How to combine several LaTeX files with equations and tables into one?

Question

I am preparing a thesis including equations, figures, and tables. I have read up on combining LaTeX files, but my problem persists. I created the main file and imported preamble, chapterfiles, and bibliography into the main file. Chapter files having tables do not get processed without the required packages, but the declared packages are only in the preamble file and not in the chapter files. On the other hand, if I declare packages in the chapter files, the compiler gives error saying I cannot declare packages before \documentclass{}. But, I have to use only one \documentclass{} right? How to solve this problem? Please help me.

Answer 1

In the main file you can insert subdocuments with \input{subdocument}.

You can also use \include{subdocument}, that roughly is equivalent to \clearpage \input{subdocument} \clearpage (see When should I use \input vs. \include?).

In any case, the subdocument.tex is any chunk of LaTeX code as:

Some \textcolor{red}{text}

It might be any part of the main document content, or the preamble, but not a standalone document as:

\documentclass{article}
\usepackage{xcolor}
\begin{document}
Some \textcolor{red}{text}
\end{document}

This is because \input is just like paste a code into another, and you cannot have a document with two \documentclass declarations, two preambles, etc.

However, you can skip this unnecessary code just loading the package standalone or docmute in the preamble of the main file:

This is a MWE of a main file:

\documentclass{article} % This in the main.tex file
\usepackage{standalone} % load only in the main file
% \usepackage{docmute}  % alternative to standalone    

% Tip: put the rest of the preamble in 'preamble.tex' 
% so you can reuse in standalone subdocuments

\input{preamble} 

\begin{document}
\maketitle
\tableofcontents
\input{SectionOne}   %  Load standalone SectionOne.tex
\input{SectionTwo}   %  Load "\section{Two} Some text ..."
\section{Three}
 Lore \input{ipsum}  %  Load simple text of ipsum.tex  
\end{document}