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

Answer 1

A little bit late to the game here is my answer, which is similar to ashkan, but more general and allows for different parameters.

First of all, why a new answer? Well, I needed an output without a table-environment (I want to write my captions etc inside my tex-document not inside my r-code) which kableExtra does not seem to provide (correct me if I am wrong). But I also wanted flexibility with the inputs (i.e., with and without line, different spans etc).

The result is a function construct_header() that constructs the header for us.

First a short example:

library(xtable)
set.seed(123)
df <- matrix(round(rnorm(16), 2), ncol = 4) 
df <- cbind(paste("Var", 1:4), df)
colnames(df) <- c("Var", rep(c("X", "Y"), 2))
df
#      Var     X       Y       X       Y      
# [1,] "Var 1" "-0.56" "0.13"  "-0.69" "0.4"  
# [2,] "Var 2" "-0.23" "1.72"  "-0.45" "0.11" 
# [3,] "Var 3" "1.56"  "0.46"  "1.22"  "-0.56"
# [4,] "Var 4" "0.07"  "-1.27" "0.36"  "1.79" 

a_header <- construct_header(
  # the data.frame or matrix that should be plotted  
  df,
  # the labels of the groups that we want to insert
  grp_names = c("", "Group A", "Group B"), 
  # the number of columns each group spans
  span = c(1, 2, 2), 
  # the alignment of each group, can be a single character (lcr) or a vector
  align = "c"
)

print(xtable(df), add.to.row = a_header, include.rownames = F, hline.after = F)
# % latex table generated in R 3.4.2 by xtable 1.8-2 package
# % Fri Oct 27 16:39:44 2017
# \begin{table}[ht]
# \centering
# \begin{tabular}{lllll}
#   \hline
#   \multicolumn{1}{c}{} & \multicolumn{2}{c}{Group A} & \multicolumn{2}{c}{Group B} \\  \cmidrule(lr){2-3} \cmidrule(lr){4-5}
#   Var & X & Y & X & Y \\ 
#   \hline
#   Var 1 & -0.56 & 0.13 & -0.69 & 0.4 \\ 
#   Var 2 & -0.23 & 1.72 & -0.45 & 0.11 \\ 
#   Var 3 & 1.56 & 0.46 & 1.22 & -0.56 \\ 
#   Var 4 & 0.07 & -1.27 & 0.36 & 1.79 \\ 
#   \hline
# \end{tabular}
# \end{table}

Note that we have to specify hline.after = FALSE (important for me, but omitted here is the possibility to specify floating = FALSE).

Which results in this table (note that this approach needs the booktabs package to be loaded in LaTeX):

You can specify to omit the lines construct_header(..., draw_line = FALSE), align the groups, and have them span in different ways, i.e.,

ugly_header <- construct_header(df, c("One", "Two", "Three"), c(2, 1, 2), c("l", "c", "r"))
print(xtable(df), add.to.row = ugly_header, include.rownames = F, hline.after = F)

which results in this:

The code for the function is this:

#' Constructs a header i.e., groups for an xtable
#'
#' @param df a data.frame or matrix
#' @param grp_names the names of the groups
#' @param span where the groups span
#' @param align the alignment of the groups, defaults to center
#' @param draw_line if the group-names should be underlined
#'
#' @return a list that can be given to the \code{add.to.row} argument of the of \code{print.xtable}
#' @export
#'
#' @examples
#' library(xtable)
#' mx <- matrix(rnorm(16), ncol = 4) 
#' mx <- cbind(paste("Var", 1:4), mx)
#' colnames(mx) <- c("Var", rep(c("X", "Y"), 2))
#' 
#' addtorow <- construct_header(mx, c("", "Group A", "Group B"), span = c(1, 2, 2), "c")
#' print(xtable(mx), add.to.row = addtorow, include.rownames = F, hline.after = F)
construct_header <- function(df, grp_names, span, align = "c", draw_line = T) {
  if (length(align) == 1) align <- rep(align, length(grp_names))
  if (!all.equal(length(grp_names), length(span), length(align)))
    stop("grp_names and span have to have the same length!")

  if (ncol(df) < sum(span)) stop("Span has to be less or equal to the number of columns of df") 

  header <- mapply(function(s, a, grp) sprintf("\\multicolumn{%i}{%s}{%s}", s, a, grp),
                   span, align, grp_names)
  header <- paste(header, collapse = " & ")
  header <- paste0(header, " \\\\")

  if (draw_line) {
    # where do we span the lines:
    min_vals <- c(1, 1 + cumsum(span)[1:(length(span) - 1)])
    max_vals <- cumsum(span)
    line <- ifelse(grp_names == "", "", 
                   sprintf("\\cmidrule(lr){%i-%i}", min_vals, max_vals))
    line <- paste(line[line != ""], collapse = " ")

    header <- paste0(header, "  ", line, "\n  ")
  }

  addtorow <- list(pos = list(-1, -1, nrow(df)),
                   command = c("\\hline\n  ", header, "\\hline\n  "))
  return(addtorow)
}