[Tex/LaTex] Using \multicolumn with beamer inside an overlay command

beameroverlaystables

I'd like to have a conditional \multicolumn command depending on the slide, so I put it inside an alt or an only command, as follows:

\documentclass{beamer}
\usepackage[latin1]{inputenc}
\begin{document}
\begin{frame}
\begin{equation*}
  \begin{array}{rlll}
  a&b&c&d\\
  A&\multicolumn{3}{l}{BCD}\\ %this is OK
   A&B&C&D\\ %as is this obviously...
  A&\only<-+>{\multicolumn{3}{l}{BCD}}\only<.->{B&C&D}\\ %but this
  A&\alt<-+>{\multicolumn{3}{l}{BCD}}{B&C&D}\\% and this do not compile
  A&\multicolumn{3}{l}{\only<-+>{BCD}\only<+->{\begin{tabular}{lll}B&C&D\\\end{tabular}}} %this line works, but, of course, gives the wrong alignment...
\end{array}
\end{equation*}
\end{frame}
\end{document}

The problem seems to be that LaTeX wants the \multicolumn command to appear as the first thing after an & or a newline…but then, as the last line shows, I don't know how to align with the other columns…

Currently I'm solving the problem using \hspace* but this is a bit of a ugly hack.

Any ideas on how I could get this to work properly??

Best Answer

I tried sprinkling some magical fairy \expandafter dust around to see if the \only part could be processed before the ampersand, but it resulted in errors.

This requires only one token of repetition:

A\alt<-+>{&\multicolumn{3}{l}{BCD}}{&B&C&D}\

I could not get your \only<-+>...\only<.-> construction to work with the +s. It does work with explicit numbers. But I assume that if the \alt construction works you'll take it as preferable.

Complete example:

\documentclass{beamer}
\usepackage[latin1]{inputenc}
\begin{document}
\begin{frame}
\begin{equation*}
  \begin{array}{rlll}
   a&b&c&d\\
   A&\multicolumn{3}{l}{BCD}\\ %this is OK
   A&B&C&D\\ %as is this obviously...
   A\only<1>{&\multicolumn{3}{l}{BCD}}\only<2->{&B&C&D} \\% this works now, but not with -+ etc.
   A\alt<-+>{&\multicolumn{3}{l}{BCD}}{&B&C&D}\\% this works even with -+
\end{array}
\end{equation*}
\end{frame}
\end{document}
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