3

Consider this MWE:

\documentclass{book}
\usepackage{xcolor}
\pagecolor{yellow!10}
\usepackage{trace}

% avoid page breaks of \part, \chapter
\makeatletter
  \renewcommand{\part}{\secdef \@part \@spart}
  \let\@endpart\relax % suppress page break after \part
  \renewcommand{\chapter}{\global \@topnum \z@ \@afterindentfalse \secdef \@chapter \@schapter}
\makeatother

\begin{document}

\def\thechapter{\roman{part}.\alph{chapter}} % this changes printout in chapter header itself, but not in the ref

Citing part \ref{part:one}, chapter \traceon \ref{chapter:one} \traceoff

\part{part one}\label{part:one}
\chapter{chapter one}\label{chapter:one}

\end{document}

... which produces this result:

test.png

... where the chapter title is printed as "Chapter i.a", and the \ref to it also prints "i.a" (as expected due to the redefinition of \thechapter as \roman{part}.\alph{chapter})

Here is the thing - I would like to not redefine \thechapter, that is, I'd like to keep the default "Chapter 1" printout for \chapter, and the default "1" for a \ref to it. Instead, I would like to define a new command, say \pcref, which given a chapter label, would print a "I.A" reference like in the example. How could I do that?

The part of the challenge, I guess, is that the printing of \ref (as shown by the trace) reduces to:

%...
\@setref #1#2#3->\ifx #1\relax \protect \G@refundefinedtrue \nfss@text {\reset@
font \bfseries ??}\@latex@warning {Reference `#3' on page \thepage \space undef
ined}\else \expandafter #2#1\null \fi 
#1<-\r@chapter:one 
#2<-\@firstoftwo 
#3<-chapter:one
{\ifx}
{false}
{\expandafter}

\r@chapter:one ->{i.a}{1}

\@firstoftwo #1#2->#1
#1<-i.a
#2<-1
{the letter i}

\null ->\hbox {}
{\hbox}
%...

... that is, when \ref is called, the \r@chapter:one command is already set, either to i.a or to 1, depending on \thechapter; that is, if there is no information about part number when \label is called, no \ref redefinition can use it either. So, I guess, the tricky part is doing all this (a new \pcref command with old \thechapter) without having to redefine \label...

sdaau
  • 17,079
  • 1
    \global\topnum=\z@ will disable top floats forever. Are you really sure that book is the right class for this job? – egreg Nov 19 '14 at 14:51
  • Thanks @egreg - I just took the book class because I have that in my original problem, and so I wanted to have \part and \chapter defined like there; I otherwise suppressed all those page breaks just to have a nice screenshot (showing all on one page) for this question :) Cheers! – sdaau Nov 19 '14 at 14:53
  • 1
    Isn't this possible with cleveref package? –  Nov 19 '14 at 14:59
  • Thanks @ChristianHupfer - didn't know about cleveref until now, thanks for the mention; cheers! – sdaau Nov 19 '14 at 15:04
  • @ChristianHupfer - just checked cleveref; the best I can figure is to use something like \creflabelformat{chapter}{#2-\thepart-#1-#3}; unfortunately, this does not change the label format when it is placed with \label, but depends on the part where the \cref command will be issued; so I guess cleveref cannot help much here... – sdaau Nov 19 '14 at 15:40

1 Answers1

4

I'm not following you into avoiding page breaks, but I'll just show how to do what you seem to want.

\documentclass{book}

\makeatletter
\renewcommand{\p@chapter}{\thepart.}
\makeatother

\begin{document}
\mainmatter
\part{Some part title}

\chapter{The chapter title}\label{A}

\section{A section title}\label{B}

``\ref{A}'' is a reference to a chapter, while ``\ref{B}'' is a reference to a section

\end{document}

enter image description here

Explanation: \p@chapter is the prefix added to the chapter number when a reference to it is requested. Whenever you do

\newcounter{foo}

LaTeX provides by default an empty \p@foo macro.

If you want to have different ways for referencing a chapter, you can do like this

\documentclass{book}

\makeatletter
\renewcommand{\p@chapter}{\maybe@part{\thepart.}}
\newcommand{\pcref}[1]{\begingroup\let\maybe@part\@iden\ref{#1}\endgroup}
\protected\def\maybe@part#1{}
\makeatother

\begin{document}
\mainmatter
\part{Some part title}

\chapter{The chapter title}\label{A}

\section{A section title}\label{B}

``\ref{A}'' is a reference to a chapter, while ``\ref{B}'' is a reference to a section
and ``\pcref{A}'' is a reference to a chapter with the part number attached to it.

\end{document}

but I'd avoid it. Chapters should be numbered independently of parts, so the problem doesn't present itself to begin with. Having two or more “Chapter 1” is just confusing.

enter image description here

egreg
  • 1,121,712
  • Many thanks for that, @egreg, was not aware about the prefix thing! This does almost what I want, except here \ref prints "I.1"; while I wanted to keep the original \ref, which prints "1", and have a new command e.g. \pcref which would print the "I.1". Is that possible? (ps: if I had page breaks, I would have had 5 pages, which as an image would either be too big or too low resolution for posting; without them, I can show all three sections on the same page, so they are readable in the screenshot posted in OP as well). Thanks again - cheers! – sdaau Nov 19 '14 at 15:08
  • Thanks for the edit, @egreg, that is fantastic - the \pcref there is exactly what I wanted! You're right about chapter numbering in general - still, this will be very useful to myself as a reminder while writing (having to refer to chapters in different parts from, say, an introduction). Cheers! – sdaau Nov 19 '14 at 15:35