[Tex/LaTex] Remove chapter number

Question

Sorry for wrong question formatting, but I'm new here and I don't know the way around these tools.
I would like to know if there is a way to remove the chapter number from the document (preferably without any preamble lines).
For example, if I have:

\chapter{chap1}
\chapter{chap2}
   \section{sec1}
       \subsection{subsec1}

Then, sec1 has number 2.1 and subsec1 has number 2.1.1. I would like the "2" removed, so I have 1.1 for subsec1 and 1 for sec1 in this particular chapter.

Answer 1

If no preamble command should be used, it has to be done inside the document body, redefining \thesection and later recall the old definition to restore it to the 1.1.1 etc. style.

Please note, that this leads to larger gaps in the ToC

\documentclass{book}

\begin{document}
\tableofcontents
\let\origthesection\thesection % Store the old definition

\chapter{chap1}
\chapter{chap2}
\renewcommand{\thesection}{\arabic{section}} % Kick out the chapter number 
   \section{sec1}
       \subsection{subsec1}

\renewcommand{\thesection}{\origthesection} % Restore to the old style
\chapter{chap 3}
   \section{sec1}
       \subsection{subsec1}

\end{document}