How can I write % into a file?

Question

How can I write % into an auxiliary file? % will not work, because LaTeX thinks that I start a comment with it (in the main file, not in the auxiliary file!), and \% will write \% into the file. How can it be accomplished?

\documentclass{article}
\begin{document}
\newwrite\outfile
\immediate\openout\outfile=example.dat
\immediate\write\outfile{%}% will not work, of course, but neither \%
\immediate\closeout\outfile
\end{document}

Answer 1

There is \@percentchar that expands to a literal % character. You need to enclose your writing operation in a \makeatletter \makeatother pair

\makeatletter
\newwrite\outfile
\immediate\openout\outfile=example.dat
\immediate\write\outfile{\@percentchar}
\immediate\closeout\outfile
\makeatother

Answer 2

use \@percentchar or \charxxx where xxx is the decimal number of %

Answer 3

Following the comment of Martin Scharrer I read up on his newverbs package. With this the following is possible:

\documentclass{article}
\usepackage{newverbs}
\Verbdef\verbpercent{%}
\begin{document}
\newwrite\outfile
\immediate\openout\outfile=example.dat
\immediate\write\outfile{\verbpercent}
\immediate\closeout\outfile
\end{document}

Comparison with the use of \@percentchar:

• Advantage: \Verbdef can generally be used for any verbatim text, and doesn't require \makeatletter \makeatother.

• Disadvantage: It uses more of TeX's memory and more time to compile (both: slightly), and theoretically there could be an incompatibility between package newverbs and some other package.

(Therefore I use \@percentchar, but reference to the newverbs package surely is useful for the general case.)