A funny bug with input, newcommand, ifthenelse in a beamer

Question

First I write a tex named thequest like:

\begin{quest}
Just a problem.
\end{quest}

Then in the main tex, I write

\newtheorem{quest}{Question}
\newcommand{\Fakeframe}[1]{\ifthenelse{\equal{#1}{}}{}{
    \begin{frame}[t]
       #1
    \end{frame}
    }}
\FakeFrame{\input{thequest.tex}}

Then xelatex. There will be "Argument of \reserved@a has an extra }." and "Paragraph ended before \reserved@a was complete." But the pdf is correct.

Why????? If I don't use the ifthenelse, or just not use the input, everything will be OK. But if I use the newcommand with a ifthenelse, and that input, there must have two errors.

That's so weird.

Actually I want to define a \Fakeframe in both beamer and article. In the beamer, it should be a frame. But in the article, it will be just a normal text. @samcarter_is_at_topanswers.xyz — Ngiap, May 28 '23 at 14:08
Fundamentally your code is wrong because because \input{thequest.tex} is not fully expandable while \ifthenelse's argument require it to be fully expandable. But do you want to check if #1 itself is empty, or the file content is empty? — user202729, May 28 '23 at 14:09
@Ngiap Can you add a bit more background what the purpose of this macro is? Your question sounds very much like an x-y-problem. Maybe using the beamerarticle package would be easier than making fake frames? — samcarter_is_at_topanswers.xyz, May 28 '23 at 14:11
See https://tex.stackexchange.com/questions/279058/check-what-kind-of-document-class-is-used — Zarko, May 28 '23 at 14:13

egreg · Answer 1 · 2023-05-28T14:21:48.350

I'm not sure what this would be useful for: it makes little sense to check for emptiness of the argument, in that case just don't use \FakeFrame{}.

Anyway, \equal tries to expand its argument and with \input this fails. Use xifthen and \isempty.

\begin{filecontents*}{\jobname-quest}
\begin{quest}
Just a problem.
\end{quest}
\end{filecontents*}
\documentclass{beamer}
\usepackage{xifthen}
\newtheorem{quest}{Question}
\newcommand{\FakeFrame}[1]{\ifthenelse{\isempty{#1}}{}{%
    \begin{frame}[t]
       #1
    \end{frame}
    }}
\begin{document}
\FakeFrame{\input{\jobname-quest.tex}}
\end{document}

If you want to load the file only if it's not empty, then use a different strategy.

\begin{filecontents*}{\jobname-quest}
\begin{quest}
Just a problem.
\end{quest}
\end{filecontents*}
\begin{filecontents*}{\jobname-empty}
\end{filecontents*}
\documentclass{beamer}
\newtheorem{quest}{Question}
\ExplSyntaxOn
\NewDocumentCommand{\FakeFrameInput}{m}
 {% #1 = filename
  \file_get:nnN { #1 } { } \l_tmpa_tl
  \tl_remove_all:Nn \l_tmpa_tl { \par }
  \tl_if_blank:VF \l_tmpa_tl
   {
    \begin{frame}[t] \input{#1} \end{frame}
   }
 }
\ExplSyntaxOff
\begin{document}
\FakeFrameInput{\jobname-quest.tex}
\FakeFrameInput{\jobname-empty.tex}
\end{document}

This will only produce one frame. The file is read into a token list variable in which we remove all \par tokens (generated by empty lines). If nothing remains (or just spaces), the file is considered empty.

A funny bug with input, newcommand, ifthenelse in a beamer

1 Answers1