\
cmd{}
and \
cmd{blah}
. This is pretty simple:
\def\cmd#1{% \def\tempa{}% \def\tempb{#1}% \ifx\tempa\tempb <empty case> \else <non-empty case> \fi }
The case where you want to ignore an argument that consists of nothing
but spaces, rather than something completely empty, is more tricky.
It’s solved in the code fragment ifmtarg, which defines
commands \
@ifmtarg
and \
@ifnotmtarg
, which examine their
first argument, and select (in opposite directions) their second or
third argument. The package’s code also appears in the LaTeX
memoir class.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=empty