Macro in Tex Programming

Question

I am confused whether the below macro is for section or what? If yes can anybody help me to explain this piece of code

 \def\Section #1 \par
    {
    \bigbreak\vskip\parskip
    \need 1.5 in
    \global\advance\secno by 1
    \subno=0\subsubno=0
    \noindent
    {\SectionTitleFont \the\secno.\hskip 1em #1}
    \par
    }

Answer 1

The macro \Section has an argument delimited by a space and a \par token, so it's supposed to be called like

\Section Whatever title I want

The text of the section following the title.

Note the blank line after the end of the section title.

The macro will store Whatever title I want (the actual title, of course) discarding the space produced by the end of line as #1.

Now the macro executes

1. \bigbreak, which issues \par and then examines the last vertical space; if this space is less than \bigskipamount, TeX removes it and inserts, in its place, a penalty of value -200 and \bigskip, making this quite a good point for breaking a page;

2. \need 1.5in that's not defined in Plain TeX, but seems to be aimed at looking into the current values of \pagegoal and \pagetotal; if the difference between the former and the latter is less than 1.5in, a \vfill\eject command is probably issued; possible definition

   \def\need{\afterassignment\checkneed\dimen0 }
   \def\checkneed{\advance\dimen0 by \pagetotal
     \ifdim\dimen0>\pagegoal \vfill\eject\fi}
   

   or just \eject if pages are to be filled up.

3. The counters \secno is advanced by 1; this is supposed to contain the section number; the counters \subno and \subsubno, that are supposed to contain the subsection and subsubsection numbers are set to 0.

4. \noindent starts a paragraph; the the title is set using \SectionTitleFont (a font choice command to be defined) preceded by the section number and a quad of space; grouping ensures the font setting does not propagate.

5. \par makes a new paragraph.

Note that 1.5in is supposed to ensure that the section title, the interline skip and at least two lines of the subsequent paragraph fit.

---

As wipet correctly remarks in a comment, this macro has a few defects.

• If the title is Good programming in \TeX, an unscrutable error would be issued

  Runaway argument?
  Good programming in \TeX 
  ! Paragraph ended before \Section was complete.
  <to be read again> 
                     \par 
  

  There is no need to have <space>\par as the delimiter, because the trailing space would be removed anyway by the final \par in the replacement text, provided we remove the space in the replacement text.

• An even more unscrutable error would be issued by a title such as Plusvalence:

  ! Missing number, treated as zero.
  <to be read again> 
                     v
  <argument> Plusv
                  alence
  

  because TeX would see \hskip 1em Plusvalence and interpret Plus as a keyword.

Fixed code:

\def\Section #1\par
  {%
   \bigbreak\vskip\parskip
   \need 1.5 in
   \global\advance\secno by 1
   \subno=0\subsubno=0
   \noindent
   {\SectionTitleFont \the\secno.\hskip 1em\relax #1}\par
  }