Place section later in document than defined

Question

Is is possible to have a section appear later in the document than where it is defined?

\documentclass{article}

\begin{document}
\section{Intro}
This section appears first.

\section{Arcane details}
This section should not appear here, but later.

\section{Interesting bits}
That people want to see.

\appendix
% The Arcane details section should appear here.

\end{document}

In this simple example, I don't want the second section I defined (Arcane details) to appear as the second section, but rather to be an appendix at the end of the document.

I know I could just move that section to the location I want it, but the case I want to do this for is for an automatically generated document (specifically Sweave from R) and the section I want to appear later involves calculations that need to be done before the rest of the document can be generated.

Answer 1

If you have one such section (more specifically, a continuous stretch of code), you could write that particular content to a file, and read it in later. File writing is supported by the filecontents package. Here's a minimal example:

\documentclass{article}
\usepackage{filecontents}% http://ctan.org/pkg/filecontents
\begin{document}
\section{Intro}
This section appears first.

\begin{filecontents*}{arcane.tex}
\section{Arcane details}
This section should not appear here, but later.
\end{filecontents*}

\section{Interesting bits}
That people want to see.

\appendix
\input{arcane}% The Arcane details section appears here.

\end{document}

that produces

Within the filecontents (starred or not) environment, everything is dumped to the file verbatim, while \input reads the contents in as if it was placed in the original source at that location. This approach should work for even large and complex constructions within your section(s). You should use a different file name for each "delayed usage", otherwise the file will be overwritten (if you're using filecontents*).

If you're interested in literally doing a cut-and-paste of the section (keeping the section number and removing it from the sequence within the document's main contents) more work is required, of course.

Answer 2

You want something like this:

\documentclass{article}

\begin{document}
\section{Intro}
This section appears first.

\def\arcanesection{%
  \section{Arcane details}
  This section should not appear here, but later.
}

\section{Interesting bits}
That people want to see.

\appendix
% The Arcane details section should appear here.
\arcanesection
\end{document}