Referencing an item using labelenumi

Question

I am writing a book in LaTeX in which I label problems m.n for Chapter m, problem n (for example, problem 6.10 is the 10th problem in Chapter 6). I am using the declaration

\renewcommand{\theenumi}{\arabic{enumi}}
\renewcommand{\labelenumi}{\thechapter.\theenumi}

and this correctly prints problem labels in the enumerate environment where the problems are listed. However, when I refer to a problem elsewhere, it only prints the problem number (e.g., "see Problem 6.10" is written "see Problem 10"). Is there a global fix for this?

Answer 1

As Christian suggests, using the enumitem package is probably the best approach. I would go one step further than the answer he refers to and define a new problems environment for your problems. To do what you want using enumitem is as simple as:

\newlist{problems}{enumerate}{1}% or 2 or 3 if you want to nest
\setlist[problems]{label=\thechapter.\arabic*, 
                   ref=Problem \thechapter.\arabic*
}

Here I am assuming that you want \ref{...} to print something like Problem 1.2. If you just want it to print 1.2 then omit the ref=.... For more details consult the very readable enumitem manual.

In fact, as shown below, I would also have the problems environment automatically add a heading for the problems, which you can do using the before= option from. enumitem. Below I have used \section{...} but perhaps \section*{...} or \subsection{...} etc would work better for you.

Here a full minimal working example:

\documentclass{book}
\usepackage{enumitem}

\newlist{problems}{enumerate}{1}% or 2 or 3 if you want to nest
\setlist[problems]{label=\thechapter.\arabic*,
                   ref=Problem \thechapter.\arabic*,
                   before={\section{Problems for Chapter \thechapter}}
}

\begin{document}

\chapter{First chapter}

\begin{problems}
  \item Is the Riemann hypothesis true? \label{Riemann}
  \item Find the first counter-example to the twin primes conjecture
  \label{twins}
\end{problems}

Actually, \ref{Riemann} and \ref{twins} are quite hard, so allow at
least an extra week.
\end{document}

For completeness, this produces:

Answer 2

Referencing counters uses \the<cntr> as it's reference content. So, instead, you should redefine \theenumi to fully contain your representation (as it will be set and therefore referenced):

\documentclass{book}

\newenvironment{questions}
  {\renewcommand{\theenumi}{\thechapter.\arabic{enumi}}%
   \begin{enumerate}}
  {\end{enumerate}}

\begin{document}

\setcounter{chapter}{9}% Just for this example

\chapter{Questions}

See questions \ref{q:first}--\ref{q:last}.

\begin{questions}
  \item \label{q:first}
  First question

  \item
  Second question

  \item
  Third question

  \item \label{q:last}
  Last question
\end{questions}

\end{document}

The above example redefines \theenumi as part of a new questions environment. This allows you to localise the change of definition without it affecting other enumerate environments (if they're used).