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From: marcel@cs.caltech.edu (Marcel van der Goot)
Newsgroups: comp.text.tex
Subject: Re: #ifdef equivalent of selective inclusion for TeX/LaTeX???
Message-ID: <1990Oct22.081716.22423@nntp-server.caltech.edu>
Date: 22 Oct 90 08:17:16 GMT
References: <1990Oct18.021309.22918@cs.umn.edu> <40303@shemp.CS.UCLA.EDU>
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In <40303@shemp.CS.UCLA.EDU> Paul Kirkaas (kirkaas@ewa.cs.ucla.edu)
wonders
> I wonder if there is an equivalent to C's "#ifdef" for TeX ---
> that is, if there is something like:
> ...

Testing whether a control sequence \cs is defined is quite easy:

   \ifx\cs\undefined
      % \cs is not defined
   \else
      % \cs is defined
   \fi

where you should make sure that \undefined is never defined.
This works because both \cs and \undefined are macros (at least,
according to TeX they cannot be anything else), and, if \cs is
not defined, they both have the same top-level "expansion" (namely,
none), so that \ifx tests true.

The only thing you need to worry about is that if you insert commands
in the place of the comments above, they must have properly nested
\if ... \fi pairs.

This method (trick?) comes straight from the TeX-book, the last paragraph
of section 4: selective loading of macros, in appendix D (p. 384).


					   Marcel van der Goot
					   marcel@vlsi.cs.caltech.edu