Setting up a paired counter for exercise / solution theorems

Question

I am trying to use two theorem environments, one for exercises and another for their solutions. The counter for the solution should be the same as the counter for the associated exercise.

An example:

Exercise 1.1. What is 2+2?
Exercise 1.2. What is 3+5?
Solution to Exercise 1.1. 2+2=4.
Solution to Exercise 1.2. 3+5=8.

Should be the output of:

\begin{exercise}\label{ex: 22} What is 2+2?\end{exercise}
\begin{exercise}\label{ex: 35} What is 3+5?\end{exercise}
\begin{solution}[ex: 22] 2+2=4.\end{solution}
\begin{solution}[ex: 35] 3+5=8.\end{solution}

Is this possible using the amsthm package?

Did you have a look into packages/classes for this type of stuff? E.g. the exam class should let you typeset the exercise and solution together and then show the solution at the end — samcarter_is_at_topanswers.xyz, Nov 16 '23 at 10:18
@samcarter_is_at_topanswers.xyz No, my searches have just been returning results on restating theorems. I cannot change the document class, but a separate package that takes care of this would also be interesting. — s.harp, Nov 16 '23 at 10:20
Some packages from https://ctan.org/topic/exercise might also be useful — samcarter_is_at_topanswers.xyz, Nov 16 '23 at 10:30
If you do not insist on the plain amsthm, even if your question suggest otherwise, I recommend having a look at xsim, "exercise sheets improved" which can handle the task you want easily. — marc, Nov 16 '23 at 11:36

Stephen · Accepted Answer · 2023-11-16T10:43:20.783

I think one counter and the label-ref mechanism is enough:

\documentclass{article}
\usepackage{amsthm}
\newcounter{exercise}
\counterwithin{exercise}{section}
\NewDocumentEnvironment{exercise}{ m +b }{%
  \noindent
  Exercise~\refstepcounter{exercise}\theexercise\label{#1}. #2}{\par\medskip}
\newenvironment{solution}[1]{%
  \begin{proof}[Solution to Exercise~\ref{#1}]}%
  {\end{proof}}
\begin{document}
\section{AAAAA}
\begin{exercise}{ex:22} What is 2+2?\end{exercise}
\begin{exercise}{ex:35} What is 3+5?\end{exercise}
\begin{solution}{ex:22} 2+2=4.\end{solution}
\begin{solution}{ex:35} 3+5=8.\end{solution}
\end{document}

You can also use amsthm to define the exercise environment which makes the code a little simpler:

\documentclass{article}
\usepackage{amsthm}
\newtheorem{exercise}{Exercise}[section]
\newenvironment{solution}[1]{%
  \begin{proof}[Solution to Exercise~\ref{#1}]}%
  {\end{proof}}
\begin{document}
\section{AAAAA}
\begin{exercise}\label{ex:22} What is 2+2?\end{exercise}
\begin{exercise}\label{ex:35} What is 3+5?\end{exercise}
\begin{solution}{ex:22} 2+2=4.\end{solution}
\begin{solution}{ex:35} 3+5=8.\end{solution}
\end{document}

Setting up a paired counter for exercise / solution theorems

1 Answers1