Beamer: including items only some slides using a relative syntax

Question

I am looking for the syntax for making an item appear on say the next three slides. I'm imagining something like

\item<+-??>

The \item should appear here and disappears 3 slides later, in the same way it would if I used \only (i.e. the item shouldn't leave any space once it vanishes).

I'm illustrating this using \item, but I'm looking for a general syntax.

Answer 1

In Relative overlay specification in beamer? I covered how to use relative specifications, but there I stuck to the visible/invisible switch. What you want is the 'extended' syntax which also allows things like altering on some slides or (here) applying \only-like effects:

\documentclass{beamer}

\begin{document}
   \begin{frame}
   \begin{itemize}
      \item<+-> item 1
      \item<+-> item 2
      \item<only@+-.(3)> item 3
      \item<+-> item 4
      \item<+-> item 5
      \item<+-> item 6
   \end{itemize}

   \end{frame}
\end{document}

The key here is that only@ prefix to the 'standard' relative specification, which switches it from doing the normal 'visible/invisible' action to the 'only' action. The part after the @ is exactly the same idea as described in my other answer.

Answer 2

I think you need to create your own macro, something like:

\documentclass{beamer}

\newcommand{\mytimeditem}[1]{\only<+>{\item #1}\only<+>{\item #1}\only<+>{\item #1}}
\begin{document}
   \begin{frame}
   \begin{itemize}
      \item<+-> item 1
      \item<+-> item 2
      \mytimeditem{item 3}
      \item<+-> item 4
      \item<+-> item 5
      \item<+-> item 6
   \end{itemize}

   \end{frame}
\end{document}

If you want to leave a blank line where "item 3" was, replace the last \only into \mytimeditem with \onslide

Second version:

\documentclass{beamer}

\newcommand{\mytimeditem}[1]{\only<+>{\item #1}\only<+>{\item #1}\only<+>{\item #1}\addtocounter{beamerpauses}{-2}}
\begin{document}
   \begin{frame}
   \begin{itemize}
      \item<+-> item 1
      \item<+-> item 2
      \mytimeditem{item 3}
      \item<+-> item 4
      \item<+-> item 5
      \item<+-> item 6
   \end{itemize}

   \end{frame}
\end{document}

A more generic version

With expl3 (code from @Joseph Wright answer) and calc

\documentclass{beamer}
\usepackage{expl3}
\usepackage{calc}

\ExplSyntaxOn
\cs_new_eq:NN \Repeat \prg_replicate:nn
\ExplSyntaxOff

\newcommand{\timeditem}[2]{
 \Repeat{#1}{
   \only<+>{\item #2}
 }\addtocounter{beamerpauses}{-#1+1}
}

\begin{document}
   \begin{frame}
   \begin{itemize}
      \item<+-> item 1
      \item<+-> item 2
      \timeditem{4}{item 3}
      \item<+-> item 4
      \item<+-> item 5
      \item<+-> item 6
      \item<+-> item 7
   \end{itemize}
   \end{frame}
\end{document}

Now the relative number of repetitions can be specified on the fly and the number of pauses is automatically adjusted.