In answering another question regarding testing for math mode, I proposed the following command
\newcommand{\testa}{%
\ifmmode
^{12}\text{CO(1-0)}%
\else
$^{12}$CO(1-0)%
\fi
}
But @egreg pointed out that I should be writing either:
\newcommand{\egrega}{%
\relax
\ifmmode
^{12}\text{CO(1-0)}%
\else
$^{12}$CO(1-0)%
\fi
}
or
\DeclareRobustCommand{\egregb}{%
\ifmmode
^{12}\text{CO(1-0)}%
\else
$^{12}$CO(1-0)%
\fi
}
and proposed I test things out in an array environment to see what happens.
\[\begin{array}{c}
\testa
\end{array}\]
Sure enough, \testa results in the following warning:
! Missing $ inserted.
<inserted text>
$
l.42 \testa
?
whereas the two command \egrega and \egregb work as expected.
I wasn't happy with just this. My understanding of robustness is quite shacky. And, while I've read various posts such as What is the difference between Fragile and Robust commands?, I still felt a bit in the dark.
So, I tried a different approach. I tried writing
\[\begin{array}{c}
\ifmmode{Math}\else{NOT MATH}\fi
\end{array}\]
Really, I don't why I did this. I expected to the same error as with the faulty version of the command. But instead I didn't get an error at all. Instead, I got the following unexpected and erroneous result:
Clearly something is going very wrong here, and not in any way I anticipated. If instead I write
\[\begin{array}{c}
\relax\ifmmode{Math}\else{NOT MATH}\fi
\end{array}\]
or precede the \ifmmode by anything else to be processed, all seems to go well.
What am I failing to understand here?
