Difficulty with \write

Question

I have a macro \Content wich contains several occurence of a macro \comm. The latter increases a counter a displays the value of the counter. Calling \Content gives the correct output. But i want to write this \Content into a file, and i encounter a problem. I guess, this is because \write does not like \stepcounter. I would like to understand why, and to find a workaround.

\documentclass{article}     
\begin{document}

\newcounter{myCounter}

\newcommand{\comm}{%
    \stepcounter{myCounter}%
    var\arabic{myCounter} %
}
\def\Content{text \comm text \comm text \comm}

\Content %%%% This works

\newwrite\tempfile
\immediate\openout\tempfile=counter.txt
\immediate\write\tempfile{\Content}
\immediate\closeout\tempfile

\end{document}

PS : by the way, the error message is Missing \endcsname inserted. which i don'T really understand...

EDIT1 I formulate more precisely my question : the macro \Content is given, and i dont' want to modify it. I just know that it contains many occurences of \comm. And i want to write the content of \COntent into a file, but replacing each \comm by something like var1, var2 which increases with some counter.

Answer 1

\write is an expansion context like \edef so only expansion happens, no assignments, so no \def or counter assignments. You need to increment the counter before the write.

If (as noted in the comments) you really want to use expansion then you can't use \advance (or equivalently) \stepcounter but you could do:

\documentclass{article}     
\begin{document}

\newcounter{myCounter}

\def\Content{text \comm text \comm text \comm}

\newwrite\tempfile
\immediate\openout\tempfile=counter.txt
{%
\def\comm#1\expandcomm#2{var#2 #1\expandcomm}
\def\expandcomm#1\relax{}
\immediate\write\tempfile{\Content\expandcomm123456789\relax}%
}%
\immediate\closeout\tempfile

\end{document}

which writes:

text var1 text var2 text var3 

Answer 2

Here's how one can do it in Plain TeX. It shouldn't be difficult to turn it into LaTeX.

\newcount\roycount

\def\writecontent{\buildcontent\immediate\write\tempfile{\commcontent}}
\def\buildcontent{\def\commcontent{\gobble}\step{text}{var}\step{text}{var}\step{text}{var}}
\def\step#1#2{\advance\roycount 1
  \edef\commcontent{\commcontent\space#1\space#2\number\roycount}}
\def\gobble#1{}

\newwrite\tempfile
\immediate\openout\tempfile=\jobname.out

\writecontent
\writecontent

\immediate\closeout\tempfile

\bye

As you see the string to write is build before the write (where assignments can't be performed).

The written file is

text var1 text var2 text var3
text var4 text var5 text var6