Redefining commands in LaTeX (\section in particular)

Question

A few days ago I asked a question about how to redefine the \section command to be able to write content just after the head of the section with no line breaking: Redefining \section command (I strongly recommend you to have a look to the link attached as it could be a good reference point to understand with this question).

Although we arrived at a satisfactory answer, I would like to ask a related question. I think that is similar but not quite the same so I have opened a new post (correct me if I'm wrong).

Here is my question:

I would like to change the \section command in the article class itself. I haven't found any place where it says that it has optional options and the only way I found that allow you to change it substantially is with the titlesec package.

How can I change it to make it work differently? The question itself is more about how to change a LaTeX command in its core than just about the \section command. But we can work with it for the sake of clarity.

Answer 1

\section is not defined in the latex kernel it is defined (or not) in each class file. The implementation in article for example is just one definition

\newcommand\section{\@startsection {section}{1}{\z@}%
                                   {-3.5ex \@plus -1ex \@minus -.2ex}%
                                   {2.3ex \@plus.2ex}%
                                   {\normalfont\Large\bfseries}}

\section is a display heading in article just because the {2.3ex \@plus.2ex} argument is positive, if you put a negative space there it will be a run-in heading (as is \paragraph so

\makeatletter

\renewcommand\section{\@startsection {section}{1}{\z@}%
                                   {-3.5ex \@plus -1ex \@minus -.2ex}%
                                   {-1em}%
                                   {\normalfont\Large\bfseries}}

\makeatother

in the preamble for example would have that effect.

The titlesec package gives a slightly more declarative interface to making the same changes, but again it basically has to assume the original definition is more or less like the one in article class.

Some other classes (such as memoir I think) start with more involved definitions that allow easier customisation.

Answer 2

Here is my way of redefining sections (with /def command):

\let\oldsection\section
\makeatletter
\def\msection{%
\@ifstar{\@Starred}{\@nonStarred}%
}
\def\@Starred{%
\@ifnextchar[%
{\GenericWarning{}{Warning: A starred section can not have parameters. I am going to ignore them!}\@StarredWith}%
{\@StarredWithout}%
}      
\def\@StarredWith[#1]#2{%
\oldsection*{#2}%
}
\def\@StarredWithout#1{
\oldsection*{#1}%
}
\def\@nonStarred{%
\@ifnextchar[%
{\@nonStarredWith}%
{\@nonStarredWithout}%
}
\def\@nonStarredWith[#1]#2{%
\oldsection[#1]{#2}%
}
\def\@nonStarredWithout#1{%
\oldsection{#1}%
}
\makeatother

The only change my section will provide is that it will accept starred sections with argument for the toc (and ignore that argument :P) Of course it isn't any real change but you can use these commands to redefine sections for any reason. A newspaper as example could add an if statement and if the section title contains words like freedom or slaves etc... could ignore these sections from table of contents and from the contents of sleeping people lives :P.

Answer 3

Supplemental Answer

Running texdoc article opens the classes.pdf file, which explains David Carlisle's answer pretty well. Look for the following section:

Chapters and Sections

where \@startsection and \secdef are explained.

Also see What does \z@ do?

Also note the texdoc source2e source2e.pdf document:

\def\@minus{minus} seemed absurd to me at first glance, but is just a way of speeding up the process by expanding once macro instead of reading 5 tokens (which ones? I wish the document would explain that better)

---

The file can also be viewed here:

http://mirror.ctan.org/macros/latex/base/classes.pdf

Other phenomenon / Headache Savers

If you decide to use the etoc package, but you have defined custom sectioning commands, you'll need to add a level to your custom sectioning command for etoc.

Why does my custom sectioning command crash on second run when using etoc?

Answer 4

The xparse package contains facilities for parsing an optional star, this is useful if you want to capture the entire \section command including its arguments.

The documentation has an example:

\NewDocumentCommand \chapter { s o m }
{
\IfBooleanTF {#1}
{ \typesetstarchapter {#3} }
{ \typesetnormalchapter {#2} {#3} }
}