\insertshortpart is never empty?

Question

I would like to use \insertshortpart (with some surrounding characters) in a Beamer theme, but only if a short part is actually available. However, if I try to check for emptiness with \ifx...\empty it turns out that this expression is never true.

Here is a minimal example:

\documentclass{beamer}
\begin{document}
\begin{frame}
\ifx\insertpart\empty
empty part
\else
non-empty part
\fi

\ifx\insertshortpart\empty
empty short part
\else
non-empty short part
\fi
\end{frame}
\end{document}

This results in "empty part" and "non-empty short part".

How can I check if the short part is actually empty?

I had a similar problem with \insertshortdate, \insertshorttitle and \insertshortsubtitle which I could solve by using \beamer@shortdate, \beamer@shorttitle and \beamer@shortsubtitle instead, respectively. For \insertshortpart, however, I couldn't find a replacement.

BTW, what's the difference between \empty and \@empty? In my style file both seem to work ...

Answer 1

The short part name is stored in \beamer@partnameshort, which however is undefined before a \part command is issued.

Here's a test for it:

\documentclass{beamer}

\makeatletter
\newcommand{\ifemptyshortpart}{%
  \@ifundefined{beamer@partnameshort}
    {\@firstoftwo}
    {\ifx\beamer@partnameshort\@empty
       \expandafter\@firstoftwo
     \else
       \expandafter\@secondoftwo
     \fi}}
\makeatother


\begin{document}

\ifemptyshortpart{\typeout{EMPTY}}{\typeout{NON EMPTY}}

\part[Short]{Long}
\ifemptyshortpart{\typeout{EMPTY}}{\typeout{NON EMPTY}}

\part[]{Long}
\ifemptyshortpart{\typeout{EMPTY}}{\typeout{NON EMPTY}}

\part{Long}
\ifemptyshortpart{\typeout{EMPTY}}{\typeout{NON EMPTY}}

\end{document}

The output on the terminal is

EMPTY
NON EMPTY
EMPTY
NON EMPTY

Note that you get empty only if no \part command has been issued or \part is issued with an explicit empty optional argument. Otherwise beamer stores in \beamer@partnameshort the long version.

The \ifemptyshortpart command should be followed by the "true" and "false" branches:

\ifemptyshortpart{short part is empty}{short part is non empty}

Answer 2

If you add

\show\insertshortpart

you will see its definition is

> \insertshortpart=macro:
->\@protected@testopt \insertshortpart \\insertshortpart {}.

which is not the same definition as \@empty even if it produces no text.

You can grab this definition at the start and use it in the test:

\documentclass{beamer}
\let\origiginsertshortpart\insertshortpart
\begin{document}
\begin{frame}
%\show\insertpart
\ifx\insertpart\empty
empty part
\else
non-empty part
\fi

%\show\insertshortpart
\ifx\insertshortpart\origiginsertshortpart
empty short part
\else
non-empty short part
\fi
\end{frame}
\end{document}