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Why can't I get what I want in the following code? MWE:
\documentclass{article}
\begin{document}
%
\def\aaa*{*}
\def\aaaAAA{AAA}
|\csname aaaAAA\endcsname|% typeset |AAA| as I desired.
|\csname aaa*\endcsname|% I want to get |*|, but get ||. So, what's wrong with my code?
%
\end{document}
That's because \csname aaa*\endcsname does not exist and therefore defaults to \relax which typesets nothing. Why does it not exist? Well, the problem is similar to No \makeatletter required?:
In \def\aaa*{*}, * does not form part of the definition, but instead forms part of the parameter text used. In order to define a macro to include a * in the definition name you'll have to use
\expandafter\def\csname aaa*\endcsname{*}% Similar to \@namedef{aaa*}{*}
Consider the following example
\def\aaa*{*}
\show\aaa*
Running TeX on it (any flavor), will report as follows on the console
> \aaa=macro:
*->*.
l.3 \show\aaa
*
?
How do we read it? The primitive command \show reports the meaning of the following token; the to lines before the question mark tell us that \aaa* are two tokens, because * appears in the continuation line.
TeX is also telling us that \aaa is a macro that has a nonempty parameter text (what's reported before ->) consisting of an asterisk. In other words, \def\aaa*{*} instructs TeX that \aaa must be followed by * and the two tokens will be replaced by *.
For instance, you can call it also as
\aaa *
because the space after \aaa is ignored when building tokens from input.
If you want to use \csname, then it should be
\csname aaa\endcsname *
(the space before * is optional).
In case you're wondering how *-variants are implemented in LaTeX, here it is:
\newcommand{\foo}{\@ifstar\foostar\foonostar}
\newcommand{\foostar}{<what we want \foo* to do>}
\newcommand{\foonostar}{<what we want \foo to do>}
Possible arguments have to be grabbed by \foostar or \foonostar, depending on the desired syntax.
The approach with xparse is slightly different, but the command will not have the * as part of the name nonetheless.
\def\aaa*{*}does not define a macro called\aaa*. It defines a macro called\aaathat must always be followed by a*.|\csname aaa\endcsname*|would give you the output of\aaa*. To define\aaa*you need\expandafter\def\csname aaa*\endcsname{*}.