Lyx table: two equivalent columns

Question

I have a simple table with two columns, which should have an equivalent width. I tried to set each column's width to 50% text width. Rightclick -> More -> Settings -> Table Settings -> Width

Now, the columns have an equivalent width,but the table does not respect the text width.

I have another table, where I have set just one column to a width in cm and the other column is not set. In this table everything is fine. The second column adapt automatically, and the table does not overflow the text width.

In both cases, the tables are placed inside a standard float element.

What do I wrong in the first case?

Answer 1

So when LaTeX makes a table it inserts a space of length \tabcolsep at the start and end of each column. The specified width of a column comes in addition to this length. That's why when you have two columns of 0.5\textwidth, the table is a bit wider (4\tabcolsep) than the text width.

There are ways around this, demonstrated in the LaTeX code below. The tabularx package and its X columns can be quite handy, but there is no support for it in the LyX GUI I believe, so you would have to write the table code by hand in an ERT.

To use the second alternative presented, calculating a length, add

p{\dimexpr0.5\linewidth-2\tabcolsep\relax}

in the LaTeX argument box in the table settings in LyX (do it for each column, of course):

This won't let you use the other column settings, so if you want centered text in the cells, use

>{\centering}p{\dimexpr0.5\linewidth-2\tabcolsep\relax}

instead. You also need to go to Document --> Settings --> LaTeX preamble and add

\usepackage{array}

as that package is required for the >{stuff} syntax, but isn't loaded by default. (>{stuff} added before a column specifier, e.g. p{1cm}, inserts stuff at the start of each cell in that column.)

Code example and output

\documentclass{article}
\usepackage{parskip} % just for example, sets zero parindent and some parskip
\usepackage{tabularx}
\begin{document}
What you're doing:

X\dotfill X % illustrates textwidth

\begin{tabular}{| p{0.5\linewidth} | p{0.5\linewidth} |}
\hline
ABC & DEF \\
ABC & DEF \\
\hline
\end{tabular}

X\dotfill X

Removing space between columns (looks bad, don't do it):

X\dotfill X

\begin{tabular}{| @{} p{0.5\linewidth} @{} | @{}p{0.5\linewidth} @{}|}
\hline
ABC & DEF \\
ABC & DEF \\
\hline
\end{tabular}

X\dotfill X

Calculate suitable length:

X\dotfill X 

\begin{tabular}{| p{\dimexpr0.5\linewidth-2\tabcolsep\relax} | p{\dimexpr0.5\linewidth-2\tabcolsep\relax} |}
\hline
ABC & DEF \\
ABC & DEF \\
\hline
\end{tabular}

X\dotfill X

Using \texttt{tabularx} (not available via GUI in LyX):

X\dotfill X

\begin{tabularx}{\linewidth}{| X | X |}
\hline
ABC & DEF \\
ABC & DEF \\
\hline
\end{tabularx}


X\dotfill X


Set the default column sep to zero with \verb|\setlength{\tabcolsep}{0pt}| (I wouldn't do this):
\setlength{\tabcolsep}{0pt}

X\dotfill X


\begin{tabular}{| p{0.5\linewidth} | p{0.5\linewidth} |}
\hline
ABC & DEF \\
ABC & DEF \\
\hline
\end{tabular}

X\dotfill X

\end{document}