xtable and header alignment

Question

Is it possible to have a header alignment in xtable which is different from the alignment used in the rest of the table? In my case, I want my header to be center aligned, but the table itself should be right aligned.

Answer 1

To do that in LaTeX you stick your headers into a \multicolumn thing to specify the alignment you want:

\begin{tabular}{rrr}
  \hline
 & \multicolumn{1}{c}{x} &\multicolumn{1}{c}{y} \\ 
  \hline
1 &   1 & 0.17 \\ 
  2 &   2 & 0.63 \\ 
  3 &   3 & 0.95 \\ 
  4 &   4 & 0.57 \\ 
  5 &   5 & 0.65 \\ 
   \hline
\end{tabular}

The print.xtable function uses the names of the xtable object as the headers. So if you rename your xtable object:

> d=data.frame(x=1:5,y=runif(5))  # sample data frame
> dx=xtable(d) # make an xtable
> names(dx)=c("\\multicolumn{1}{c}{x}","\\multicolumn{1}{c}{y}")

then that's most of the work done, you just have to print it overriding the sanitization function of print.xtable:

> print.xtable(dx,sanitize.colnames.function=function(x){x})
% latex table generated in R 2.15.1 by xtable 1.7-0 package
% Thu Feb 21 15:28:11 2013
\begin{table}[ht]
\begin{center}
\begin{tabular}{rrr}
  \hline
 & \multicolumn{1}{c}{x} & \multicolumn{1}{c}{y} \\ 
  \hline
1 &   1 & 0.78 \\ 
  2 &   2 & 0.34 \\ 
  3 &   3 & 0.88 \\ 
  4 &   4 & 0.45 \\ 
  5 &   5 & 0.54 \\ 
   \hline
\end{tabular}
\end{center}
\end{table}

otherwise it does

& $\backslash$multicolumn\{1\}\{c\}\{x\} & $\backslash$multicolumn\{1\}\{c\}\{y\} \\ 

How's that?

Answer 2

Just to follow up on the answer by Spacedman (could not add a comment as I am reputationless ;)

Instead of doing sanitize.colnames.function=function(x){x} you can do:

sanitize.colnames.function=function(x){paste0("\\multicolumn{1}{c}{",x,"}")}

This way you can skip renaming step. If you do/want to do other header "beautifications," they should be done before paste0 or between the commas (if short)