Self loop in an undirected graph using TikZ

Question

How can I create a self loop without a direction on the edge?

\path[every node/.style={font=\sffamily\small}]
(3)   edge[loop] node  {$e_{1}$} (3);

creates the loop I want but has a direction.

Answer 1

For some reason, the TikZ every loop style contains an arrow. So you simple have to remove that:

\begin{tikzpicture}[every loop/.style={}]
    \node (3) {(3)};
    \path[every node/.style={font=\sffamily\small}]
        (3)   edge[loop] node  {$e_{1}$} (3);
\end{tikzpicture}

or remove it globally with

\tikzset{every loop/.style={}}

in the preamble.

Answer 2

There is another possibility. loop and every loop are simple styles and it's possible to redefine loop instead of every loop. It's interesting if you have a lot of loops and if you want some specific options

The code from pgf

% Loops

\tikzstyle{loop}=                    [to path={
  \pgfextra{\let\tikztotarget=\tikztostart}
  [looseness=8,min distance=5mm,every loop]
  \tikz@to@curve@path
  }]

\tikzstyle{every loop}=              [->,shorten >=1pt]

My modification

\documentclass{article}
\usepackage{tikz}
\makeatletter
\tikzset{my loop/.style =  {to path={
  \pgfextra{\let\tikztotarget=\tikztostart}
  [looseness=12,min distance=10mm]
  \tikz@to@curve@path},font=\sffamily\small
  }}  
\makeatletter 

 \begin{document}     
\begin{tikzpicture}
    \node (3) {(3)} ;
    \path  (3)   edge[my loop] node[above]  {$e_{1}$} (3);
\end{tikzpicture}

\end{document}