replacement that surround each negative number with \textcolor{red}{}. Finally, use cat() with sep="\n" to write the modified lines out to the *.tex file.

\documentclass{article}
\begin{document}
<<simpleExamp, results="asis", echo=FALSE>>=
library(knitr)
library(xtable)
testMatrix <- matrix(c(sample(-10:10,10)), ncol = 2)
## I added the following three lines
xt <- capture.output(xtable(testMatrix))
xt_mod <- gsub("(\\s|^)(-\\d*)", "\\1\\\\textcolor{red}{\\2}", xt)
cat(xt_mod, sep="\n")
@
\end{document}

(Note also that I replaced results=tex with results="asis", which knitr 'prefers' and which it will more quickly process.)

---

Edit: Adding an image of the resulting table. (Getting it in an SO-ready form required a few tweaks to the code, which is also included below.)

\documentclass{standalone}
\renewenvironment{table}{}{}% Ignore `table` environment in standalone mode.
\begin{document}
<<simpleExamp, results="asis", echo=FALSE>>=
library(knitr)
library(xtable)
cat("\\Huge\n\n")
testMatrix <- matrix(c(sample(-10:10,10)), ncol = 2)
## I added the following three lines
xt <- capture.output(print.xtable(xtable(testMatrix), table.placement=NULL))
xt_mod <- gsub("(\\s|^)(-\\d*)", "\\1\\\\textcolor{red}{\\2}", xt)
cat(xt_mod, sep="\n")
@
\end{document}

Answer 2

Copied Josh O'Brien's answer with a little tweak to color a table with decimals:

\documentclass{article}
\begin{document}
<<simpleExamp, results="asis", echo=FALSE>>=
library(knitr)
library(xtable)
testMatrix <- matrix(c(sample(-10:10,10)), ncol = 2)*1.1
## I added the following three lines
xt <- capture.output(xtable(testMatrix))
xt_mod <- gsub("(\\s|^)(-\\d\\.\\d*)", "\\1\\\\textcolor{red}{\\2}", xt)
cat(xt_mod, sep="\n")
@
\end{document}