possible to create latex multicolumns in xtable?

Question

I am using xtable with R Markdown and knitr to produce .tex files that I call with \input{}. Works great, but I have not figured out how to create multicolumns like the one shown here. Does anyone know how to to this?

So far I am using:

tbl <- xtable(data, align="l r r r r r")
colnames(tbl) <- c("Variable", 
                  "Mean", "Std Dev", 
                  "Mean", "Std Dev", 
                  "Difference")
caption(tbl) <- c("Table Title")
print(tbl, 
    include.rownames=FALSE,
    caption.placement="top",
    booktabs=TRUE,
    type="latex", 
    file="output.tex")

I'd like to have a different grouping header over each "Mean" and "Std Dev" ("Treatment" and "Control").

Alternatively, is there a better method for using R Markdown/knitr to automatically generate tables? I don't want to manually edit the tables because the report needs to generate automatically.

UPDATE: @agstudy: I'm new to latex, but I think this is the output I am looking to produce automatically with xtable (or something like xtable):

\begin{tabular}{lrrrrr}
  \toprule
      & \multicolumn{2}{c}{Treatment} & \multicolumn{2}{c}{Control} & \\
  \cmidrule(lr){2-3} \cmidrule(lr){4-5}
  Variable & Mean & Std Dev & Mean & Std Dev & Difference \\ 
  \midrule
  var1 & 1 & 2 & 3 & 4 & 5 \\ 
  \bottomrule
\end{tabular}

UPDATE 2: @Jonathan: it took me a few reads to understand what you were suggesting. I took your recommendation, and it worked.

In the R markdown chunk I now use:

tbl <- xtable(data)
print(tbl,
    only.contents=TRUE,
    include.rownames=FALSE,
    type="latex",
    digits(tbl) <- c(0,1,1,1,1,1),
    file="output/tblout.tex")

Then in the text, I use:

\begin{tabular}{lddddd}
\toprule
    & \multicolumn{2}{c}{Treatment} & \multicolumn{2}{c}{Control} & \\
    \cmidrule(lr){2-3} \cmidrule(lr){4-5}
    Variable  &  \multicolumn{1}{r}{Mean}  &  \multicolumn{1}{r}{Std Dev}  &       \multicolumn{1}{r}{Mean}  &  \multicolumn{1}{r}{Std Dev}  &  \multicolumn{1}{r}{Difference}  \\
\midrule
\input{../output/tblout}
\bottomrule
\end{tabular}

I'll see if anyone has any other suggestions for a native xtable (or other package) solution. Otherwise, I will accept your answer. Thanks!

Answer 1

I think the add.to.row option in xtable achieves this perfectly.

Example code here:

require(xtable)
age <- sample(c('30-50', '50-70', '70+'), 200, replace=T)
sex <- sample(c('Male', 'Female'), 200, replace=T)
val <- table(age, sex)
val <- rbind(val, formatC(prop.table(val)*100, format='f', digits=1))
val <- structure(val, dim=c(3, 4))
val <- rbind(c('n', '%'), val)
rownames(val) <- c('', sort(unique(age)))
val <- xtable(val)


addtorow <- list()
addtorow$pos <- list(0)
addtorow$command <- paste0(paste0('& \\multicolumn{2}{c}{', sort(unique(sex)), '}', collapse=''), '\\\\')

print(val, add.to.row=addtorow, include.colnames=F)

Answer 2

Assuming the form of the table is the same across runs (i.e., only the numbers are changing), my suggestion would be to use the only.contents argument to print.xtable and code the multi-column headers in by hand. To the best of my knowledge xtable is not capable of doing multi-column cells itself.