# Add the chapter number when referencing a section (without the chapter number in the section name)

I am using the book class and I have the following structure for my chapter and sections:

\renewcommand\thechapter{\Roman{chapter}}
\renewcommand\thesection{\arabic{section}}


so that I got for the chapter and sections

I
1
2
...
II
1
2
...


which is fine since I don't want to have an heavy structure with I.1, I.2, ..., II.1 in the section title and ToC.

But when I reference let say the section 1 of chapter II I would like to get as a result II.1 and not just 1 (which is confusing when I am referencing from another chapter).

Is there a way to add this chapter number automatically for the section references? I could not find satisfying answer so far.

NB: I don't know if it is of any interest but my that my figures, tables, equations are all named with the chapter number so when I ref them I have already the chapter number which is fine for me.

Thanks

A reference can be prefixed a the time of its definition in \label by \p@<counter>:

\makeatletter
\renewcommand*{\p@section}{\thechapter.}
\makeatother


Example:

\documentclass{book}
\renewcommand\thechapter{\Roman{chapter}}
\renewcommand\thesection{\arabic{section}}

\makeatletter
\renewcommand*{\p@section}{\thechapter.}
\makeatother

\begin{document}
\tableofcontents
\chapter{First chapter}
\label{chap:first}
\section{First section}
\label{sec:first}
\chapter{Second chapter}
\label{chap:second}
\section{Second section}
\label{sec:second}

Section \ref{sec:first} in chapter \ref{chap:first},
section \ref{sec:second} in chapter \ref{chap:second}.
\end{document}