[Tex/LaTex] Problem referencing/labeling while using the subfiles package

Question

I have a problem using labels together with the subfiles package. When I'm writing in one section and I try to reference to different section (or for that matter anything within that section) I get an undefined label error. My main.tex:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage{graphicx}
\graphicspath{{images/}{../images/}}

\usepackage{subfiles}

\begin{document}
\maketitle

\section{Introduction}
\label{sec:intro}
\subfile{sections/CH1_Introduction}

\section{Theory of Ultrasound Anemometer}
\label{sec:theory}
\subfile{sections/CH2_Theory}

Then, for example, when I write in my 'introduction' section file:

In section \ref{sec:theory} the theory will be discussed ...

Which results in an undefined reference error. Can someone help me? Is the problem of the label command? Or am I doing something else wrong?

Answer 1

This can be done with the package xr. Here is an MWE that makes some assumptions about the layout of the project. In the root folder, under a file called main.tex:

\documentclass{article}
\usepackage{xr}
\usepackage{subfiles}

\begin{document}
\section{Introduction}
\subfile{sections/CH1_Introduction}
\label{sec:intro}

\section{Theory of Ultrasound Anemometer}
\subfile{sections/CH2_Theory}
\label{sec:theory}
\end{document}

Then, in a subfolder called sections, there are two additional tex files called CH1_Introduction:

\documentclass[../main.tex]{subfiles}
\externaldocument{../main.tex}

\begin{document}
This is a text written in the introduction file that discusses the rest of the paper. We will discuss the theory of ultrasound traducers in section \ref{sec:theory}.
\end{document}

and CH2_Theory.tex:

\documentclass[../main.tex]{subfiles}
\externaldocument{../main.tex}
\begin{document}
Here we talk about the ultrasound traducers that we said we would discuss in section \ref{sec:intro}. 
\end{document}

compiling main.tex than returns:

Now assuming that all your sections are called in the main document this should allow you to cite the sections (but not subsections with the current layout) from the introduction.