# How to change equation numbering style

Here is my MWE:

\documentclass[a4paper, 12pt]{report}
\usepackage{amssymb, amscd}
\begin{document}
\chapter{Test}

\begin{table}[h]
\center
\caption{Test}
\begin{tabular}{l | r}
Two & 2 \\
Four & 4
\end{tabular}
\end{table}

Test equation:
$$a+b=c^2$$
\end{document}


It produces this:

Now I would like to change equation numbering style. I want (1-1) instead of (1.1). How can I do that? Also, I want to preserve numbering style for tables and figures (Table 1.1:) , I don't want (Table 1-1:)

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You can try to redefine \theequation with \renewcommand{\theequation}{\thechapter--\arabic{equation}}. :) – Paulo Cereda Jun 30 '12 at 14:20
@PauloCereda: That's it, thanks :) Please post it as answer so I can accept it :) – xx77aBs Jun 30 '12 at 15:58

I was looking for some hints in the amsmath documentation and found the following trick (adapted, of course):

\usepackage{amsmath}
\renewcommand{\theequation}{\thechapter--\arabic{equation}}


If we look at the original definition of \theequation, we can find that it's more robust than our first attempt:

\long macro:->\ifnum \c@chapter >\z@ \thechapter .\fi \@arabic \c@equation


we can easily redefine it accordingly:

\makeatletter
\long\def\theequation{\ifnum \c@chapter > \z@ \thechapter --\fi \@arabic \c@equation}
\makeatother


Interestingly enough, there's another approach with the etoolbox package:

\usepackage{etoolbox}
\patchcmd{\theequation}{.}{--}{}{}


There we go. :)

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