Print only existing footnotes / suppress empty footnotes

Question

This is a follow-up to my earlier question Merge separate footnotes into text.

I now have a similar case, where I need to check whether there actually are footnotes relating to a particular number:

\documentclass[a4paper]{article}

\usepackage{lipsum}
\newcommand{\mylips}[1]{\printfootnote#1 \lipsum[#1]}

\def\printfootnote#1{\footnote{\csname extfootnote#1\endcsname}}
\def\definefootnote#1 #2\endfootnote{%
  \expandafter\def\csname extfootnote#1\endcsname{#2}}
\input{footnotefile}


\begin{document}
 \mylips{1}
 \mylips{2}
 \mylips{3}
 \mylips{4}
 \mylips{5}
 \mylips{6}
 \mylips{7}
\end{document}

footnotefile.tex would be

\definefootnote1 Some footnote\endfootnote
\definefootnote3 Another footnote\endfootnote
\definefootnote7 And another footnote\endfootnote 

The challenge is that I don't want the empty notes to appear at all.

How would I write a test for whether the result of a certain \printfootnote\arabic{somecounter} will be empty?

Answer 1

Well, your \printfootnote should check whether the footnote was defined:

\def\printfootnote#1{%
  \expandafter\ifx\csname extfootnote#1\endcsname\relax % by default \csname FOO\endcsname is \relax
 \stepcounter{footnote}\else % Even if we do not produce footnote, we step the number
 \footnote{\csname extfootnote#1\endcsname}\fi}

This works for me in your example.