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Question

I'd to be able to set a value at the start of my document that sets in which chapters examples will have solutions. My attempt at this is as follows.

\newcommand{\solutionsupto}{2}

\newenvironment{solution}{
\ifnum \thechapter < \solutionsupto
\underline{\bf Solution}

}{\fi}

The intention here is that solutions should be displayed in chapter 1 but not chapter 2 (or later chapters).

However, I get a compilation error if I have a solution in chapter 2. For example,

\documentclass[a4paper,12pt]{book}
\usepackage[british]{babel}


\newcommand{\solutionsupto}{2}

\newenvironment{solution}{
\ifnum\value{chapter} < \solutionsupto
\underline{\bf Solution}

}{\fi}


\begin{document}

\chapter{One}
First question.

\begin{solution}
First answer
\end{solution}
\chapter{Two}
Second question

\end{document}

works as expected, but

\documentclass[a4paper,12pt]{book}
\usepackage[british]{babel}


\newcommand{\solutionsupto}{2}

\newenvironment{solution}{
\ifnum\value{chapter} < \solutionsupto
\underline{\bf Solution}

}{\fi}


\begin{document}

\chapter{One}
First question.

\begin{solution}
First answer
\end{solution}

\chapter{Two}
Second question
\begin{solution}
Second answer
\end{solution}

\end{document}

results in an Incomplete \ifnum error.

On the other hand, if I replace the use of the solution environment with what I think it should evaluate to, that is

\documentclass[a4paper,12pt]{book}
\usepackage[british]{babel}

\newcommand{\solutionsupto}{2}

\newenvironment{solution}{
\ifnum\value{chapter} < \solutionsupto
\underline{\bf Solution}

}{\fi}

\begin{document}

\chapter{One}
First question.

\ifnum \thechapter < \solutionsupto
\underline{\bf Solution}

First answer
\fi
\chapter{Two}
Second question
\ifnum \thechapter < \solutionsupto
\underline{\bf Solution}

Second answer
\fi

\end{document}

it works exactly as expected with the first answer displayed but the second answer omitted.

Any help on this would be very much appreciated.

Answer 1

You can't have \fi in the end part, because \end{solution} isn't expanded if it's in the false branch of a conditional.

You can instead absorb the contents of the environment using the +b argument type.

\documentclass[a4paper,12pt]{book}
\usepackage[british]{babel}

\newcommand{\solutionsupto}{2}

\NewDocumentEnvironment{solution}{+b}{%
  \ifnum\value{chapter} < \solutionsupto \relax
    \par\noindent\underline{\bfseries Solution}\par
    #1
  \fi
}{}


\begin{document}

\chapter{One}
First question.

\begin{solution}
First answer
\end{solution}

\chapter{Two}
Second question
\begin{solution}
Second answer
\end{solution}

\end{document}

Beware: commands such as \bf or \it have been deprecated for 30 years and aren't defined in the kernel. Some classes may define it for back compatibility with 30+ year old documents.