Foreach inside a TikZ matrix

Question

Consider this MWE:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{matrix}

\begin{document}
\begin{tikzpicture}
\matrix [matrix of nodes] {
a \\
b \\
c \\
};
\end{tikzpicture}
\end{document}

It might be convenient to fill the matrix with a foreach loop:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{matrix}

\begin{document}

\begin{tikzpicture}
\matrix [matrix of nodes] {
\foreach \c in {a,b,c}{
\c \\
}
};
\end{tikzpicture}
\end{document}

This however fails with the message

 ! Extra }, or forgotten
 \endgroup.

It seems like the ending brace of \foreach bothers the matrix node. Can a matrix be populated with \foreach and how?

Answer 1

\foreach isn't expandable and the grouping is most likely causing some issues as well. I would recommend to use the loop outside the matrix and accumulate the rows in a macro which then only has to be expanded. The etoolbox package provides \gappto (global append to; \xappto would cause issues with fragile content) which can be used here. There is also \g@addto@macro provided by the LaTeX kernel, if you don't want to rely on e-TeX and don't mind to have to use \makeatletter/\makeatother.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{matrix}
\usepackage{etoolbox}

\begin{document}

\begin{tikzpicture}
    \let\mymatrixcontent\empty
    \foreach \c in {a,b,c}{%
        \expandafter\gappto\expandafter\mymatrixcontent\expandafter{\c\\}%
        % or
        %\xappto\mymatrixcontent{\expandonce{\c\\}}
    }
    \matrix [matrix of nodes] {
        \mymatrixcontent
    };
\end{tikzpicture}

\end{document}