Inserting a frame every n frames

Question

I am using beamer for a presentation (of sorts) and want to add a frame every n frames (let's keep things general). As an additional challenge, I want these extra slides to not be counted in the regular numbering (although this is maybe better solved by using a custom counter? See below).

What I've tried

I'm really close to getting this done, but my approach I guess can be improved.

What I've been trying so far is to use everypage to perform a check at every page to see if an extra slide should go after (I do this check using pgf), and if it is, I insert a frame with afterpage.

This works, but it adds one too many additional slides, because in a 10 page presentation it adds an extra one at the end (after page 10, which technically is not wrong, but is not what I want). Also, it means I need to add two packages, just for this (and that's just because I get pgf for free with beamer)!

Also, for this I've been using framenumber, but it's very possible that it would be better to use a different, custom counter (since beamer, you know... uses framenumber).

\documentclass{beamer}

\usepackage{afterpage}
\usepackage{everypage}

% Regular slide
\newcommand{\baseslide}[1]{
  \begin{frame}{\phantom{bg}}
    \begin{center}%
      \LARGE #1
    \end{center}
  \end{frame}
}

% Extra slide
\newcommand{\extraslide}{
  \baseslide{This slide was inserted}
  \addtocounter{framenumber}{-1}
}

% Modulo test using pgfmathparse
% Based on http://tex.stackexchange.com/a/15303/18982
\newcommand\modulo[4]{ 
  \begingroup
  \pgfmathsetmacro{\a}{#1}
  \pgfmathsetmacro{\b}{#2}
  \pgfmathparse{mod(\a,\b)==0} 
  \ifdim\pgfmathresult pt= 1 pt 
    #3
  \else 
    #4
  \fi 
  \endgroup
}

% At every page, check if the next should come before an inserted slide
\AddEverypageHook{
  \modulo{\value{framenumber}+1}{5}{
    % If so, add a slide after that page
    \afterpage{\extraslide}
  }{}
}

\begin{document}

\baseslide{This is test \insertframenumber}
\baseslide{This is test \insertframenumber}
\baseslide{This is test \insertframenumber}
\baseslide{This is test \insertframenumber}
\baseslide{This is test \insertframenumber}
% Slide should go here
\baseslide{This is test \insertframenumber}
\baseslide{This is test \insertframenumber}
\baseslide{This is test \insertframenumber}
\baseslide{This is test \insertframenumber}
\baseslide{This is test \insertframenumber}
% But it also goes here, and it shouldn't!

\end{document}

So, my questions:

1. Is there a better way to do this?
2. How can I make sure that these pages are only added within the document (not as the first or last page)?

Additional, perhaps not necessarily useful background

I'm using beamer to generate a series of slides with some text that I can show to people one at a time to elicit certain responses. In this particular case, they are sentences to be read during a recording session.

The extra slides I'm inserting are "breaks", so that I can give the participant (performer?) some time to rest, drink some water, or do whatever it is people do when they relax.

Answer 1

A non-answer using pdftk

With the existing solution (the MWE), the last spurious slide (in the cases where it exists) can be easily removed with the help of pdftk (although this sort of post-production makes me feel dirty inside):

pdftk "input.pdf" cat 1-r2 output "output.pdf"

Another possibility would be to go with an entirely pdftk solution.

I put together this bash script which works fine... but it messes with the pagination (of course) in such a way that in a file which originally had 10 pages, in which you added the slide every 5 pages, the page that was originally page 6 will be now page 7 (and labeled as such) in the final pdf. The whole reason for me messing with framenumber in the first place was to keep the page numbering ignore the extra slides, so this is not really a solution in my book. But for completion's sake, here it is:

#/bin/bash

file=$1
every=$2
out=$3

added=$(mktemp PDFXXXX.pdf)
pages=$(
  echo "$(
    pdftk "$file" dump_data |
    grep NumberOfPages   |
    cut -f2 -d" "
  )-1" | bc -l
)
n=$(echo "($pages - 1) / $every" | bc)

pdftk "$file" cat end output "$added"
pdftk "$file" cat 1-r2 output "$out"

for ((i=$n;i>0;i--)); do
  start=$(echo "($i * $every) + 1" | bc)
  end=$(echo "$start - 1" | bc)
  tmp=$(mktemp PDFXXXX.pdf)

  pdftk A="$out" B="$added" cat A1-$end B1 A$start-end output "$tmp"
  mv "$tmp" "$out"
done
rm "$added"

Ran as ./script.sh "input.pdf" 5 "output.pdf", it will extract the last page of input.pdf and create a new output.pdf file in which this page is inserted every 5 pages in the original.

Answer 2

The following example adds a slide before a new frame if needed.

• no additional frame at the beginning is ensured by \ifnum\value{framenumber}=0
• per construction an additional frame at the end is not possible

Con's

The additional frame has to use the \frame{} syntax and cannot be written as \begin{frame}

---

\documentclass{beamer}

\usepackage{etoolbox} 
\BeforeBeginEnvironment{frame}{
\ifnum\value{framenumber}=0
\else
    \pgfmathparse{int(mod(\value{framenumber},3))}
    \ifnum\pgfmathresult=0
        \addtocounter{framenumber}{-1}
        \frame{%
            whatever
        } 
    \fi
\fi
}


\usepackage{pgffor}
\usepackage{tikz}

\begin{document}

\foreach \i in {1,...,30} {
    \begin{frame}
        \insertframenumber
    \end{frame}
}

\end{document}