TeX/expandafter
Synopsis edit
\expandafter <macro> <tokens>
Description edit
The \expandafter command delays expanding a macro until its arguments have been expanded.
Examples edit
\def\a[#1]{A's argument was `#1'}
\def\args{[FOO]}
\a\args
will not work, saying "! Use of \a doesn't match its definition." This is because while defining \a, the first argument (denoted #1) appeared in square braces. Therefore, \a expects its first argument to appear in square braces, while in \a\args, the immediate character after \a is not a left square bracket.
Using \expandafter, we can write
\expandafter\a\args
This expands \args before \a, as if we had written the following in the first place:
\a[FOO]
As a result, TeX will print A's argument was `FOO'.
The \expandafter command first expands the tokens following <macro>, and then expands <macro>, with the expanded <tokens> following it as if they had been typed in the file.
Multiple invocations edit
One sometimes sees long chains of expandafters, which can normally be read as "expand after N", e.g.
% Chain of expandafters leads to \pra having definition ``\onelevelexpanded''
\def\onelevelexpanded{Page={\the\count0} }
\def\mycommandcontents{\onelevelexpanded}
\ea\def\ea\pra\ea{\mycommandcontents}
This expands \mycommandcontents only once (in contrast to TeX/edef).