algorithm2e + cleveref: problem with wrong reference to line numbers

Question

For some reason I do not understand, cleverefproduces wrong references to line numbers of listings produces with the algorithm2e package. I hope that I did not miss a simple solution ...

My example is the following:

\documentclass[paper=A4, fontsize=11pt]{scrbook}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[linesnumbered]{algorithm2e}
\usepackage{hyperref}
\usepackage{cleveref}

\begin{document}

\begin{algorithm}
Do something\;
Do something else\;\label{line}
\end{algorithm}

line~\ref{line} vs.\ \cref{line}.
\end{document}

The output (which you can also check online) is

The \ref command works as expected (and refers to line 2) but the \cref command always refers to line 1 (no matter how many lines there are and which line I try to reference).

Does anybody know a solution to this problem?

Answer 1

You found an oversight in the cleveref package. First, algorithm2e numbers the lines with the AlgoLine counter, and cleveref does not track that. So we need a

\crefalias{AlgoLine}{line}

to get that sorted.

Next, cleveref hacks into the \label command to write another label for its own use. In your example, the .aux contains

\newlabel{line}{{3}{1}{}{AlgoLine.1.3}{}}
\newlabel{line@cref}{{[line][1][]1}{1}}

The second line is written by cleveref. The offending part here is {[line][1][]1}, which is actually the content of \cref@currentlabel. Why is it wrong? While \label looks at \@currentlabel, cleveref uses its own \cref@currentlabel to determine what is been labeled. In order to keep that up to date, cleveref hacks into \refstepcounter, and updates \cref@currentlabel there. This works fine when algorithm2e is used without hyperref, but when hyperref is also loaded, the AlgoLine counter is updated with \stepcounter, and cleveref does not hack into that! This also means that cleveref will fail on all counters that are not (only) manipulated with \refstepcounter.

The solution is to also hack into \stepcounter, and update \cref@currentlabel there, too. The code for doing so is the same as for \refstepcounter. The complete code is then

\documentclass[paper=A4, fontsize=11pt]{scrbook}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[linesnumbered]{algorithm2e}
\usepackage{hyperref}
\usepackage{cleveref}

\crefalias{AlgoLine}{line}%

\makeatletter
\let\cref@old@stepcounter\stepcounter
\def\stepcounter#1{%
  \cref@old@stepcounter{#1}%
  \cref@constructprefix{#1}{\cref@result}%
  \@ifundefined{cref@#1@alias}%
    {\def\@tempa{#1}}%
    {\def\@tempa{\csname cref@#1@alias\endcsname}}%
  \protected@edef\cref@currentlabel{%
    [\@tempa][\arabic{#1}][\cref@result]%
    \csname p@#1\endcsname\csname the#1\endcsname}}
\makeatother

\begin{document}
\begin{algorithm}
Do something\;
Do more\;
Do something else\;\label{line}
\end{algorithm}

line~\ref{line} vs.\ \cref{line}
\end{document}

Answer 2

mafp's solution is a bad idea, and should be avoided. Hacking \stepcounter is definitely the wrong thing to do, as it's used to step all kinds of counters unrelated to cross-references. Cross-reference counters ought to always be stepped by \refstepcounter, which additionally takes care of setting \@currentlabel appropriately. Thus "only monitoring \refstepcounter" is precisely the right thing to do. Most cross-referencing-related packaged (hyperref, varioref...) work this way.

Unfortunately, when hyperref is loaded, algorithm2e by-passes both \refstepcounter and \H@refstepcounter (hyperref's version of the same), for reasons of its own (related to avoiding creating duplicate hyperlink anchors, I believe). This is an unfortunate choice of implementation, since by-passing the standard cross-reference machinery means packages like cleveref have to maintain fragile, specially-crafted support for algorithm2e. But that's LaTeX2e for you.

With hyperref loaded, algorithm2e has to take care of setting \@currentlabel itself (since it by-passes \refstepcounter). The correct solution is to modify algorithm2e so that it sets \cref@currentlabel at the same time as \@currentlabel. The latest version of cleveref (0.18.8) contains a fix for this.

It would be helpful if people reported bugs like this to the package author, instead of just posting them to stackexchange and using whatever ugly work-arounds are suggested in the replies. That way bugs get fixed in the package itself, and everyone benefits.

Answer 3

I found a simple solution for the incompatibility of hyperref+algorithm2e line numbering. Just use \nllabel instead of \label. Worked for me. Credits to Karthik: http://martinharrigan.blogspot.com.br/2007/11/using-hyperref-and-algorithm2e-line.html