# [Tex/LaTex] Problem with beamer’s \pause in alignments

alignbeamerpause

I have the following code, (edit:) now without aligned.

\documentclass{beamer}
\mode<presentation> { \setbeamercovered{transparent} }

\begin{document}
\begin{frame}
\begin{cases} %\begin{aligned} v(t,x) &= g(t,x), \\ \pause u(x) &= f(x) %& (x\in\mathbb R) %\end{aligned} \end{cases}
\end{frame}
\end{document}


(In the first version of the question, nothing was commented out.)

It compiles nicely, but everything except "u(x)" already shows up on the first slide if I use beamer.cls v 1.70 2007/03/11 18:50:44. On another computer at work we have an older version installed, namely v 1.55 2004/10/11 16:09:50; there everything works as expected: On the first slide, the full second line is shown in transparent style.

The same happens if I use plain TeX's \halign, so cases is not the culprit. Moreover, if I don't use \setbeamercovered{transparent}, then everything works as expected. In other words, \pause doesn't work together with \setbeamercovered{transparent} and \halign.

Is this a bug in beamer introduced between versions 1.55 and 1.70? (If yes, then I'm surprised since it'd regard it quite a serious one.) Does anyone know a workaround that doesn't involve as much markup as Matthew's answer to my original question?

From the beamer manual, in the middle of a use case:

Euclid finds that he can also add a \pause between the definition and the example. So, \pauses seem to transcede environments, which Euclid finds quite useful. After some experimentation he finds that \pause only does not work in align environments. He immediately writes an email about this to beamer’s author, but receives a polite answer stating that the implementation of align does wicked things and there is no fix for this. Also, Euclid is pointed to the last part of the user’s guide, where a workaround is described. (emphasis added)

Here is my workaround, using \action:

\documentclass{beamer}
\mode<presentation> { \setbeamercovered{transparent} }

\begin{document}
\begin{frame}
\begin{cases} \begin{aligned} \action<+->{v(t,x) &= g(t,x), \\} \action<+->{u(x)} &\action<.->{=f(x)} & \action<.->{(x\in\mathbb R)} \end{aligned} \end{cases}
\end{frame}
\end{document}


It's surprising that the second line has to have each cell of the align enclosed in an action specification while the first line can go in a single one (maybe that has something to do with the fact that it's shown by default on all slides). But I remember Till's advice to Euclid:

align does wicked things and there is no fix for this.

Added: \action is a generic \only if I recall correctly. And the incremental overlay specifications like <+-> are very much worth learning. See Section 8 of the beamer manual, or at least it's Section 8 of my 3.06 manual: "Creating Overlays."