Use of forloop-counter inside beamer \only

Question

I'm using LaTeX beamer and the forloop-package to create a for loop, and I want to use the counter inside an \only-command. Here is a MWE (or more like non-working example):

\documentclass[12pt]{beamer}

\usepackage{forloop}

\begin{document}
  \begin{frame}
  \begin{center}
    \newcounter{slideno}
    \forloop{slideno}{1}{\value{slideno} < 20}{
      \only<\value{slideno}>{\arabic{slideno}}%
    }
  \end{center}
\end{frame}
\end{document}

The error message is:

! LaTeX Error: Command \c@slideno already defined.
               Or name \end... illegal, see p.192 of the manual.

Now, the post over here[0] tells me that using counters like this in \only should be possible, and in fact, if I comment out the for-loop (and add a \setcounter), it works like a charm.

Does the for-loop some magic to the counter? Can I somehow circumvent this?

Thanks a lot,

Lukas

[0] usage of \only with counter, pdfcomment and beamer class

Answer 1

Since frame itself does replicate the content somehow with each slide, a \newcounter command inside a frame with more than slide is of course leading to a multiple definition of slideno, which is wrong.

Put the \newcounter{slideno} definition into the preamble (which is always a good idea for counters.)

There's also missing % in the \forloop

   \forloop{slideno}{1}{\value{slideno} < 20}{
      \only<\value{slideno}>{\arabic{slideno}}%
    }

will lead to numbers moving to the right, whereas

   \forloop{slideno}{1}{\value{slideno} < 20}{%
      \only<\value{slideno}>{\arabic{slideno}}%
    }

will provide 'stationary' numbers.

\documentclass[12pt]{beamer}

\usepackage{forloop}

\newcounter{slideno}

\begin{document}
  \begin{frame}
  \begin{center}
    \forloop{slideno}{1}{\value{slideno} < 20}{%
      \only<\value{slideno}>{\arabic{slideno}}%
    }
  \end{center}
\end{frame}
\end{document}