Renaming section levels?

Question

I have several reports I've written up in article format.

I want to keep them in separate files but combine them into a larger report. I've removed the headers, but don't want to go through each file and relabel each section as a subsection, and so on. Is there a way to handle this programatically? For instance, in my main file I might have

\section{Section1}
\input{file1}

and in file1.tex I have

\section{File1Section1}
...
\section{File1Section2}

I think some solution might be the redefinition of \section before and after each \input invocation, or an environment in which I can wrap the \input invocation where each \section is renamed as \subsection and so on. Help?

Answer 1

The following redefinition of \input redefines \section to be \subsection (as well as for the subsidiary sectioning commands) via \demotesections. Of course, this assumes that there are no deeper levels inside the included files than \subparagraph. However, it would be possible to set these to \relax as a "no-op":

\documentclass{article}
\usepackage{filecontents,lipsum}% http://ctan.org/pkg/{filecontents,lipsum}
% Create some dummy files
\begin{filecontents*}{file1}
\section{File1Section1}\lipsum[1]
\section{File1Section2}\lipsum[2]
\end{filecontents*}
\begin{filecontents*}{file2}
\section{File2Section1}\lipsum[1]
\section{File2Section2}\lipsum[2]
\end{filecontents*}

\let\oldinput\input
\newcommand{\savesections}{%
  \let\oldsection\section% Store \section
  \let\oldsubsection\subsection% Store \subsection
  \let\oldsubsubsection\subsubsection% Store \subsubsection
  \let\oldparagraph\paragraph% Store \paragraph
  \let\oldsubparagraph\subparagraph% Store \subparagraph
}\AtBeginDocument{\savesections}
\newcommand{\restoresections}{%
  \let\section\oldsection% Restore \section
  \let\subsection\oldsubsection% Restore \subsection
  \let\subsubsection\oldsubsubsection% Restore \subsubsection
  \let\paragraph\oldparagraph% Restore \paragraph
  \let\subparagraph\oldsubparagraph% Restore \subparagraph
}
\newcommand{\demotesections}{%
  \let\section\subsection% Modify \section to be \subsection
  \let\subsection\subsubsection% Modify \subsection to be \subsubsection
  \let\subsubsection\paragraph% Modify \subsubsection to be \paragraph
  \let\paragraph\subparagraph% Modify \paragraph to be \subparagraph
  %\let\subparagraph\relax% Make \subparagraph a no-op
}
\renewcommand{\input}[1]{%
  \demotesections% Demote sections
  \oldinput{#1}% Input file
  \restoresections}% Restore sections
\begin{document}
\section{Section1}
\input{file1}
\input{file2}
\subsection{Just a subsection}
\section{Section2}
\input{file1}
\input{file2}
\end{document}

The sectioning units are first saved using \savesections. Redefinition via \demotesections is "localized" within the new macro \input as a result of a subsequence \restoresections, allowing to use the default sectional units outside without loss of functionality.

Answer 2

For what it's worth, before I came back here to see @Werner's elegant solution, I took a stab at it. I will incorporate it into his example.

\documentclass{article}
\usepackage{filecontents,lipsum}% http://ctan.org/pkg/{filecontents,lipsum}
% Create some dummy files
\begin{filecontents*}{file1}
\section{File1Section1}\lipsum[1]
\section{File1Section2}\lipsum[2]
\end{filecontents*}
\begin{filecontents*}{file2}
\section{File2Section1}\lipsum[1]
\section{File2Section2}\lipsum[2]
\end{filecontents*}

\newcounter{toplevel}
\setcounter{toplevel}{1}
% 
\newcommand{\open}{
  \setcounter{section}{0}
  \renewcommand*{\thesection}{\arabic{toplevel}.\arabic{section}}
}
%
\newcommand{\close}{
  \addtocounter{toplevel}{1}
  \setcounter{section}{\value{toplevel}}
  \renewcommand*{\thesection}{\arabic{toplevel}}
}
% 
\renewcommand*{\thesubsection}{\arabic{toplevel}.\arabic{section}.\arabic{subsection}}

\begin{document}
\section{Section1}
\open
\input{file1}
\input{file2}
\subsection{Just a subsection}
\close
\section{Section2}
\open
\input{file1}
\input{file2}
\close
\end{document}

It sort of works except that the it needs better formatting so the \thesubsection is in a smaller font and also the \tableofcontents doesn't recognize the demotion of level so the printing is not very pretty. And apart from aesthetics, any \thesubsection in the main file will be further demoted (in my kludgey way) when included between the \open and \close statements. Anyway it's what I could do with my knowledge of \LaTeX (basically, \newcounter, \setcounter, and \renewcommand).