How to read counter value from every section

Question

I'm trying to read a counter value for a specific section. For example

\documentclass{article}
\usepackage{amsmath}

\newcounter{M}[section]
\numberwithin{M}{section}
\newcommand{\foo}{\refstepcounter{M}(\theM)\\}

\begin{document}

\section{First}
\foo
\foo
\foo
\section{Second}
\foo
\foo

\countervalue{M}{1}
\countervalue{M}{2}

\end{document}

I want \countervalue{M}{1} to return 3 and \countervalue{M}{2} to return 2 (or 1.3 and 2.2 respectively), is it possible?

Answer 1

Okay I've found a simpler way to do it using etoolbox

\documentclass{article}
\usepackage{amsmath}
\usepackage{etoolbox}

\newcounter{M}[section]
\numberwithin{M}{section}
\newcommand{\foo}{
    \refstepcounter{M}
    \csxdef{M\thesection}{\arabic{M}}
    (\theM)\\}

\begin{document}
    \section{First}
    \foo
    \foo
    \foo
    \section{Second}
    \foo
    \foo
    First section has: \csuse{M1} foos\\
    Second section has: \csuse{M2} foos
\end{document}

Answer 2

One counter isn't enough because you need to record a different value for each section. Here is one way:

\documentclass{article}
% \usepackage{xparse} % uncomment for LaTeX kernels older than 2020-10-01
\usepackage{etoolbox}

\makeatletter
\NewDocumentCommand{\arystoDef}{s O{section} m}{%
  \newtoggle{arysto@#3@within}%
  \IfBooleanT{#1}% if the starred version was used
    {\toggletrue{arysto@#3@within}%
     \csdef{arysto@#3@parentctr}{#2}% record name of the parent counter for #3
    }%
}

\newcommand*{\arystoIncr}[1]{%
  \edef\arysto@ctrname{arysto@#1@ctr@\number\value{section}}%
  \@ifundefined{c@\arysto@ctrname}
    {\newcounter{\arysto@ctrname}%
     \iftoggle{arysto@#1@within}{%
         \csedef{the\arysto@ctrname}{%
           \arabic{\csuse{arysto@#1@parentctr}}.%
           \unexpanded\expandafter\expandafter\expandafter{%
             \csname the\arysto@ctrname\endcsname}%
         }%
     }{}%
    }{}%
  \refstepcounter{\arysto@ctrname}%
  (\csuse{the\arysto@ctrname})%
}

\newcommand*{\countervalue}[2]{%
  \csuse{thearysto@#1@ctr@#2}%
}
\makeatother

\arystoDef{foo}
\newcommand*{\foo}{\arystoIncr{foo}}

\begin{document}

\section{First}
\foo
\foo
\foo
\section{Second}
\foo
\foo

\bigskip\noindent
\countervalue{foo}{1}\\
\countervalue{foo}{2}

\end{document}

If you use \arystoDef*{foo} or \arystoDef*[section]{foo} instead of \arystoDef{foo}, you'll get this: