Why is escaped # breaking my command?

Question

I have created the following two commands:

\newcommand{\termExplanation}[1]{\item[#1]\label{#1}}
\newcommand{\newTerm}[1]{\hyperref[#1]{\emph{#1}}}

In my text, I use \newTerm{Term 3}, and somewhere else in the document is my term explanation:

\begin{description}
    \termExplanation{Term 1} description...
    \termExplanation{Term 2} description...
    \termExplanation{Term 3} description...
    ...
\end{description}

Working fine so far. But now I wanted to introduce the new Term C#. Of course I used \newTerm{C\#} and \termExplanation{C\#}.
But it fails:

% occurs at \newTerm{C\#}
Missing \endcsname inserted. ..., die Programmiersprache ist \newTerm{C\#}
Extra \endcsname. ..., die Programmiersprache ist \newTerm{C\#}
% occurs at \termExplanation{C\#}
Missing \endcsname inserted. ....1}{8}{Begriffserklärung}{section.A.1}{}}
Extra \endcsname. ....1}{8}{Begriffserklärung}{section.A.1}{}}

When compiling for the first time, the text has two C#, the second one is emphasized and there is no link on either one. The explanation output is just fine. When recompiling there is no pdf output.

What am I doing wrong?

You can't use \# in a label. – egreg Feb 26 '16 at 12:26 — egreg, Feb 26 '16 at 12:26
that's sad.... thought i could avoid writing C-Sharp – TheFlow0360 Feb 26 '16 at 12:29 — TheFlow0360, Feb 26 '16 at 12:29

score 6 · Answer 1 · answered Feb 26 '16 at 14:58

You can't generally use \# in a label, because it doesn't expand to a character. However, in this particular situation a workaround is possible:

\documentclass{article}
\usepackage{hyperref}

\newcommand{\termExplanation}[1]{%
  \item[#1]%
  \begingroup
  % locally disable \#
  \def\#{?hashmark?}%
  \phantomsection\label{#1}%
  \endgroup
}
\newcommand{\newTerm}[1]{%
  \begingroup
  % locally disable \#
  \def\#{?hashmark?}%
  \edef\x{\endgroup\noexpand\hyperref[#1]}\x{\emph{#1}}%
}


\begin{document}
\begin{description}
\termExplanation{C\#} Hello, World!
\termExplanation{lorem} Lorem Ipsum!
\end{description}

\newTerm{C\#} \newTerm{lorem}
\end{document}

The trick is to tell LaTeX that \# in a label writes ?hashmark?; the same trick is used to retrieve the label before \hyperref does its job.

Don't forget \phantomsection that creates a suitable anchor.

\phantomsection is a really good hint, i was missing this - but since I now use glossries anyways, it wasn't that important. Still, thx for your answer — TheFlow0360, Feb 26 '16 at 15:00

score 5 · Accepted Answer · edited Apr 13 '17 at 12:35

The problem is that the content of \label doesn't handle these special characters well. The easiest alternative would be to simply not use the special characters and define variants of your commands that allow an arbitrary label in such case:

\documentclass{article}
\usepackage{hyperref}

\makeatletter
\def\termExplanation{%
  \@ifnextchar[{\termExplanation@opt}{\termExplanation@noopt}
}
\def\termExplanation@opt[#1]#2{%
  \item[#2]\label{#1}%
}
\def\termExplanation@noopt#1{\termExplanation@opt[#1]{#1}}

\def\newTerm{%
  \@ifnextchar[{\newTerm@opt}{\newTerm@noopt}%
}
\def\newTerm@opt[#1]#2{%
  \hyperref[#1]{\emph{#2}}%
}
\def\newTerm@noopt#1{\newTerm@opt[#1]{#1}}
\makeatother

\begin{document}
\begin{description}
\termExplanation[chash]{C\#} Hello, World!
\termExplanation{lorem} Lorem Ipsum!
\end{description}

\newTerm[chash]{C\#} \newTerm{lorem}
\end{document}

To understand what this bit of code those, I recommend you have a look at:

as they provide very comprehensive answers. Otherwise, the \def command is the TeX primitive behind \newcommand and offers a little more flexibility in the way arguments can be given, though definitely in most cases use \newcommand.

A better option alternative, as suggested by egreg in the comments, is:

\makeatletter
\newcommand\termExplanation{\@dblarg\termExplanation@opt}
\def\termExplanation@opt[#1]#2{\item[#2]\label{#1}}
\newcommand\newTerm{\@dblarg\newTerm@opt}
\def\newTerm@opt[#1]#2{\hyperref[#1]{\emph{#2}}}
\makeatother

As a last note, it really seems to me like you are creating a glossary. If so, then you should also have a look at the glossaries package. I provide a small MWE of the glossaries package below:

Main file:

\documentclass{article}

\usepackage{hyperref}
\usepackage[toc]{glossaries}

\renewcommand*{\glstextformat}[1]{\emph{#1}}
\loadglsentries{glossary}

\makeglossaries

\begin{document}

You can now refer to glossary terms with \gls{permittivity} and
\gls{permeability}.  You can even get the plural with \glspl{permittivity} and
get the capitalized version with \Gls{permittivity} and \Glspl{permeability}.

\clearpage
\printglossaries

\end{document}

and glossary.tex:

\newglossaryentry{permeability}{
    name={permeability},
    plural={permeabilities},
    symbol={\(\mu\)},
    parent={},
    description={A measure of how easy it is to form a magnetic field in a medium.}
}

\newglossaryentry{permittivity}{
    name={permittivity},
    plural={permittivities},
    symbol={\(\varepsilon\)},
    parent={},
    description={A measure of the resistance encountered when forming an electric field in a medium}
}

and the output:

Note that the glossaries package requires an additional run of makeglossaries in order to process the glossary entries.

is glossaries easy to use? I struggled with integrating bibtex before, so now I hesitate to use big packages when i can achieve my wanted result with some easy macros — TheFlow0360, Feb 26 '16 at 12:33
and also, could you give a little explanation to your answer? I' m not that familiar with LaTeX, e.g. I don't know what \makeatletter and \makeatother do ^^ — TheFlow0360, Feb 26 '16 at 12:35
your solution doesn't work: Illegal parameter number in definition of \termExplanation@noopt. ...anation@noopt#1{\termExplanation@opt[#1]#2 - I tried to resolve this myself bad had no luck, don't know the syntax that well — TheFlow0360, Feb 26 '16 at 12:43
@TheFlow0360 I updated my answer to include a minimal example of how to use glossaries. Also, I there was a typo in one of the definitions where it should've been [#1]#1 but instead I wrote [#1]#2. — JP-Ellis, Feb 26 '16 at 12:48
still not working properly for me, but I think I'll use glossaries anyways, so not a problem - thx for the extensive answer — TheFlow0360, Feb 26 '16 at 12:56
@TheFlow0360 I added a more complete example showing how the code goes in the document and how the function should be used. — JP-Ellis, Feb 26 '16 at 13:02
I tried it like you wrote there, and it didn't work - maybe i just misstyped somewhere, but I just rewrote everything to use glossaries, so I won't try it out again — TheFlow0360, Feb 26 '16 at 13:08
By the way, it should be \newcommand\termExplanation{\@dblarg\@termExplanation} followed by \def\@termExplanation[#1]{#2}{\item[#2]\label{#1}} — egreg, Feb 26 '16 at 14:40

Why is escaped # breaking my command?

2 Answers2