Numbering slides within a frame

Question

In trying to answer this question, I encounter the following problem. Consider the example below. I have three slides in Frame 1 which I want to print consecutively on three pages in a handout, and number them 1(a), 1(b), and 1(c). Following Martin Scharrer's comment in this answer, I define a counter slideinframe that has the same value as \beamer@slideinframe, which keeps track of the current slide number within a frame.

The problem is when I use the counter in the body text, it produces the correct number; however, when used in the footline, the number is not correct. What's wrong here?

\documentclass[handout]{beamer}
\usetheme{Madrid}

% Define counter "slideinframe"
\makeatletter
\def\c@slideinframe{\beamer@slideinframe}
\makeatother

% Define new template style "frame renumbering"
\defbeamertemplate*{footline}{frame renumbering}
{
  \leavevmode%
  \hbox{%
  \begin{beamercolorbox}[wd=\paperwidth,ht=2.25ex,dp=1ex,right]{date in head/foot}%
    \usebeamerfont{date in head/foot}
    \insertframenumber(\alph{slideinframe}) / \inserttotalframenumber\hspace*{2ex} 
  \end{beamercolorbox}}%
  \vskip0pt%
}

\begin{document}

\begin{frame}{Frame 1}
This is frame \insertframenumber, slide (\alph{slideinframe})
\begin{itemize}
  \item<1| handout:1> Handout 1
  \item<2| handout:2> Handout 2
  \item<3| handout:3> Handout 3
\end{itemize}
\end{frame}

\end{document}

Answer 1

The problem is that beamer steps the counter for 'another frame' before doing the 'decoration':

\long\def\beamer@doseveralframes#1{%
  \ifx\beamer@againname\@empty\else%
    % Again stuff
    \expandafter \gdef\csname beamer@again@\beamer@againname\endcsname{{#1}}%
  \fi%
  \beamer@slideinframe=1\relax%
  \loop%
  \beamer@anotherslidefalse%
  \def\beamer@doifnotinframe{}%
  \def\beamer@doifinframe{\begin{beamer@frameslide}%
    #1%
  \end{beamer@frameslide}}%
  {\beamer@masterdecode{\beamer@whichframes}}%
  \beamer@donow%
  \ifbeamer@twoscreenstext%
    \beamer@dosecondscreennow{#1}% two screen stuff
  \fi%
  \ifbeamer@anotherslide
  \advance\beamer@slideinframe by 1\relax Slide number increased here
  \repeat%
  \beamer@writeslidentry% Footlines added here
  \beamer@reseteecodes}

At least with your example moving this fixes the issue

\documentclass[handout]{beamer}
\usetheme{Madrid}

% Define counter "slideinframe"
\makeatletter
\def\c@slideinframe{\beamer@slideinframe}
\long\def\beamer@doseveralframes#1{%
  \ifx\beamer@againname\@empty\else%
    % Again stuff
    \expandafter \gdef\csname beamer@again@\beamer@againname\endcsname{{#1}}%
  \fi%
  \beamer@slideinframe=1\relax%
  \loop%
  \beamer@anotherslidefalse%
  \def\beamer@doifnotinframe{}%
  \def\beamer@doifinframe{\begin{beamer@frameslide}%
    #1%
  \end{beamer@frameslide}}%
  {\beamer@masterdecode{\beamer@whichframes}}%
  \beamer@donow%
  \ifbeamer@twoscreenstext%
    \beamer@dosecondscreennow{#1}% two screen stuff
  \fi%
  \beamer@writeslidentry% Moved this line
  \ifbeamer@anotherslide
  \advance\beamer@slideinframe by 1\relax
  \repeat%
  \beamer@reseteecodes}
\makeatother

% Define new template style "frame renumbering"
\defbeamertemplate*{footline}{frame renumbering}
{
  \leavevmode%
  \hbox{%
  \begin{beamercolorbox}[wd=\paperwidth,ht=2.25ex,dp=1ex,right]{date in head/foot}%
    \usebeamerfont{date in head/foot}
    \insertframenumber(\alph{slideinframe}) / \inserttotalframenumber\hspace*{2ex} 
  \end{beamercolorbox}}%
  \vskip0pt%
}
\begin{document}

\begin{frame}{Frame 1}
This is frame \insertframenumber, slide (\alph{slideinframe})
\begin{itemize}
  \item<1| handout:1> Handout 1 
  \item<2| handout:2> Handout 2 
  \item<3| handout:3> Handout 3 
\end{itemize}
\end{frame}

\end{document}

I need to see if this is a general solution: the line does look misplaced to me.