xtable and knitr: How to center table on full page width?

Question

(This question does not solve this because that was about centering within linewidth. I'm interested in the full page width.)

I'm producing a table like this:

\documentclass{article}

\begin{document}

<<tabletest, message=F, warning=F, echo = F, results = "asis">>=
library(readr)
library(xtable)
# Create dummy data.frame
values <- 1:14
df <- t(data.frame(values, values))
colnames(df) <- paste("column", values, sep="")
rownames(df) <- c("row1", "row2")
ltable <- xtable(
  x = df
)
print(ltable)
@

\end{document}

The result looks like this:

I want to center this horizontally on the page. My first try was wrapping the chunk in \centerline{}, but that got me this:

Runaway argument?
{ \begin {table}[ht] \centering \begin {tabular}{rrrrrrrrrrrrrrr} \hline \ETC.
! Paragraph ended before \centerline was complete.
<to be read again> 
                   \par

The interesting part in the resulting .tex file looks like this:

\centerline{
% latex table generated in R 3.2.5 by xtable 1.8-2 package
% Mon Apr 25 16:40:48 2016
\begin{table}[ht]
\centering
\begin{tabular}{rrrrrrrrrrrrrrr}
  \hline
 & column1 & column2 & column3 & column4 & column5 & column6 & column7 & column8 & column9 & column10 & column11 & column12 & column13 & column14 \\ 
  \hline
row1 &   1 &   2 &   3 &   4 &   5 &   6 &   7 &   8 &   9 &  10 &  11 &  12 &  13 &  14 \\ 
  row2 &   1 &   2 &   3 &   4 &   5 &   6 &   7 &   8 &   9 &  10 &  11 &  12 &  13 &  14 \\ 
   \hline
\end{tabular}
\end{table}

}

If I manually edit that to this:

% latex table generated in R 3.2.5 by xtable 1.8-2 package
% Mon Apr 25 16:40:48 2016
\begin{table}[ht]
\centerline{
\begin{tabular}{rrrrrrrrrrrrrrr}
  \hline
 & column1 & column2 & column3 & column4 & column5 & column6 & column7 & column8 & column9 & column10 & column11 & column12 & column13 & column14 \\ 
  \hline
row1 &   1 &   2 &   3 &   4 &   5 &   6 &   7 &   8 &   9 &  10 &  11 &  12 &  13 &  14 \\ 
  row2 &   1 &   2 &   3 &   4 &   5 &   6 &   7 &   8 &   9 &  10 &  11 &  12 &  13 &  14 \\ 
   \hline
\end{tabular}
}
\end{table}

Then it works. But how can I accomplish this automatically? After reading this and this, I tried:

x <- capture.output(print(xtable(ltable)))
x <- gsub("\\\\centering", "", x, fixed=TRUE) # Remove \centering
x <- gsub("\\begin{table}[ht]", "\\begin{table}[ht]\\centerline{", x, fixed=TRUE) # Add \centerline{
x <- gsub("\\end{table}", "\\end{table}}", x, fixed=TRUE) # Add ending }

But the format of x is not as I got when calling print(ltable) earlier:

> print(x)
 [1] "% latex table generated in R 3.2.5 by xtable 1.8-2 package"                                                                                                                                                                                                                                                                             
 [2] "% Mon Apr 25 16:51:02 2016"

That is, this will output the brackets with numbers in front of every line and it will not produce a table. Can anyone help me with this?

Answer 1

Case 1: Non-floating table

If the table is not supposed to float, it is easy to enclose the tabular in a makebox as suggested here:

\documentclass{article}
\begin{document}

<<echo = F, results = "asis">>=
library(xtable)

cat("\\makebox[\\textwidth]{")
print(xtable(t(cars)[, 1:16]), floating = FALSE)
cat("}")

@
\end{document}

Case 2: Floating table

In this case, the best solution I found uses the adjustwidth environment from the changepage package.

First, define a new environment that encloses its content in an adjustwidth environment to remove the left margin.

\newenvironment{widestuff}{\begin{adjustwidth}{-4.5cm}{-4.5cm}\centering}{\end{adjustwidth}}

Second, pass this new environment as latex.environments to print.xtable. (If there was a way to pass arguments to the environment in latex.environments, the custom environment would not be necessary at all.)

\documentclass{article}
\usepackage{changepage}
\newenvironment{widestuff}{\begin{adjustwidth}{-4.5cm}{-4.5cm}\centering}{\end{adjustwidth}}
\begin{document}
<<echo = F, results = "asis">>=
library(xtable)
print(xtable(t(cars)[, 1:16]), latex.environments = "widestuff")

print(xtable(t(cars)[, 1:12]), latex.environments = "widestuff")
@
\end{document}

(The screenshots contain some text that is not present in the code example. It is only there to visualize the page dimensions.)