Listing inside loop body

Question

I want a listing in the body of a loop. E.g. write something like

\documentclass{article}

\usepackage{listings}

\begin{document}
    \makeatletter
    \lstset{escapechar=|}
    \newcounter{test}
    \@whilesw{\ifnum\value{test}<3}\fi{
        \stepcounter{test}
        \begin{lstlisting}
|\roman{test}|
        \end{lstlisting}
    }
\end{document}

instead of

\documentclass{article}

\usepackage{listings}

\begin{document}
    \lstset{escapechar=|}
    \newcounter{test}
    \stepcounter{test}
    \begin{lstlisting}
|\roman{test}|
    \end{lstlisting}
    \stepcounter{test}
    \begin{lstlisting}
|\roman{test}|
    \end{lstlisting}
    \stepcounter{test}
    \begin{lstlisting}
|\roman{test}|
    \end{lstlisting}
\end{document}

Is that possible?

score 3 · Accepted Answer · answered Oct 30 '19 at 02:38

When (La)TeX scans the parameter of a macro, it fixes the catcodes of the characters/tokens it reads. So when the second parameter for \@whilesw is scanned, all special characters will be processed as if they were normal LaTeX code. This clashes with verbatim environments which rely on changing catcodes later on to typeset special characters properly. In this example this is especially problematic because listings can't recognoize \end{lstlisting} as the end of the listing anymore.

A solution is to read the listing's code under a catcode regime where every character has catcode 12 (other) and thus handles them in no special way. We can use the filecontentsdefmacro environment from the filecontentsdef package for this purpose to store the code in a macro \lsttemplate. The second step is to re-apply the normal catcode regime when that code is actually processed to make the lstlisting environment actually do its job. This can be done using the ε-TeX primtitive \scantokens

The complete solution looks like this:

\documentclass{article}
\usepackage{listings}
\usepackage{filecontentsdef}

\begin{filecontentsdefmacro}\lsttemplate
\begin{lstlisting}
\roman{test} --> |\roman{test}|
\end{lstlisting}
\end{filecontentsdefmacro}

\begin{document}
    \makeatletter
    \lstset{escapechar=|, basicstyle=\ttfamily}
    \newcounter{test}
    \@whilesw{\ifnum\value{test}<3}\fi{
        \stepcounter{test}
        \scantokens\expandafter{\lsttemplate}
    }
\end{document}

outputs

Thanks, this did solve my problem. I'd like to point out one caveat though. The filecontentsdefmacro may not contain a comment. The problem with comments in scantokens is outlined here. Comments work in the scontents solution by Pablo González L. — Malte, Nov 15 '19 at 15:05

score 2 · Answer 2 · answered Nov 01 '19 at 00:14

Same result as @siracusa, but using the scontents package (inspired by filecontentsdef):

\documentclass{article}
\usepackage{listings}
\usepackage[store-env=lsttemplate]{scontents}

\begin{scontents}
\begin{lstlisting}
\roman{test} --> |\roman{test}|
\end{lstlisting}
\end{scontents}

\begin{document}
    \makeatletter
    \lstset{escapechar=|, basicstyle=\ttfamily}
    \newcounter{test}
    \@whilesw{\ifnum\value{test}<3}\fi{
        \stepcounter{test}
        \getstored[1]{lsttemplate}
    }
\end{document}

Listing inside loop body

2 Answers2