How do I expand a macro into a tabular head?
Better define it as a new column type using the array
package:
\documentclass{article}
\usepackage{array}
\newcolumntype{L}{|l|l|l|}
\begin{document}
\begin{tabular}{L}
1 & 2 &3\\
\end{tabular}
\end{document}
This is probably not practical but it's a nice demonstration of \expandafter
and gets around the problem of macros in the table preamble with the array
package.
\documentclass{article}
\usepackage{array}
\begin{document}
\def\buildtabularhead{|c|c|c|}
\expandafter\tabular\expandafter{\buildtabularhead}
1 & 2 &3\\
\endtabular
\end{document}
The two \expandafter
s make sure that the macro \buildtabularhead
is expanded before LaTeX knows it should be expecting a tabular
preamble. Notice also that instead of \begin{tabular}
...\end{tabular}
I had to use \tabular
...\endtabular
. Internally the former does the same as the latter but surrounding it with a group and doing important checks, including proper nesting (See source2e Section 54.1).
I had a very dirty answer, but I found theneater solution below while reading the documentation of the array
package. In particular, this is inspired from how the *
column specifier is defined there.
The idea is to introduce the new column type \expand
(\newcolumntype
sets up most of the necessary ingredients), and then redefine \NC@rewrite@\expand
to expand the next token with \expandafter
, and then continue the action that array
is doing at that time (namely finding user-defined column types and replacing them by their definition).
\documentclass{article}
\usepackage{array}
\makeatletter
\newcolumntype{\expand}{}
\long\@namedef{NC@rewrite@\string\expand}{\expandafter\NC@find}
\makeatother
\begin{document}
\def\mypream{c|*{2}{c|||}}
\begin{tabular}{c\expand\mypream cc}
a & b & d &e & f & apsdoi \\
cde & def & erasd &arp & sefoi & wp
\end{tabular}
\end{document}