change the numbering in Beamer's table of content

Question

Beamer's table of contents starts with bullet 1. How can I change that to bullet 0? The command \setcounter{section}{-1} in the preamble does not help.

Answer 1

One would expect that setting \beamer@tocsectionnumber=-1 would be enough (since \beamer@tocsectionnumber is the TeX counter controlling the section number typset in the ToC); however, this is not the case, since \beamer@sectionintoc (defined in beamerbasetoc.sty) has a conditional test using \ifnum\beamer@tempcount>0 which won't allow typesetting non positive numbers in the ToC. The solution is to patch \beamer@sectionintoc and use as conditional test \ifnum\beamer@tempcount>-1:

\documentclass{beamer}
\usetheme{CambridgeUS}
\usepackage{etoolbox}

\makeatletter
\patchcmd{\beamer@sectionintoc}
  {\ifnum\beamer@tempcount>0}
  {\ifnum\beamer@tempcount>-1}
  {}
  {}
\beamer@tocsectionnumber=-1
\makeatother

\begin{document}

\begin{frame}
\tableofcontents
\end{frame}

\section{First test section}
\begin{frame}
test
\end{frame}
\section{Second test section}
\begin{frame}
test
\end{frame}
\section{Third test section}
\begin{frame}
test
\end{frame}

\end{document}