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I would like to implement a command \drawColoredGraph which accepts a comma separated list of n colours and draws a graph on the vertices 1,...,n, in which the vertex k is coloured with the colour provided as the k-th argument.

I tried to implement this functionality using the \forcsvlist command from the very useful etoolbox package. I came up with the following

%!TEX program = lualatex
\documentclass{article}
\usepackage{tikz,etoolbox}
\usetikzlibrary{graphs,graphdrawing}
\usegdlibrary{circular}

\newcounter{countNodes}

\newcommand\drawColoredGraph[1]{%
  \setcounter{countNodes}{0}
  \begin{tikzpicture}
  \graph[simple necklace layout] {
    \forcsvlist{\drawNode}{#1} 1;
  };
  \end{tikzpicture}
}
\newcommand{\drawNode}[1]{\stepcounter{countNodes}\thecountNodes[fill = #1] -- }%

\newcommand\writeColoredNodes[1]{%
  \setcounter{countNodes}{0}
  \forcsvlist{\writeNode}{#1}1
}
\newcommand{\writeNode}[1]{\stepcounter{countNodes}{\color{#1}\thecountNodes} -- }


\begin{document}

\begin{tikzpicture}
\graph[simple necklace layout] {
  1[fill = red] -- 2[fill = blue] -- 3[fill = yellow] -- 4[fill = blue] -- 1;
};
\end{tikzpicture}

\writeColoredNodes{red,blue,yellow,blue} % writes 1–2–3–4–1 in the correct colors 

% \drawColoredGraph{red,blue,yellow,blue} % DOES NOT COMPLILE

\end{document}

The tikzpicture environment contains the example graph I am trying to create automatically. The writeColoredNodes command shows that the forcsvlist command principally does exactly what I want it to do.

For some reason \drawColoredGraph produces an endless list of error messages (either Undefined control sequence. or Missing \endcsname inserted., both of which I don't understand). What am I doing wrong?

I could imagine that the problem lies in the fact that the compiler first converts -- into a dash, after which Tikz does not recognise -- anymore as an edge in the graph but I neither know whether this is the core of the problem, nor how to fix it.

Any suggestions? I would also be open to other approaches for the above problem.

1 Answer 1

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Rather than using \forcsvlist I would use \foreach from tikz/pgf but I think that the real problem is building the path and then giving it to \graph, which involves expansion issues.

Here is a way around both of these problems:

\documentclass{article}
\usepackage{tikz}
\usepackage{etoolbox}% for \xappto
\usetikzlibrary{graphs,graphdrawing}
\usegdlibrary{circular}

\newcommand\drawColoredGraph[1]{%
  % build the graph specifications
  \def\graphspecs{}%
  \foreach \colour [count=\g] in {#1} {
      \xappto\graphspecs{\g [fill=\colour] -- }
  }%
  % expand specs but not \graph -- and close off graph specs by adding 1
  \xdef\graphspecs{\noexpand\graph[simple necklace layout]{\graphspecs 1}}%
  \begin{tikzpicture}% draw the graph
    \graphspecs;
  \end{tikzpicture}%
}

\begin{document}

   \drawColoredGraph{red,blue,yellow,blue}

\end{document}

Here is the output:

enter image description here

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