Of course, they have different syntax.
\ifx\cyrdash\undefined is traditional way to test if a macro is undefined. Of course, \undefined sould not be defined.
There are \ifdefined and \ifcsname primitives in eTeX. They have no other side effects.
LaTeX's \@ifundefined test if a macro is defined or it has the same meaning of \relax. Furthermore, \@ifundefined{undefinedfoo}{...}{...} will makes \undefinedfoo to be \relax.
Normally, there is no difference between
\ifx\foo\undefined A \else B \fi
and
\ifdefined\foo B \else A \fi
eTeX primitives may be a little safer. Say, we don't need to worry about whether \undefined is really undefined.
However, these two usage are different:
% wrong
% \expandafter\ifx\csname foo\endcsname\undefined A \else B \fi % This is always false
\expandafter\ifx\csname foo\endcsname\relax A \else B \fi % This is \@ifundefined
and
\ifcsname foo\endcsname B \else A \fi
In fact, \csname undefined\endcsname makes \undefined to be \relax, while \ifcsname undefined\endcsname makes \undefined unchanged. That's why \ifcsname in eTeX is necessary.
Test code:
\documentclass{minimal}
\long\def\themeaning#1{\string#1: \meaning#1\par}
\begin{document}
\makeatletter\ttfamily
\def\known{abc}
\let\empty\relax
% initial
\themeaning\known
\themeaning\empty
\themeaning\unknown
\hrulefill
% useful eTeX extension
\ifdefined\known yes\else no\fi\par
\ifdefined\empty yes\else no\fi\par
\ifdefined\unknown yes\else no\fi\par
\hrulefill
% Or
\ifcsname known\endcsname yes\else no\fi\par
\ifcsname empty\endcsname yes\else no\fi\par
\ifcsname unknown\endcsname yes\else no\fi\par
\hrulefill
% the meanings are unchanged
\themeaning\known
\themeaning\empty
\themeaning\unknown
\hrulefill
% LaTeX2e kernel
\@ifundefined{known}{yes}{no}\par
\@ifundefined{empty}{yes}{no}\par
\@ifundefined{unknown}{yes}{no}\par
\hrulefill
% \unknown is changed
\themeaning\known
\themeaning\empty
\themeaning\unknown
\end{document}
It's one of the uncountable ways to check whether an argument is empty. Almost all of them suffer of some drawback: in the case at hand, the argument mustn't be \\.
What does \ifx do? It compares the two tokens following it, without expanding them. The test return success if both tokens are equal in the sense of \meaning or \show:
if they are characters (or control sequences \let to a character), they must represent the same (character code, category code) pair
if they are control sequences (but not \let to a character), they must have the same meaning; the cases are too many to list them all.
Before analyzing the macro, we should know something about TeX conditional: the abstract form is
\ifZ<test><true text>\else<false text>\fi
where \ifZ denotes one of the primitive conditionals. The <test> can take many forms, depending on \ifZ (it's empty, in some cases); what is important is that the <true text> is everything that goes from the end of <test> to \else (or \fi if \else is missing).
So let's see what happens when we call \mycommand{abc}, that is, the default argument, which is empty, is substituted to #1: the tokens that TeX sees are
\ifx\\\\<true>\else<false>\fi
Of course the test is true, because the tokens that follow \ifx are both \\.
When we call \mycommand[xyz]{abc}, instead, TeX sees
\ifx\\xyz\\<true>\else<false>\fi
and TeX compares \\ to x and they are different. But now the <true text> is
yz\\<true>
(remember: the <true text> is what goes from the test to the \else) and it will be ignored altogether.
This test is quite safe, because it doesn't do expansion; however it might fail miserably if the user calls \mycommand[\\]{abc}, so often different delimiter tokens are used, which are supposed not to creep into user's input (for instance \uchyph or \vfuzz). Another similar test might be
\if\relax\noexpand#1\relax
with the caveat that \if does expand what's next until two unexpandable tokens remain. The first token is \relax, which is unexpandable. The second one will be what becomes of the expansion of \noexpand, that is, the first token of #1 (if non empty) but made unexpandable; if #1 is empty, then \noexpand applies to \relax, resulting again in \relax because it's unexpandable.
In both cases some token is reserved (\\ in the first case, \relax in the second one). The safest test is
\if\relax\detokenize{#1}\relax
because the expansion of a non empty #1 can never give a first token equivalent to \relax: even if #1 is \relax, \detokenize{\relax} will give the string \relax (the first token of which is the character \).
Again, if #1 is empty, the test compares \relax to \relax; if #1 is not empty, everything will be ignored up to the \else. Of course it requires that an engine with the e-TeX extensions is run: it won't work with "Knuth TeX".
Best Answer
The conditional
\ifxdoes no expansion and it compares the next two tokens. So in your test it compares\myargwith the first token in#1.Therefore the test will be always false, because
#1is the name of a section level counter.Adding
\expandafterbefore\ifxwould not help, even adding braces around#1:would become
which returns false, as it compares
swithe.You could use
\pdfstrcmp(or, better,\pdf@strcmpwith thepdftexcmdspackage):Complete example:
Note that the attempt of doing the comparison with
\ifxin the following waywouldn't work either. This is because
\@seccntformatis passed through an\edef(in the form\protected@edef), so we'd get an errorfor
\@argument. Adding\noexpandin front of\@argumentwouldn't help, because the conditional is expanded during the\edef. This instead would workbecause the argument to
\unexpandedis not changed during an\edef, so it will be processed afterwards, when typesetting is involved.Note that the
\protected@edefis necessary because we must get the expansion of\thesectionor\thesubsectionat the moment the macro\@seccntformatis executed. An\unexpandedfree solution is possible, by hiding the part in a macro: