How to use `refcount` for chapter-wise counters?

Question

I'm using the refcount package to set some counters to the same values as counters from another .tex file. The labels from the other .tex file are imported with the zref-xr package.

This method fails for chapter-wise numbering. For example if a figure in the external .tex is numbered "3.2", the \setcounterref will add "3" to whatever counter and print ".2" into the document.

How can I extract only the rear part ("2") and add this to a counter?

---

\documentclass{book}

\usepackage{filecontents}
\begin{filecontents*}{external.aux}
\newlabel{fig:external}{{3.2}{6}{external figure}{figure.3.2}{}}
\end{filecontents*}

\usepackage{refcount}
\setrefcountdefault{1}

\usepackage{zref-xr}
\zxrsetup{toltxlabel}
\zexternaldocument*[external-]{external}

\setcounter{chapter}{1}
\usepackage{hyperref}

\begin{document}

\setcounterref{figure}{external-fig:external}

\thefigure

\end{document}

Answer 1

Assuming that all reference numbers are divided by periods, you can define a new command for accessing the last item.

\begin{filecontents*}{external.aux}
\newlabel{fig:external}{{3.2}{6}{external figure}{figure.3.2}{}}
\newlabel{thm:x}{{2}{8}{Theorem}{theorem.2}{}}
\end{filecontents*}

\documentclass{book}
\usepackage{xparse}
\usepackage{refcount}
\usepackage{zref-xr}
\usepackage{hyperref}

\setrefcountdefault{1}
\zxrsetup{toltxlabel}
\zexternaldocument*[external-]{external}
\setcounter{chapter}{1}

\newtheorem{theorem}{Theorem}

\ExplSyntaxOn
\NewDocumentCommand{\xsetcounterref}{mm}
 {% #1 = counter, #2 = label
  \seq_set_split:Nnx \l_tmpa_seq { . } { \getrefnumber{#2} }
  \setcounter{#1}{\seq_item:Nn \l_tmpa_seq { -1 }}
 }
\cs_generate_variant:Nn \seq_set_split:Nnn { Nnx }
\ExplSyntaxOff


\begin{document}

\xsetcounterref{figure}{external-fig:external}
\thefigure

\xsetcounterref{theorem}{external-thm:x}
\thetheorem

\end{document}

The expanded value of \getrefnumber{<label>} is split at periods and the last item is retrieved (with \seq_item:Nn \l_tmpa_tl { -1 }).

The output (you can check) will be

1.2
2

Answer 2

In case numbers always are of pattern ⟨chapter number⟩.⟨figure number⟩, don't use \setcounterref but apply \setcounter on a combination of \getrefnumber and removal of the dot and everything in front of it.

\documentclass{book}

\usepackage{filecontents}
\begin{filecontents*}{external.aux}
\newlabel{fig:external}{{3.2}{6}{external figure}{figure.3.2}{}}
\end{filecontents*}

\usepackage{refcount}[2010/12/01 v3.2] % issue warning if refcount 
                                       % older than version 3.2 is 
                                       % loaded.
\setrefcountdefault{1}

\usepackage{zref-xr}
\zxrsetup{toltxlabel}
\zexternaldocument*[external-]{external}

\setcounter{chapter}{1}
\usepackage{hyperref}

\newcommand\gobbletodot{}\long\def\gobbletodot#1.{}%

\begin{document}

% refcount manual, section 1.2 Expandable commands, states:
%   These commands that can be used in expandible contexts
%   ...
%   Since version 3.2 the expandable macros described before
%   in this section are expandable in exact two expansion steps.
%
\setcounter{figure}{\expandafter\expandafter\expandafter\gobbletodot\getrefnumber{external-fig:external}}

\thefigure

\end{document}