siunitx ‘S’ table column ‘table-format’ option

2021-08-01

I can’t believe it’s taken me this long to figure out how to use the table-format option of the S column type from the siunitx package in a LaTeX table!

A basic table in LaTeX, showing part of the mtcars dataset in R:

\begin{table}[h]
\centering
\begin{tabular}{lrrrrr}
  \hline
 & {mpg} & {cyl} & {disp} & {drat} & {qsec} \\ 
  \hline
Mazda RX4 & 21 & 6 & 160 & 3.9 & 16.46 \\ 
Mazda RX4 Wag & 21 & 6 & 160 & 3.9 & 17.02 \\ 
Datsun 71 & 22.8 & 4 & 108 & 3.85 & 18.61 \\ 
Hornet 4 Drive & 21.4 & 6 & 258 & 3.08 & 19.44 \\ 
Hornet Sportabout & 18.7 & 8 & 360 & 3.15 & 17.02 \\ 
Valiant & 18.1 & 6 & 225 & 2.76 & 20.22 \\ 
   \hline
\end{tabular}
\end{table}
A basic table

It’s nice to align numeric columns by decimal point, which can be achieved using the S column type:

\begin{tabular}{lSSSSS}

But now the columns are all off centre:

Table with misaligned 'S' columns

It turns out, that the table-format option can be used to tell siunitx how many digits to expect both before and after the decimal point:

\begin{tabular}{l
	S[table-format=2.1]
	S[table-format=1.0]
	S[table-format=3.0]
	S[table-format=1.2]
	S[table-format=2.2]}
Table with properly aligned 'S' columns

I had always assumed that table-format referred to a version number, v3.2 for example, and not knowing what the differences were I had avoided using the option altogether.

As an addendum, while I was figuring this out I also did some experiments in exporting nice looking LaTeX tables directly from R using the xtable package. Here is a basic xtable export snippet, the resulting table, and the table rendered by LaTeX:

# Packages
library(xtable)

# Data
data(mtcars)
dat <- head(mtcars)[,c(1,2,3,5,7)]

# Basic xtable, no formatting
xtab_bad <- xtable(dat)

# Write basic xtable to file
fileConn <- file("xtab_bad.tex")
writeLines(print(xtab_bad),
  fileConn)
close(fileConn)
% latex table generated in R 4.1.0 by xtable 1.8-4 package
% Mon Aug  2 08:50:05 2021
\begin{table}[ht]
\centering
\begin{tabular}{rrrrrr}
  \hline
 & mpg & cyl & disp & drat & qsec \\ 
  \hline
Mazda RX4 & 21.00 & 6.00 & 160.00 & 3.90 & 16.46 \\ 
  Mazda RX4 Wag & 21.00 & 6.00 & 160.00 & 3.90 & 17.02 \\ 
  Datsun 710 & 22.80 & 4.00 & 108.00 & 3.85 & 18.61 \\ 
  Hornet 4 Drive & 21.40 & 6.00 & 258.00 & 3.08 & 19.44 \\ 
  Hornet Sportabout & 18.70 & 8.00 & 360.00 & 3.15 & 17.02 \\ 
  Valiant & 18.10 & 6.00 & 225.00 & 2.76 & 20.22 \\ 
   \hline
\end{tabular}
\end{table}
standard xtable table output

By default, xtable renders all columns as right-aligned, with two decimal places after columns identified as numeric, with no caption or label. Below is an improved table, created by adding some customisations to xtable:

xtab_good <- xtable(dat,
  label = "table_a",
  align = c("l", "S[table-format=2.1]", "S[table-format=1.0]", 
    "S[table-format=3.0]", "S[table-format=1.2]", "S[table-format=2.2]"), 
  display = c("s", "f", "d", "d", "f", "f"),
  digits = c(  0,   1,   0,   0,   2,   2),
  caption = c("Test caption, this is the full caption.", "Testing short"))

colSanit <- function(x){
  paste0("{", x, "}") 
}

rowSanit <- function(x){
  paste0("{\\emph{", x, "}}") 
}

# Write better xtable to file
fileConn <- file("xtab_good.tex")
writeLines(print(xtab_good, 
    include.rownames = TRUE, 
    caption.placement = "top",
    table.placement = "",
    booktabs = TRUE,
    sanitize.colnames.function = colSanit, 
    sanitize.rownames.function = rowSanit, 
    sanitize.text.function = function(x) {x}), 
  fileConn)
close(fileConn)
% latex table generated in R 4.1.0 by xtable 1.8-4 package
% Mon Aug  2 08:53:54 2021
\begin{table}
\centering
\caption[Test caption, this is the full caption.]{Testing short} 
\label{table_a}
\begin{tabular}{lS[table-format=2.1]S[table-format=1.0]S[table-format=3.0]S[table-format=1.2]S[table-format=2.2]}
  \toprule
 & {mpg} & {cyl} & {disp} & {drat} & {qsec} \\ 
  \midrule
{\emph{Mazda RX4}} & 21.0 & 6 & 160 & 3.90 & 16.46 \\ 
  {\emph{Mazda RX4 Wag}} & 21.0 & 6 & 160 & 3.90 & 17.02 \\ 
  {\emph{Datsun 710}} & 22.8 & 4 & 108 & 3.85 & 18.61 \\ 
  {\emph{Hornet 4 Drive}} & 21.4 & 6 & 258 & 3.08 & 19.44 \\ 
  {\emph{Hornet Sportabout}} & 18.7 & 8 & 360 & 3.15 & 17.02 \\ 
  {\emph{Valiant}} & 18.1 & 6 & 225 & 2.76 & 20.22 \\ 
   \bottomrule
\end{tabular}
\end{table}
A better xtable

The changes made: