beamer's \only command, equation counter increment does not get retained outside of \only

Question

I've got a problem where I am using beamer's \only command, but within \only, the equation number incrementation doesn't survive. I.e., I'd like to do

\only<1>{
\begin{align}
  equation 1
\end{align}
}

\only<2>{
\begin{align} 
  equation 2 
\end{align} 
}

The problem is that both equations show up as having equation number 1, rather than the first being equation 1 and the 2nd being equation 2. One way to deal with this seems to be to add a \refstepcounter{equation} after the first only, but that is not ideal, and it depends on how many equations are in the first \only environment (the same problem would occur if defining a macro, say, that called the \refstepcounter{equation} at the end of the only, i.e,. the user would need to keep track of how many equations are in each \only).

Is there a better way to do this? thanks very much!

Answer 1

The following code adds a command to remove the equation counter from the list of counters that are reset after each overlay. (Warning: I don't know a lot about Beamer's internals, so it is possible that it breaks something that I am not aware of (though I did some testing).) Just add the definitions to your preamble (don't forget the \makeatletter) and use the commands as in the example.

\documentclass{beamer}

\makeatletter
% The following two commands should only be given between frames

% Remove the equation counter from the list of counters that are reset after
% each overlay.
\def\donotresetequations{{%
    \let\@@elt\relax
    \def\@elt##1{%
        \expandafter\ifx\csname ##1\endcsname\c@equation%
        \else%
            \@@elt {##1}%
        \fi%
    }%
    \edef\beamer@overlaycounterresets{\beamer@overlaycounterresets}%
    \let\@elt\relax%
    \def\@@elt{\@elt}%
    \xdef\beamer@overlaycounterresets{\beamer@overlaycounterresets}%
}}

% Add the equation counter from the list of counters that are reset after
% each overlay.
\def\resetequations{\resetcounteronoverlays{equation}}
\makeatother

% Example document
\begin{document}

\begin{frame}
    \only<1>{
        \begin{align}equation\ 1.1\end{align}
    }
    \only<2>{
        \begin{align}equation\ 1.2 \end{align}
    }
\end{frame}

\donotresetequations

\begin{frame}
    \only<1>{
        \begin{align}equation\ 2.1\end{align}
    }
    \only<2>{
        \begin{align}equation\ 2.2\end{align} 
    }
\end{frame}

\resetequations

\begin{frame}
    \only<1>{
        \begin{align}equation\ 3.1\end{align}
    }
    \only<2>{
        \begin{align}equation\ 3.2 \end{align}
    }
\end{frame}
\end{document}

Answer 2

You can use the \alt environment to include the equation when you want and increment the counter when you don't:

\documentclass{beamer}

\begin{document}
\begin{frame}
\alt<1>{% do this in frame 1
   \begin{align}
      equation 1
   \end{align}
}{% do this not in frame 1
   \stepcounter{equation}
}

\only<2>{
   \begin{align} 
      equation 2 
   \end{align}
}

\end{frame}

\end{document}

Answer 3

I think I might have stumbled upon an answer to my own question that does not require any beamer modification. It uses the beamer overprint feature and the \onslide feature. Critical is that you use the \onslide feature without any braces (i.e., if you were to change the below to use \onslide<1>{ stuff goes in here }, then the example fails to work and the contents continues to occupy space. Not using the braces, however, will (like the \only command) not occupy any space, but will increment the equation numbers for some reason. I am not sure if this is intentional beamer behavior or not, but it appears to work.

\begin{frame}{test slide}
This is some front text to exist on all slides.
\begin{overprint}
\onslide<1>
this goes on slide 1
\begin{align}
  eq 1
\end{align}
\onslide<2>
this goes on slide 2
\begin{align}
  eq 2
\end{align}
\end{overprint}
\end{frame}