How to span over variable number of cells?

Question

I'm looking for a macro to span all cells in a tabu environment to get category headers in tables.

This example does what I need for two columns.

\begin{tabu} {XX}
   abc   & def \\
   \multicolumn{2}{c}{category header} \\
   ghi   & jkl \\
\end{tabu}

What I want is a macro without the need to specify the number of columns. The category header should always span over all columns of the tabu environment.

I tried to access the tabu@nbcols constant but I've got no clue how to do that. These two don't work:

\multicolumn{\tabu@nbcols}{c}{header}
\multicolumn{\the\tabu@nbcols}{c}{header}

What I want could look like:

\begin{tabu} {XX}
   abc   & def \\
   \mycatheader{category header} \\
   ghi   & jkl \\
\end{tabu}

Answer 1

\makeatletter
\def\mycatheader#1{\multicolumn{\tabu@nbcols}{c}{#1}}
\makeatother

\begin{document}


\begin{tabu} {XX}
   abc   & def \\
   \mycatheader{category header}\\
   \multicolumn{2}{c}{category header} \\
   ghi   & jkl \\
\end{tabu}

Answer 2

You've fast got it.

Your problem is the use of macro with @ letter, that is normally not allowed; you need to write \makeatletter to be able to use @ and \makeatother to make the situation normal again.

Here is what you can do:

In your preamble, define a new macro to access the number of columns:

\makeatletter
\edef\mycol{\tabu@nbcols}
\makeatother

And in a tabu environment:

\begin{tabu} {XX}
abc   & def \\
\multicolumn{\mycol}{c}{header}\\
ghi   & jkl \\
\end{tabu}

Your macro could be written like this:

\def\mycatheader#1{\multicolumn{\mycol}{c}{#1}}

Answer 3

\documentclass[a4paper]{article}
\usepackage{tabu}
\begin{document}

\begin{tabu} {XX}
 abc   & def \\
 \multicolumn{2}{c}{category header} \\
 ghi   & jkl \\
\end{tabu}

\bigskip
\newcommand{\mycatheader}[1]{\noalign{\medskip\noindent\centerline{\textbf{#1}}\smallskip}}

\begin{tabu} {XX}
abc   & def \\
\mycatheader{category header}
ghi   & jkl \\
\end{tabu}

\end{document}