How can I position the text within a node of a graph?

Question

I would like to create a visualization of the Bellman-Ford algorithm. To do so, I need to add the information of the weight (number or $\infty$) and the predecessor (letter) into every node. This should look like that (done with Gimp):

How can I make that with LaTeX (Tikz)?

I am currently create my graph:

\documentclass{beamer}
\usepackage{tikz}
\usepackage{verbatim}
\usetikzlibrary{arrows,shapes}

\begin{document}
\pgfdeclarelayer{background}
\pgfsetlayers{background,main}

\begin{frame}
\tikzstyle{vertex}=[circle,fill=black!25,minimum size=20pt,inner sep=0pt]

    \begin{figure}
        \begin{tikzpicture}[scale=2.5, auto,swap]
            % First we draw the vertices
            \foreach \pos/\name in {{(0,2)/a}, {(1,2)/b}, {(2,2)/c},
                                    {(0,1)/d}, {(1,1)/e}, {(2,1)/f}, 
                                    {(0,0)/g}, {(1,0)/h}, {(2,0)/i}}
                \node[vertex] (\name) at \pos {$\name$};
            % Connect vertices with edges and draw weights
            \foreach \source/ \dest /\weight/\style in {a/b/0/, b/c/1/, a/d/2/, e/d/5/,
                                                b/e/3/bend right, e/b/-1/bend right, b/f/4/above, f/i/3/,
                                                i/e/1/, g/h/1/}
                \path (\source) edge[->,\style, thick] node {$\weight$} (\dest);
        \end{tikzpicture}
        \end{figure}
    \end{frame}
\end{document}

Answer 1

You can place the text using label={[<options>]center:<text>}, where center puts the label in the middle of the "host" node, so you'll have to move it using xshift=..., yshift=... in the <options> of the label.

\documentclass{beamer}
\usepackage{tikz}
\usepackage{verbatim}
\usetikzlibrary{arrows,shapes}

\begin{document}
\pgfdeclarelayer{background}
\pgfsetlayers{background,main}

\begin{frame}
\tikzset{
    vertex/.style={
        circle,
        fill=black!25,
        minimum size=20pt,
        inner sep=0pt,
        text height=1ex,
        text depth=1ex,
        label={[weight label]center:\weight},
        label={[pred label]center:\pred}
    },
    pred label/.style={
        font=\tiny,
        xshift=0.3em,
        yshift=-0.8ex,
        text height=1ex,
        text depth=0pt
    },
    weight label/.style={
        font=\tiny,
        xshift=-0.3em,
        yshift=-0.8ex,
        text height=1ex,
        text depth=0pt
    }
}


    \begin{figure}
        \begin{tikzpicture}[scale=2.5, auto,swap]
            % First we draw the vertices
            \foreach \pos/\name/\weight/\pred in {%
                {(0,2)/a/0/g},%
                {(1,2)/b/$\infty$/a}%
            }
                \node[vertex] (\name) at \pos {$\name$};
            % Connect vertices with edges and draw weights
            \foreach \source/ \dest /\weight/\style in {a/b/0/}
                \path (\source) edge[->,\style, thick] node {$\weight$} (\dest);
        \end{tikzpicture}
        \end{figure}
    \end{frame}
\end{document}