Numeration of theorems considering the depth (section, subsection ...)

Question

I want the theorems to be numerated considering the depth, namely:

Chapter 1.

Theorem 1.1.

Section 1.1.

Theorem 1.1.1.

Subsection 1.1.1.

Theorem 1.1.1.1.

Theorem 1.1.1.2.

Theorem 1.1.1.3.

I have used different theorem types like

\newtheorem{th}{Theorem}[chapter]
\newtheorem{thh}{Theorem}[section]
\newtheorem{thhh}{Theorem}[subsection]

but if I change part of a section into a subsection I have to change all the theorem types within.

Is there any way to unify this?

Answer 1

The following code updates \thetheorem - the counter representation of the theorem counter/environment - with a prefix that depends on the current sectional unit:

\documentclass{report}

\newtheorem{theorem}{Theorem}

\makeatletter
\newcommand{\updatetheoremprefixfor}[1]{%
  \expandafter\expandafter\expandafter\let\expandafter\csname old#1\expandafter\endcsname\csname #1\endcsname
  \@namedef{#1}{\renewcommand{\thetheorem}{\csname the#1\endcsname.\arabic{theorem}}
  \setcounter{theorem}{0}%
  \csname old#1\endcsname}}
\makeatother

\updatetheoremprefixfor{chapter}
\updatetheoremprefixfor{section}
\updatetheoremprefixfor{subsection}
\updatetheoremprefixfor{subsubsection}

\begin{document}

\chapter{A chapter}
\begin{theorem}
This is a theorem within a \verb|\chapter|.
\end{theorem}

\section{A section}
\begin{theorem}
This is a theorem within a \verb|\section|.
\end{theorem}

\subsection{A subsection}
\begin{theorem}
This is a theorem within a \verb|\subsection|.
\end{theorem}
\begin{theorem}
This is a theorem within a \verb|\subsection|.
\end{theorem}
\begin{theorem}
This is a theorem within a \verb|\subsection|.
\end{theorem}

\section{A section}
\begin{theorem}
This is a theorem within a \verb|\section|.
\end{theorem}
\begin{theorem}
This is a theorem within a \verb|\section|.
\end{theorem}

\subsection{A subsection}
\begin{theorem}
This is a theorem within a \verb|\subsection|.
\end{theorem}

\end{document}