How to avoid that xtable flips the x and y axis of a R table in the final tex output?

Question

I work with xtable which turns a table of r into a functionable latex code.

    <<echo=false>>=
attach(warpbreaks)
p2 <- round(tapply(breaks, list(tension), mean), digits=2) %>%
  as.table(.)
@

p2 looks like this:

  L     M     H 
36.39 26.39 21.67 

But the output of the created pdf of this code:

<<echo=false, results=tex>>=
library("xtable")
xtable(p2,
       align = "rr")
@ 

looks (approximately) like this:

      x 
L 36.39 
M 26.39 
H 21.67

Why does it not look like the inital table in r and how can I fix it?

Edit: this is the Tex code I get through this

\begin{table}[ht]
\centering
\begin{tabular}{rr}
  \hline
 & . \\ 
  \hline
L & 36.39 \\ 
  M & 26.39 \\ 
  H & 21.67 \\ 
   \hline
\end{tabular}
\end{table}

Answer 1

When you apply xtable on objects of type table the xtable.table method is called. This method is defined as follows (code obtained with getAnywhere(xtable.table)):

function (x, caption = NULL, label = NULL, align = NULL, digits = NULL, 
    display = NULL, auto = FALSE, ...) 
{
    if (length(dim(x)) == 1) {
        return(xtable.matrix(matrix(x, dimnames = list(rownames(x), 
            names(dimnames(x)))), caption = caption, label = label, 
            align = align, digits = digits, display = display, 
            auto = auto, ...))
    }
    else if (length(dim(x)) == 2) {
        return(xtable.matrix(matrix(x, ncol = dim(x)[2], nrow = dim(x)[1], 
            dimnames = list(rownames(x), colnames(x))), caption = caption, 
            label = label, align = align, digits = digits, display = display, 
            auto = auto, ...))
    }
    else {
        stop("xtable.table is not implemented for tables of > 2 dimensions")
    }
}

So, the table is converted to a matrix and then xtable is called on the matrix. When you convert a list to a matrix the values appear in columns:

> matrix(p2,dimnames=list(rownames(p2)))
   [,1]
L 36.39
M 26.39
H 21.67

To change the output you can transpose the table using t(). For this it is not necessary to use as.table, you can transpose the result of tapply directly.

The transpose function creates a rowname [1,] in the output. To remove this you can use include.rownames=FALSE. This option is an argument of print.xtable, not of xtable itself. Therefore you need to use the print function explicitly in the code.

R code:

p2 <- round(tapply(breaks, list(tension), mean), digits=2)
p2trans <- t(p2)
p2tab <- xtable(p2trans)
print(p2tab,include.rownames=FALSE)

Full MWE:

\documentclass{article}
\begin{document}
<<echo=FALSE,results='asis'>>=
library(xtable)
attach(warpbreaks)
p2 <- round(tapply(breaks, list(tension), mean), digits=2)
p2trans <- t(p2)
p2tab <- xtable(p2trans)
print(p2tab,include.rownames=FALSE)
@
\end{document}

Result: