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Trimming extra horizontal rule in LaTeX tables from R's xtable

Q: How can I trim the extra horizontal rule length in LaTeX tables from R's xtable?

I'd like to do something like you see in my R code below. But that code throws and error, not producing the desired result. 

% table 1
    \begin{table}[ht]
      \centering
      \begin{tabular}{@{}lrr@{}}
        \toprule  
        & a\_a & b\_b \\ 
        \midrule 
        A & 0.35 & -1.05 \\ 
        B & 0.64 & 0.92 \\ 
        \bottomrule 
      \end{tabular}
    \end{table}

% table 2 
    \begin{table}[ht]
      \centering
      \begin{tabular}{lrr}
        \toprule  
        & a\_a & b\_b \\ 
        \midrule 
        A & 0.36 & -1.05 \\ 
        B & 0.64 & 0.92 \\ 
        \bottomrule 
      \end{tabular}
    \end{table}

tables

# Begin R code
library(xtable)
library(stringr)
options(xtable.comment = FALSE)
set.seed(10)
mat <- matrix(rnorm(4), nrow=2)
rownames(mat) <- LETTERS[1:2]
colnames(mat) <- str_c(letters[1:2], "_", letters[1:2])
#mat.x <- xtable(mat, caption="My Caption!", align="lrr", digits=2) # No Error!
mat.x <- xtable(mat, align="@{}lrr@{}", digits=2) # Error!
print(mat.x, 
      sanitize.text.function = function(x){x},
      sanitize.colnames.function = function(x){str_replace_all(x, "_", "\\\\_")},
      floating=T,
      hline.after=NULL,
      math.style.negative=F,
      add.to.row=list(pos=list(-1,0,nrow(mat.x)),
                      command=c("\\toprule ", "\\midrule ", "\\bottomrule ")))

# Error in `align<-.xtable`(`*tmp*`, value = "@{}lrr@{}") : 
#  "align" must have length equal to 3 ( ncol(x) + 1 )
# In addition: Warning message:
#  In .alignStringToVector(value) : Nonstandard alignments in align string
Solution

  • You need to pass either a string with the same number of characters as the number of columns + 1, or a string of this length. So try (amongst other permutations) 

    align=c("@{}l", "r", "r@{}")

    To see why have a look at the functions.

    library(xtable)

set.seed(10)
mat <- matrix(rnorm(4), nrow=2)
rownames(mat) <- LETTERS[1:2]
colnames(mat) <- paste0(letters[1:2], "_", letters[1:2])
xtable(mat, align="@{}lrr@{}", digits=2)

    "align" must have length equal to 3 ( ncol(x) + 1 )

    The warning suggests passing a vector of length three, so try

    xtable(mat, align=c("@{}l","r", "r@{}"), digits=2)
# \begin{table}[ht]
# \centering
# \begin{tabular}{@{}lrr@{}}
# ---
# ---

    So that works as expected. Also the align argument is robust to how you pass the string, as long as it is of the required length.

    xtable(mat, align=c("@{}lrr@", "{", "}"), digits=2)
# \begin{table}[ht]
# \centering
# \begin{tabular}{@{}lrr@{}}
# ---
# ---

    To see why the first string didn't work look at the relevant parts of xtable:::xtable.data.frame function: must be to do with align

    align(x) <- switch(1 + is.null(align), align, c("r", c("r", "l")[(characters | factors) + 1]))

    This leads to xtable:::`align<-.xtable, and then xtable:::.alignStringToVector