Extract Section number from Equation reference

Question

How can I obtain the number of the Section in which an Equation (or Figure) appears? For instance, I have the reference 3.2, which refers to the second (2nd) Equation in Section 3. How can I extract the 3 (or the 2), such that it is still clickable (using hyperref)?

Preferably it should return "2nd" when it is the second Equation, "3d" when it's the third one, and so on.

MWE:

\documentclass[10pt,a4paper,fleqn]{article}
\usepackage{amsmath}
\usepackage{lipsum}
\usepackage{hyperref}
\hypersetup{colorlinks}
\numberwithin{equation}{section}
\setlength{\parindent}{0pt}
\begin{document}

\section{Start}
\lipsum[1]

\section{Halfway}

The equation \eqref{ThisOne} is the (1st, 2nd, 3d, ...?) Equation in Section (1, 2, 3, ...?).

\section{End}

A famous formula:

\begin{equation}
x_1, x_2 = \frac{ -b \pm \sqrt{b^2 - 4ac} }{ 2a }
\end{equation}

And another one:

\begin{equation}
\sin^2(\varphi) + \cos^2(\varphi) = 1
\label{ThisOne}
\end{equation}

\end{document}

Answer 1

You can do this easily with the help of the refcount (to turn the reference number into a string), xstring (to extract the number before and after the dot), and engord (to get the desired format as ordinal number) packages.

In the following example, I defined the commands \SecNum and \EqNum; the first one gives the section number of the reference and the second one gives the equation number in ordinal notation and turns it into a hyperlink to the given equation:

\documentclass[10pt,a4paper,fleqn]{article}
\usepackage{amsmath}
\usepackage{refcount}
\usepackage{xstring}
\usepackage{engord}
\usepackage{xspace}
\usepackage{lipsum}
\usepackage{hyperref}

\hypersetup{colorlinks}
\numberwithin{equation}{section}
\setlength{\parindent}{0pt}

\newcommand\equ{}
\newcommand\EqNum[1]{%
  \StrBehind{\getrefnumber{#1}}{.}[\equ]%
  \hyperref[#1]{\engordnumber{\equ}\xspace}%
}
\newcommand\SecNum[1]{%
  \StrBefore{\getrefnumber{#1}}{.}\xspace%
}

\begin{document}

\section{Start}
\lipsum[1]

\section{Halfway}

The equation \eqref{ThisOne} is the \EqNum{ThisOne}~Equation in Section \SecNum{ThisOne}.

\section{End}

A famous formula:

\begin{equation}
x_1, x_2 = \frac{ -b \pm \sqrt{b^2 - 4ac} }{ 2a }
\end{equation}

And another one:

\begin{equation}
\sin^2(\varphi) + \cos^2(\varphi) = 1
\label{ThisOne}
\end{equation}

\end{document}

I added \xspace from the xspace package to take care of proper spacing.

Answer 2

The zref package allows for an extention of the regular 2-part \label-\ref system into any number of properties/elements. The following MWE creates a new property list (called special) and adds section and equation counters (in \arabic format) to this list for referencing:

\documentclass[10pt,a4paper,fleqn]{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\usepackage{hyperref}% http://ctan.org/pkg/hyperref
\usepackage{zref}% http://ctan.org/pkg/zref

\makeatletter
\zref@newlist{special}% Create a new property list called special
\zref@newprop{section}{\arabic{section}}% Section property holds \arabic{section}
\zref@addprop{special}{section}% Add a section property to special
\zref@newprop{equation}{\arabic{equation}}% Equation property holds \arabic{equation}
\zref@addprop{special}{equation}% Add an equation property to special
\newcommand*{\eqnref}[1]{\zref@extractdefault{#1}{equation}{??}}
\newcommand*{\secref}[1]{\zref@extractdefault{#1}{section}{??}}
\newcommand*{\spref}[2][section]{\zref@extractdefault{#2}{#1}{??}}
\newcommand*{\splabel}[1]{\zref@labelbylist{#1}{special}}%
\makeatother
\hypersetup{colorlinks}
\numberwithin{equation}{section}
\setlength{\parindent}{0pt}
\begin{document}

\section{Start}
\lipsum[1]

\section{Halfway}

The equation \eqref{ThisOne} is Equation~\eqnref{ThisOne} in Section~\secref{ThisOne}.

\section{End}

A famous formula:

\begin{equation}
x_1, x_2 = \frac{ -b \pm \sqrt{b^2 - 4ac} }{ 2a }
\end{equation}

And another one:

\begin{equation}
\sin^2(\varphi) + \cos^2(\varphi) = 1
\label{ThisOne}\splabel{ThisOne}
\end{equation}

\end{document}

zref labels are set using \splabel, while equations are referenced using \eqnref and sections using \secref. These are specific implementations of a more general \spref[<type>]{<refname>} (where <type> defaults to section). Since the macros used are expandable, fmtcount can also be used to provide ordinal references:

\usepackage{fmtcount}% http://ctan.org/pkg/fmtcount
%...
\newcommand*{\eqnref}[1]{\ordinalnum{\zref@extractdefault{#1}{equation}{??}}}

Hyper-referencing is also possible, if needed.

---

With the addition of

\let\oldlabel\label
\renewcommand{\label}[1]{\oldlabel{#1}\splabel{#1}}%

in your preamble, you only have to use one \label command to obtain the desired referencing output rather than \label + \splabel.