# Drawing a graph with pgfmathparse+matrix

I want to draw a graph. I use tikz matrix to draw the vertices, and then would like to add edges from an array. I tried the following code:

\documentclass{article}
\usepackage{tikz,pgfmath}
\usetikzlibrary{matrix}
\begin{document}
\begin{tikzpicture}
\matrix (w) [nodes={circle,draw,scale=0.66}, matrix of nodes, row sep=2em, column sep=0.5em]{
1&2&3\\
&4&5\\
&6\\
};
\def\V{{"w-1-1","w-1-2","w-1-3","w-2-2","w-2-3","w-3-2"}}
%\draw[->] (\V[1]) to (\V[2]); %Line A
%\node at (1,1) {\pgfmathparse{\V[2]}\pgfmathresult}; %Line B
\draw[->] (\pgfmathparse{\V[1]}\pgfmathresult) to (\pgfmathparse{\V[2]}\pgfmathresult); %Line C
\end{tikzpicture}
\end{document}


I tried Line A first, and it didn't work. It turned out that in order to access the array correctly I have to use pdfmathparse. But the code doesn't compile (although Line B works fine). (It seems that the output of pgfmathresult is a string which is not what draw expects.)

P.S.: The original motivation was to illustrate graph pebbling, succinctly. I have attached the final code, along with the result, in any case.

\documentclass{article}
\usepackage{tikz,pgfmath,xstring,algorithm}
\usetikzlibrary{matrix}

\newcommand{\Configuration}[6]{%A fixed graph
\begin{minipage}{.2\textwidth}%1
\begin{tikzpicture}
\matrix (w) [nodes={circle,draw,scale=0.6}, matrix of nodes, row sep=2em, column sep=0.5em,ampersand replacement=\&]{
1\&2\&3\\
\&4\&5\\
\&6\\
};
\def\V{{"w-1-1","w-1-2","w-1-3","w-2-2","w-2-3","w-3-2"}}%the mapping
\foreach \x/\y/\z in {{1/4/#1},{2/4/#2},{3/4/#3},{3/5/#4},{4/6/#5},{5/6/#6}}%pebbling
\pgfmathsetmacro{\X}{\V[\x-1]}
\pgfmathsetmacro{\Y}{\V[\y-1]}
\draw[->] (\X) to node {\ifthenelse{\z=0}{}{$\bullet$}} (\Y);
\end{tikzpicture}
\end{minipage}
}

\begin{document}
\begin{figure}
\centering
\Configuration{0}{0}{0}{0}{0}{0}
\Configuration{1}{0}{0}{0}{0}{0}
\Configuration{1}{1}{0}{0}{0}{0}
\Configuration{1}{1}{1}{0}{0}{0}
\Configuration{1}{1}{1}{0}{1}{0}
\Configuration{1}{1}{0}{0}{1}{0}
\Configuration{1}{0}{0}{0}{1}{0}
\Configuration{0}{0}{0}{0}{1}{0}
\Configuration{0}{0}{0}{1}{1}{0}
\Configuration{0}{0}{0}{1}{1}{1}
\Configuration{0}{0}{0}{0}{1}{1}
\end{figure}
\end{document}


## 1 Answer

Not sure about what do you want, but I think that \pgfmathsetmacro solves your problem:

\documentclass [border=2mm]{standalone}
\usepackage{tikz}
\usepackage{tikz,pgfmath}
\usetikzlibrary{matrix}

\begin{document}
\begin{tikzpicture}
\matrix (w) [nodes={circle,draw,scale=0.66}, matrix of nodes, row sep=2em, column sep=0.5em]{
1&2&3\\
&4&5\\
&6\\
};
\def\V{{"w-1-1","w-1-2","w-1-3","w-2-2","w-2-3","w-3-2"}}
\pgfmathsetmacro{\start}{\V[1]}
\pgfmathsetmacro{\finish}{\V[2]}
\draw[->] (\start) to (\finish); %Line C
\end{tikzpicture}
\end{document}