How to use \only or similar to align content inside `align` environment?

Question

Please consider the following MWE:

\documentclass{beamer}
\usetheme{Copenhagen}
\begin{document}
\begin{frame}
\begin{block}{}
Text.
\begin{align}
    A&=B\
    \only<2>{
    &\color{red}{C=D+1-5M\implies E=1}\
    }
    \uncover<3->{&=C\}
    \uncover<4->{&=B}
\end{align}
\end{block}
\end{frame}
\end{document}

The problem is on slide 2, where A=B is not in the same position as in slides 1, 3 and 5:

I want the red because is a note for a property that I will use in the next step, however it is the only slide that is not aligned with respect to the others.

I would like to have the following output (see slide 2.):

How can we achieve the last image not using commands like \hspace?

just put it in \[...\] after the alignment? (since you don't want it aligned) — David Carlisle, Mar 04 '21 at 20:58
@DavidCarlisle hi! Sorry if I have not explained well, I want A=B of slide 2 aligned with the rest of lines (=C and =B). Also, the red part should be aligned to the middle. — manooooh, Mar 04 '21 at 21:18
You said I don't want it aligned, what do you mean by that? I want to align the slide 2 properly. — manooooh, Mar 04 '21 at 21:26
the red part has no alignment with the black so no need to put it in the align just make it a \[..\[ — David Carlisle, Mar 04 '21 at 21:27
Do you mean break the align into two like this?: https://imgur.com/a/WVsYj3C It creates a lot of space around and the black lines are not aligned w.r.t. =. — manooooh, Mar 04 '21 at 21:32
actually what I meant doesn't work as well as I intended, so forget it:-) — David Carlisle, Mar 04 '21 at 21:34

score 2 · Accepted Answer · answered Mar 04 '21 at 21:02

In order for the elements to stop flickering between slides, they need to have the same width. One can assure they have the same width by using components from eqparbox (with a modification to allow for math content via \eqmathbox[<tag>][<align>]{<stuff>}). Additionally, I use \alt<os>{<default>}{<other>} which puts <default> on the provided overlay specification and <other> otherwise.

\documentclass{beamer}
\usetheme{Copenhagen}
\usepackage{eqparbox}
%\usepackage{xparse}% If you have LaTeX2e < 2020-10-01
% https://tex.stackexchange.com/a/34412/5764
\makeatletter
% \eqmathbox[<tag>][<align>]{<math>}
\NewDocumentCommand{\eqmathbox}{o O{c} m}{%
  \IfValueTF{#1}
    {\def\eqmathbox@##1##2{\eqmakebox[#1][#2]{$##1##2$}}}
    {\def\eqmathbox@##1##2{\eqmakebox{$##1##2$}}}
  \mathpalette\eqmathbox@{#3}
}
\makeatother
\begin{document}
\begin{frame}
\begin{block}{}
  Text.
  \begin{align}
    \eqmathbox[LHS][r]{A} &\eqmathbox[RHS][l]{{} = B} \
    \alt<2>{\eqmathbox[LHS][r]{\color{red}C = D + 1}}{} &
      \alt<2>{\eqmathbox[RHS][l]{\color{red}{} - 5M \implies E = 1}}{\uncover<3->{= C}} \
    \uncover<4->{&= D}
  \end{align}
\end{block}
\end{frame}
\end{document}

Thanks for the answer! It seems that my LaTeX distribution is having some issues with respect to eqparbox. I use pdfLaTeX. In Overleaf it throws me a lot of errors, and in TeXworks says: "! Undefined control sequence. l.29 \NewDocumentCommand {\eqmathbox}{o O{c} m}{%" — manooooh, Mar 04 '21 at 21:25
@manooooh add \usepackage{xparse} for older releases. (as Werner has already put in a comment in the code) — David Carlisle, Mar 04 '21 at 21:28

How to use \only or similar to align content inside `align` environment?

1 Answers1