[Tex/LaTex] Petersen graph with new tikz graph library

graphstikz-pgftkz-graph

I'm considering moving from tkz-berge to the new tikz graph library
for drawing my graphs (in the sense of graph theory). I have produced
the Petersen graph, but is there is a more elegant way of
coding it? I'm specifically interested in a way to avoid having to
define a new counter.

\documentclass{article}
\usepackage{tikz}

\usetikzlibrary{graphs}
\usetikzlibrary{graphs.standard}

\begin{document}

\begin{tikzpicture}[every node/.style={draw,circle,very thick}]
  \graph[clockwise, radius=2cm] {subgraph C_n [n=5,name=A]};
  \graph[clockwise, radius=1cm] {subgraph I_n [n=5,name=B]};

  \foreach \i in {1,2,3,4,5}{\draw (A \i) -- (B \i);}
  \newcounter{j}
  \foreach \i in {1,2,3,4,5}{%
  \pgfmathsetcounter{j}{ifthenelse(mod(\i+2,5),mod(\i+2,5),5)}
  \draw (B \i) -- (B \thej);
  }
\end{tikzpicture}

\end{document}

enter image description here

Best Answer

Instead of explicitly defining a new counter \j you can use evaluate. Of course, this isn't much of a saving as you still need to define \j inside the evaluate statment, but it does save a loop:

enter image description here

\documentclass{article}
\usepackage{tikz}

\usetikzlibrary{graphs}
\usetikzlibrary{graphs.standard}

\begin{document}

\begin{tikzpicture}[every node/.style={draw,circle,very thick}]
  \graph[clockwise, radius=2cm] {subgraph C_n [n=5,name=A] };
  \graph[clockwise, radius=1cm] {subgraph I_n [n=5,name=B] };

  \foreach \i [evaluate={\j=int(mod(\i+2+4,5)+1)}]% using Paul Gaborit's optimisation
     in {1,2,3,4,5}{
    \draw (A \i) -- (B \i);
    \draw (B \j) -- (B \i);
  }
\end{tikzpicture}
\end{document}

Note that you need to take int(...) of the mod statement because otherwise you are asking to draw edges like (B 1.0) -- (B 3.0), which is not what you want.

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