[Tex/LaTex] Reset custom counter in new frame

beamercountersoverlays

I'm using a custom counter to cycle bullet point colours in beamer:

\documentclass{beamer}
\usepackage{graphicx,color}
\usepackage[T1]{fontenc}
\usepackage[latin9]{inputenc}
\usepackage{marvosym}

\newcommand{\myitem}[1]{\item[{\textcolor{#1}\Laserbeam}]}
\newcounter{myc}%[frame] <--if I uncomment [frame] it won't compile
\makeatletter
\def\newitem{
\stepcounter{myc}%
\ifnum\value{myc}=1
  \myitem{red}
\else\ifnum\value{myc}=2
  \myitem{green}
\else\ifnum\value{myc}=3
  \myitem{blue}
  \setcounter{myc}{0}
\fi\fi\fi
}
 
\begin{document}
\begin{frame}
    \begin{itemize}
        \newitem An item
        \newitem Another item
        \newitem Yet another item
        \newitem These items...
        \newitem ...are rather boring
    \end{itemize}
\end{frame}

\begin{frame}
    \begin{itemize}
        \newitem An item on a second slide
        \newitem Another item
        \newitem Yet another item
        \newitem These items...
        \newitem ...are rather boring
    \end{itemize}
\end{frame}
\end{document}

with the following result (2 slides combined in to 1 image)
example, non resetting

However I would like the colours to reset at the start of a new frame, without having to insert \setcounter{myc}{0} after each \begin{frame} Based on this answer I thought I might be able to use \newcounter{myc}[frame], but I can't: ! LaTeX Error: No counter 'frame' defined.

So how do I reset a counter per frame?

Best Answer

You were near: the counter is called framenumber. There's an easier way to branch for numeric cases.

\documentclass{beamer}
\usepackage{graphicx,color}
\usepackage[T1]{fontenc}
\usepackage[latin9]{inputenc}
\usepackage{marvosym}

\newcommand{\myitem}[1]{\item[{\textcolor{#1}\Laserbeam}]}
\newcounter{myc}[framenumber]
\newcommand\newitem{%
  \stepcounter{myc}%
  \ifcase\value{myc}%
    % no value for 0
  \or
    \myitem{red}%
  \or
    \myitem{green}%
  \or
    \myitem{blue}%
    \setcounter{myc}{0}%
  \fi
}

\begin{document}
\begin{frame}
    \begin{itemize}
        \newitem An item
        \newitem Another item
        \newitem Yet another item
        \newitem These items...
        \newitem ...are rather boring
    \end{itemize}
\end{frame}

\begin{frame}
    \begin{itemize}
        \newitem An item on a second slide
        \newitem Another item
        \newitem Yet another item
        \newitem These items...
        \newitem ...are rather boring
    \end{itemize}
\end{frame}
\end{document}

An easier way for extending the number of colors is using xparse; you just call \assigncolors{red,green,blue} or with any other list. You can issue this command any times you want during the document, but it's better to do it outside frames, to avoid multiple evaluations.

\documentclass{beamer}
\usepackage{graphicx,color}
\usepackage[T1]{fontenc}
\usepackage[latin9]{inputenc}
\usepackage{marvosym}
\usepackage{xparse}

\newcounter{myc}[framenumber]

\ExplSyntaxOn
\NewDocumentCommand{\newitem}{}
 {
  \chris_newitem:
 }

\NewDocumentCommand{\assigncolors}{m}
 {
  \chris_assign_colors:n { #1 }
 }

\seq_new:N \g_chris_colors_seq
\int_new:N \l_chris_color_int
\cs_new_protected:Npn \chris_assign_colors:n #1
 {
  \seq_gset_from_clist:Nn \g_chris_colors_seq { #1 }
 }
\cs_new_protected:Npn \chris_newitem:
 {
  \stepcounter{myc}
  \int_set:Nn \l_chris_color_int
   {
    \int_mod:nn { \value{myc} } { \seq_count:N \g_chris_colors_seq }
   }
  \int_compare:nT {\l_chris_color_int = 0 }
   {
    \int_set:Nn \l_chris_color_int { \seq_count:N \g_chris_colors_seq }
   }
  \chris_do_item:n
   {
    \seq_item:Nn \g_chris_colors_seq { \l_chris_color_int }
   }
 }
\cs_new:Npn \chris_do_item:n #1
 {
  \item[ \textcolor { #1 }{ \Laserbeam } ]
 }
\ExplSyntaxOff

\assigncolors{red,green,blue}

\begin{document}
\begin{frame}
    \begin{itemize}
        \newitem An item
        \newitem Another item
        \newitem Yet another item
        \newitem These items...
        \newitem ...are rather boring
    \end{itemize}
\end{frame}

\begin{frame}
    \begin{itemize}
        \newitem An item on a second slide
        \newitem Another item
        \newitem Yet another item
        \newitem These items...
        \newitem ...are rather boring
    \end{itemize}
\end{frame}
\end{document}
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