Cross-reference two numbered environments

Question

Consider the following code:

\documentclass[a4paper,12pt]{book}
\usepackage{amsthm}
\usepackage{hyperref}

\theoremstyle{plain}
\newtheorem{exercise}{Exercise}
\newtheorem{solution}{Solution}

\begin{document}
  \begin{exercise}
    This is an exercise.
  \end{exercise}

  \begin{solution}
    This is a solution.
  \end{solution}
\end{document}

I'd like to:

1. Assign the same number to each "exercise" and "solution" pair (possibly via some sort of unique ID they share?);
2. Automatically generate hyperlinks from an "exercise" title to its corresponding "solution" and vice versa, so one can easily navigate from a problem to its solution and vice versa.

I'm probably asking for something fairly difficult -- I'd be happy with just the first question. I'm not sure if they should be two separate questions but they seem quite related to me.

Answer 1

\documentclass[a4paper,12pt]{book}
\usepackage{amsthm}
\usepackage{hyperref}

\theoremstyle{plain}
\newcounter{exercise}[chapter]
\renewcommand\theexercise{\thechapter.\arabic{exercise}}
\newtheorem{xexercise}[exercise]{Exercise}
\newtheorem*{xsolution}{Solution}

\newenvironment{exercise}[1]
{\begin{xexercise}\label{#1}\def\thissoln{\hyperref[sol-#1]{Solution}}}
{\par\thissoln\end{xexercise}}

\newenvironment{solution}[1]
{\begin{xsolution}[Solution \ref{#1}]\label{sol-#1}}
{\end{xsolution}}

\begin{document}

\chapter{aaa}
  \begin{exercise}{z}
    This is an exercise.
  \end{exercise}
\chapter{bbb}
  \begin{exercise}{a1}
    This is an exercise.
  \end{exercise}
  \begin{exercise}{thing}
    This is an exercise.
  \end{exercise}

\chapter{solutions}
  \begin{solution}{z}
    This is a solution.
  \end{solution}
  \begin{solution}{a1}
    This is a solution.
  \end{solution}
  \begin{solution}{thing}
    This is a solution.
  \end{solution}
\end{document}

Answer 2

You can define the solution environment to have the same number as the last exercise, but with the possibility of having a different number grabbed through the \label-\ref mechanism.

The solution number will be a link to the exercise. If the solution doesn't immediately follow the corresponding exercise, because you want to state two exercises next to each other, specify the label to the exercise and use it as an optional argument to \begin{solution}.

\documentclass[a4paper,12pt]{book}
\usepackage{amsthm}
\usepackage{hyperref}

\theoremstyle{definition} % body text is upright
\newtheorem{innerexercise}{Exercise}
\newtheorem*{innersolution}{Solution of \exerciseref}
\newenvironment{exercise}
  {\innerexercise\label{exercise@@\theinnerexercise}}
  {\endinnerexercise}
\newenvironment{solution}[1][]
  {\if!#1!%
     \def\exerciseref{\ref{exercise@@\theinnerexercise}}%
   \else
     \def\exerciseref{\ref{#1}}%
   \fi
   \innersolution}
  {\endinnersolution}

\begin{document}
\begin{exercise}
This is an exercise.
\end{exercise}

\begin{solution}
This is a solution.
\end{solution}

\begin{exercise}\label{another}
This is another exercise.
\end{exercise}

\begin{exercise}\label{ohwell}
This is yet another exercise.
\end{exercise}

\begin{solution}[another]
A solution.
\end{solution}

\begin{solution}[ohwell]
Too much.
\end{solution}

\end{document}

Answer 3

I found the package exercise.sty and It fulfill all requirements.

You define, exercises like that:

\begin{Exercise} [⟨key val list⟩] ... \end{Exercise}

\begin{Exercise*} [⟨key val list⟩] ... \end{Exercise*}

And Answers:

\begin{Answer} [⟨key val list⟩] ... \end{Answer}

All above with these possible key values:

label={⟨string ⟩} 
title={⟨string ⟩} 
difficulty={⟨number ⟩} 
origin={⟨string ⟩} 
name={⟨string ⟩} 
counter={⟨counter ⟩} 
number={⟨string ⟩}

you can use label key and then in an answer referencing with ref and number key values. Btw, you can use these label as a normal label latex with \ref{label}.

By example:

\documentclass[12pt]{article}
\usepackage{hyperref}
\usepackage{exercise}
\begin{document}

\begin{Exercise}[label=ex1]
This is an exercise.
\end{Exercise}

\begin{Answer}
This is an answer.
\end{Answer}

\end{document}

So, the result looks like this:

This package exercise has really others options, like environments of exercises and other stuffs.

Answer 4

Based on the collective input I developed a couple alternatives:

1. Without answers package

---

\documentclass[a4paper,12pt]{book}
\usepackage{amsthm}
\usepackage{hyperref}

\theoremstyle{definition}
\newtheorem{innerexercise}{Exercise}
\newtheorem*{innersolution}{Solution \exerciseref}
\newenvironment{exercise}[1]
  {\innerexercise\label{#1} (p\pageref{sol:#1})}
  {\endinnerexercise}
\newenvironment{solution}[1]
  {\def\exerciseref{\ref{#1}}%
   \innersolution\label{sol:#1} (p\pageref{#1})}
  {\endinnersolution}

\begin{document}

\chapter{Exercise One}

\begin{exercise}{labelone}
 Exercise one.
\end{exercise}

\chapter{Exercise Two}

\begin{exercise}{labeltwo}
 Exercise two.
\end{exercise}

\chapter{Solutions}

\begin{solution}{labeltwo}
 Solution two.
\end{solution}

\begin{solution}{labelone}
 Solution one.
\end{solution}

2. With answers package

---

\documentclass[a4paper,12pt]{book}
    \usepackage{amsthm}
    \usepackage{answers}
      \Newassociation{sol}{solution}{ans}
    \usepackage{hyperref}

\newtheorem{innerexercise}{Exercise}
\newenvironment{exercise}[1]
  {\begin{innerexercise}\label{#1} (p.~\pageref{sol:#1})\renewcommand{\Currentlabel}{#1}}
  {\end{innerexercise}}
\newtheorem*{innersolution}{Solution \exerciseref}
\renewenvironment{solution}[1]
  {\def\exerciseref{\ref{#1}}%
   \innersolution\label{sol:#1} (p.~\pageref{#1})}
  {\endinnersolution}

\begin{document}
\Opensolutionfile{ans}[solutionsfile]
\chapter{Problems}
\begin{exercise}{First}
 First exercise
 \begin{sol}
  First solution.
 \end{sol}
\end{exercise}
\begin{exercise}{Second}
 Second exercise
 \begin{sol}
  Second solution.
 \end{sol}
\end{exercise}
\Closesolutionfile{ans}

\chapter{Solutions}
\input{solutionsfile}
\end{document}

In any case I can manage numbered exercise-solution pairs with the added benefit of \pageref that serves both as a hyperlink for people navigating this content on a PDF and as a page marker for those who read it on paper.

The second solution is more manageable because I don't have to keep solutions ordered. It's also better style as solutions are kept inside their respective "parents" and you just need to define one label per pair.