How to number theorems depending on whether there is a subsection

Question

I am trying to typeset a exam subject with the same numbering as this document by Concours CentraleSupélec.

Sections are numbered with Roman, subsections are numbered with Alph and questions are numbered within each subsection with arabic : that is, question number 2 within subsection A of section I has number I.A.2.

I know how to do this with ntheorem and KOMA scrartcl : it suffices to do

\renewcommand\thesection{\Roman{section}}
\renewcommand\thesubsection{\thesection.\Alph{subsection}}
\newtheorem{question}{Question}[subsection]

And this works.

However, not all sections have subsections. In this case, questions are numbered within the section and with Alph, not arabic. So the third question in a section IV that does not have subsections is numbered IV.C, not IV..3.

I know I can do

\theoremnumbering{Alph}
\renewtheorem{question}{Question}[section]

I am trying to automate this. Clearly, it is not possible, when encountering a new section, to know whether there will be a subsection or not, so one has to change the numbering at each section and again at each subsection (or at least each first subsection within a section).

I could not find a AtBeginSection or AtBeginSubsection in the KOMA-script manual. Is there some hook that I can use to execute some code at each section/subsection without hacking the whole thing ?

Answer 1

The following does what you want. I just redefined \thequestion to check for the current \value of subsection (which is \c@subsection) and based on that print it differently.

\documentclass[]{scrartcl}

\usepackage{ntheorem}

\renewcommand*\thesection{\Roman{section}}
\renewcommand*\thesubsection{\thesection.\Alph{subsection}}

\newtheorem{question}{Question}[subsection]
\makeatletter
\renewcommand*\thequestion{%
  \ifnum\c@subsection=0
    \thesection.\Alph{question}%
  \else
    \thesubsection.\arabic{question}%
  \fi
}
\makeatother

\begin{document}
\section{Foo}
\begin{question}
  foo
\end{question}

\section{Foo}
\subsection{Bar}
\begin{question}
  foo
\end{question}
\end{document}

Issue: What if you have something like the following structure:

I. Section
  A. Question
  B. Subsection
    1. Question

Solution: Add a Boolean switch to the \section and \subsection command which indicates whether we are inside a subsection and if so use thesubsection.\arabic{question}. If not step the subsection counter and use \thesection.\Alph{question}. For the patching part I used etoolbox:

\documentclass[]{scrartcl}

\usepackage{ntheorem}
\usepackage{etoolbox}

\newif\ifInsideSubsection
\pretocmd\section{\InsideSubsectionfalse}{}{}
\pretocmd\subsection{\InsideSubsectiontrue}{}{}

\renewcommand*\thesection{\Roman{section}}
\renewcommand*\thesubsection{\thesection.\Alph{subsection}}

\newtheorem{question}{Question}[subsection]
\makeatletter
\renewcommand*\thequestion{%
  \ifInsideSubsection
    \thesubsection.\arabic{question}%
  \else
    \thesection.\Alph{question}%
    \global\advance\c@subsection by 1\relax
  \fi
}
\makeatother

\begin{document}
\section{Foo}
\begin{question}
  foo
\end{question}

\section{Foo}
\begin{question}
  foo
\end{question}
\subsection{Bar}
\begin{question}
  foo
\end{question}
\end{document}