# Identifying the section an equation is in

Suppose in my document in section 1 I have a labeled equation, say \label{eq1}. Then much later, say in section 8, I want to reference this equation and the section it is in. I know \ref{eq1} will give me the equation number. But is there a command (or a way to write a command) that would let me say something like \refsec{eq1} and have it return the section number that the equation is in?

I'm trying to avoid having to always look up the section labels. It would be great if this worked across chapters as well.

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Why not build your equation counters to depend on chapter/section? –  Alan Munn Dec 30 '11 at 18:32
You can consider numbering equations by section: with amsmath it's sufficient to say \numberwithin{equation}{section}. –  egreg Dec 30 '11 at 18:33
This looks like we would need to change the labels on each equation. Since we are nearing the end of the project (15 chapter, 1200 pages) that isn't feasible. We're looking for a way to speed up the final editing/cross-referencing process. –  HTG Dec 30 '11 at 18:45
Which documentclass are you using? If your document uses chapters then the default labelling should be (1.1), (1.2), (1.3), etc where 1 is the chapter number, and 1, 2, 3 etc are the equation numbers. If you use @egreg's suggestion, the labels and cross references should change automagically, to 1.1.1, 1.1.2, 1.1.3 –  cmhughes Dec 30 '11 at 18:53
We're using the report document class. We are trying to avoid having the three-number references because we think it looks clunky. Right now the references show up as section.equation. The problem occurs where we currently have a \pageref, and the publisher has asked us to reference the section instead. That get's tedious, so we were looking for a fast change. –  HTG Dec 30 '11 at 19:18

This following approach based on the idea of label defined by the LaTeX-Kernel. First you have to define a new kind of label which write the needed information to the aux file. Normally the command \label writes the following in the aux file

\newlabel{#1}{{\@currentlabel}{\thepage}}


where \@currentlabel is the current number (in your case the equation number).

Now you need a the current section number instead of the equation number so the information in the aux file should be:

\newlabel{sec@#1}{{\thesection}{\thepage}}


The prefix sec@ is used to avoid multiple labels.

amsmath uses his own definition of \label named \label@in@display. To make sure that you don't have to type two different labels the definition will expand by the new label command.

Now I have to define a new reference command which is able to handle the new label. The idea based on the definition of \ref (see latex.ltx).

The explanation of the commands \@bsphack a.s.o. can be found in the document macros2e.

\documentclass{article}
\usepackage{amsmath}
\makeatletter
\newcommand\sec@label[1]{%
\@bsphack
\protected@write\@auxout{}%
{\string\newlabel{sec@#1}{{\thesection}{\thepage}}}%
\@esphack}

\def\label@in@display#1{%
\ifx\df@label\@empty\else
\@amsmath@err{Multiple \string\label's:
label '\df@label' will be lost}\@eha
\fi
\sec@label{#1}\gdef\df@label{#1}%
}
\def\secref#1{\expandafter\@setref\csname r@sec@#1\endcsname\@firstoftwo{#1}}
\def\secpageref#1{\expandafter\@setref\csname r@sec@#1\endcsname\@secondoftwo{#1}}
\makeatother

\begin{document}
\section{foo}
Text
\section{foo}
Text
\section{foo}
Text
$$a+b=c\label{eq1}$$

\section{bar}
See equation in section \secref{eq1} on page \secpageref{eq1}
\end{document}


The same with hyperref

\documentclass{article}
\usepackage{amsmath}
\usepackage{hyperref}
\makeatletter
\newcommand\sec@label[1]{%
\@bsphack
\if@filesw
\begingroup
\edef\@currentlabstr{%
\expandafter\strip@prefix\meaning\@currentlabelname
}%
\protected@write\@auxout{}{%
\string\newlabel{seceq@#1}{%
{\@currentlabel}%
{\thepage}%
{\@currentlabstr}%
{\@currentHref}%
{\theHsection}%%%%%%%%<- Define the reference counter
}%
}%
\endgroup
\fi
\@esphack}

\def\label@in@display#1{%
\ifx\df@label\@empty\else
\@amsmath@err{Multiple \string\label's:
label '\df@label' will be lost}\@eha
\fi
\sec@label{#1}\gdef\df@label{#1}%
}
\newcommand*\@refsecstar{}
\newcommand*\T@secref{}

\long\def\@fifthoffive#1#2#3#4#5{#5}
\def\@refsecstar#1{%
\HyRef@StarSetRef{seceq@#1}\@fifthoffive
}
\def\T@secref#1{\hyperref[{#1}]{\secref*{#1}}}%
\DeclareRobustCommand\secref{%
\@ifstar\@refsecstar\T@secref
}%
\makeatother

\begin{document}

\section{foo}
Text
\section{foo}
Text

\section{foo bar}\label{sec}
Text

\begin{table}[!ht]
\caption{table cpation}
\label{tab}
\end{table}

$$a+b=c\label{eq1}$$

\begin{align}
a+b=c\label{eq2}
\end{align}
\clearpage
\section{bar}
See equation in section \secref{eq1} on page \pageref{eq1}

See equation in section \secref*{eq1} on page \pageref*{eq1}
\end{document}

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Wow! That's amazing! I've got to learn more about the detailed coding. Right now it's magic, but it works great! –  HTG Dec 30 '11 at 18:55
Absolutely brilliant! :) –  Paulo Cereda Dec 31 '11 at 13:03
@Marco: Does this code work only for equation labels? If so, how difficult would it be to make it generic? so that you could say \secref{any label} and have it return the section number where the figure, or example, or table, or any other labeled item is? –  HTG Dec 31 '11 at 17:37
@HTG: very simple. \let\origlabel\label then \def\label#1{\origlabel{#1}\sec@label{#1}} –  Marco Daniel Dec 31 '11 at 17:44
@HTG: You are interested in referencing -- Maybe this answer is also helpful for you: tex.stackexchange.com/questions/14981/… –  Marco Daniel Dec 31 '11 at 17:45