I'd like to have a macro where I can insert a key as argument and get a value which I have defined before. This should be similar to Create an array of variables but with strings/text instead of numbers.
The usage should look like:
\setprefix{foaf}{http://xmlns.com/foaf/0.1/}
and later
\getprefix{foaf}
which should return http://xmlns.com/foaf/0.1/.
When I try to use the code from the before mentioned question I get ! Illegal parameter number in definition of.
\def\setprefix#1#2{\expandafter\def\csname MY@#1\endcsname{#2}}
\def\getprefix#1{\csname MY@#1\endcsname}
Should be all you need. The MY@ internal prefix is arbitrary and just needs to be distinct from other prefixes (so don't use LT@ unless you want to break longtable). By using different strings there you can have different arrays of macros.
You can use \csname...\endcsname:
\documentclass{article}
\makeatletter
\newcommand\setprefix[2]{\global\@namedef{gecko@\detokenize{#1}}{#2}}
\newcommand{\getprefix}[1]{%
\@ifundefined{gecko@\detokenize{#1}}
{\@latex@warning{No prefix `\detokenize{#1}' defined}UNKNOWN}%
{\@nameuse{gecko@\detokenize{#1}}}%
}
\makeatother
\setprefix{foaf}{http://xmlns.com/foaf/0.1/}
\setprefix{\just#stupid@input}{http://xmlns.com/foaf/0.1/}
\begin{document}
\getprefix{foaf}
\getprefix{fuaf}
\getprefix{\just#stupid@input}
\end{document}
In the first argument of \setprefix you're basically limited in using only balanced braces.
\documentclass{article}
\newcommand\setprefix[2]{\expandafter\def\csname#1\endcsname{#2}}
\newcommand\getprefix[1]{\csname#1\endcsname}
\begin{document}
\setprefix{foaf}{http://xmlns.com/foaf/0.1/}
\getprefix{foaf}
\end{document}
Note also that you can recall the prefix with \key, in the example \foaf (only as long as the key is alphabetic).
ROLLBACK to original answer, removing caveat.
\setprefix{?.3$a}{foo}\useprefix{?.3$a} will work just fine
\expandafter\def\csname ?.3$a\endcsname{foo} and then \csname ?.3$a\endcsname does work without problems! Your initial solution did suffice just fine! (I'd had upvoted but I'm out of votes for today...)
\@namedefand\@nameusedo. Oretoolbox'\csdefand\csuse.#in one URI which resulted in the! Illegal parameter number in definition of.urlpackage and change#2in David's answer to\url{#2}.