Including an optional answer in an exam paper

Question

I would like to include answers in an exam paper so that the paper could be compiled in two different ways: with and without the answers. The way this is usually done in British universities (or at least some of them) is to use the comment package, like this:

\documentclass{article}
\usepackage{comment}
\excludecomment{answer}
\newcommand{\showanswers}{\renewenvironment{answer}{\begin{answershown}}{\end{answershown}}}
\newenvironment{answershown}{\textbf{Answer:} }{}
% \showanswers  % comment out to hide the answer
\begin{document}
  Is the following command correct?
  \begin{verbatim}
    public class
  \end{verbatim}
  \begin{answer} % do not indent!
    The following command is not correct:
    \begin{verbatim}
      public class
    \end{verbatim}
  \end{answer} % do not indent!
\end{document}

Here \showanswer is a toggle between the clean version of the paper (without the answers) and the version containing the answers. The disadvantage is that the comment package does not allow indentation (which is important in that it clearly shows the structure, potentially complicated, of the paper), and so compiling the source code above produces an error message:

Runaway argument?
! File ended while scanning use of \next.
<inserted text>
                \par
<*> ./mwe.tex
?

To allow indentation, I can use the environ package:

\documentclass{article}
\usepackage{environ}
\NewEnviron{answer}{}
\newcommand{\showanswers}{\RenewEnviron{answer}{\textbf{Answer:} \BODY}}
\showanswers  % comment out to hide the answer
\begin{document}
  Is the following command correct?
  \begin{verbatim}
    public class
  \end{verbatim}
  \begin{answer}
    The following command is not correct:
    \begin{verbatim}
      public class
    \end{verbatim}
  \end{answer}
\end{document}

But environ does not like verbatim in the answer, and compiling the latter source code produces:

Runaway argument?
 public class \end {verbatim}\env@ignore  The following command is no\ETC.
! File ended while scanning use of \@xverbatim.
<inserted text>
                \par
<*> ./mwe.tex
?

Is there a way to have both indentation and verbatim (in answers)?

Answer 1

Seems that lstlisting and verbatim plays with catcodes and can't really be used inside environments, at least when we want to be able not to display them. I'm not sure if it's the best solution, but you can save your code before your environment inside a box using the package realboxes and the command \Sbox{\mylstbox}{your content} (make sure to create your box using \newsavebox{\mylstbox}) , and reuse it after inside your environment via \usebox{\mylstbox}. It works for both verbatim and lstlisting.

mode exam:

mode answers:

By the way, I think that tcolorbox also has a module for answers, you may like it (and I guess this solution also applies to tcolorbox mode).

%%% Comment/uncomment, or compile with:
%%%
%%% pdflatex -jobname=exam_answers "\def\showanswers{} \input{exam.tex}" && pdflatex -jobname=exam_no_answers "\input{exam.tex}"
%%%
%%% to produce automatically two versions without changing the file.
% \def\showanswers{}
\documentclass{article}
\usepackage{listings}
\usepackage{realboxes} % https://tex.stackexchange.com/questions/29971/why-can-lstlisting-not-be-saved-in-a-command
\usepackage{changepage} % For adjustwidth and indentation
\NewDocumentEnvironment{answer}{+b}{
  \ifdefined\showanswers
  \textbf{Answer}:
\begin{adjustwidth}{1cm}{0cm} % Indent
    #1
  \end{adjustwidth}
  \else\fi}{}
\newsavebox{\mylstbox}
\begin{document}
\noindent Is the following command correct?
\begin{lstlisting}
  public class
\end{lstlisting}
\Sbox{\mylstbox}{%
\begin{lstlisting}
public class {
  hello
}
\end{lstlisting}
}
\begin{answer}
  The following command is not correct:
\usebox{\mylstbox}
\end{answer} % do not indent!
\end{document}

Answer 2

Okay, there are several possible solutions. (you can use either one of the first 2)

I rewrite them into separate questions (future readers are more likely to search for these terms).

---

How can I define a "comment" environment such that the environment delimiter can be indented?

Easy, use verbatim package.

\usepackage{verbatim}
\let\answer=\comment
\let\endanswer=\endcomment

Reference: Redefine environment to be a synonym for another

Note that it's actually possible to indent environments defined by comment package with tab, only space fails.

Alternatively:

• use version package. See https://tex.stackexchange.com/a/325928/250119
• use memoir documentclass. See https://tex.stackexchange.com/a/29061/250119

---

How can I redefine an environment defined by the environ package, such that it allows catcode-changing commands (e.g. \verb, \begin{verbatim}) in its body?

There's nothing special with environments defined by the environ package. You can still use \renewenvironment to redefine it as usual.

\usepackage{environ}
\NewEnviron{answer}{}
\newcommand{\showanswers}{\renewenvironment{answer}{\begin{answershown}}{\end{answershown}}}
\newenvironment{answershown}{\textbf{Answer:} }{}

---

Not that you need this one, but for reference:

How can I define an environment with the environ package, such that it allows catcode-changing commands in its body?

You can't. There are alternatives like savebox or scontents package.

Reference: Verbatim in environ’s \BODY