How to use \input inside \setlength?

Question

I'm trying this:

\documentclass{article}
\usepackage{calc}
\newlength{\foo}
\newcommand{\zzz}{\input{size.txt}}
\setlength{\foo}{2in * \zzz}
\usepackage[paperwidth=\foo,paperheight=\foo]{geometry}
\begin{document}
\end{document}

But I'm getting:

! Missing number, treated as zero.
<to be read again>
                   \let
l.5 \setlength{\foo}{2in * \zzz}

How to fix this?

Answer 1

The LaTeX \input command is not expandable: this is partly due to the fact it does 'safe' check for the existence of a file, and partly due to supporting both TeX primitive (\input <file> ) and LaTeX (\input{<file>}) syntaxes. This means that if you want to work by expansion, as you do here, you need to use the primitive

\begin{filecontents*}[overwrite]{size.txt}
5%
\end{filecontents*}
\documentclass{article}
\usepackage{calc}
\newlength{\foo}
\makeatletter
\let\primitiveinput\@@input
\makeatother
\newcommand{\zzz}{\primitiveinput size.txt }
\setlength{\foo}{2in * \zzz}
\usepackage[paperwidth=\foo,paperheight=\foo]{geometry}
\begin{document}
\end{document}

You'll need to make sure your input file doesn't have a \par token in it: I've added an example using a comment char.