A set implementation in LaTeX?

Question

Hi,

Consider the following straightforward implementation of a list in latex:

\newcommand{\add@to@list}[2]{%
  \ifx#2\@empty%
    \xdef#2{#1}%
  \else%
    \xdef#2{#2,#1}%
  \fi%
}%

I wonder if there is a simple way to implement a set (list with no repeated elements) ?

Answer 1

Try taking a look at the l3clist module in the expl3 bundle. It provides a basic programming interface to comma-separated lists.

---

Now that I'm back on a real machine, here's an example:

\documentclass{article}
\usepackage{expl3}
\begin{document}
\ExplSyntaxOn
\clist_new:N \l_my_clist
\clist_put_right:Nn \l_my_clist {hello}
\clist_put_right:Nn \l_my_clist {\unknown}
\clist_put_right:Nn \l_my_clist {hello}
\clist_remove_duplicates:N \l_my_clist
\clist_show:N \l_my_clist
\ExplSyntaxOff
\end{document}

Answer 2

This seems to work:

\newcommand{\add@to@set}[2]{%
   \ifx#2\@empty%
      \xdef#2{#1}%
   \else%
      \@expandtwoargs\@removeelement{#1}{#2}{#2}%
      \xdef#2{#2,#1}%
   \fi%
}%