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From: marcel@cs.caltech.edu (Marcel van der Goot)
Newsgroups: comp.text.tex
Subject: Re: TeX and marks
Keywords: mark
Message-ID: <1990Oct22.075424.22214@nntp-server.caltech.edu>
Date: 22 Oct 90 07:54:24 GMT
References: <41700@eerie.acsu.Buffalo.EDU>
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In  <41700@eerie.acsu.Buffalo.EDU> Mary Deck (mdeck@sybil.cs.Buffalo.EDU)
writes
> ... It *really* surprised me to find out that \if\topmark\topmark,
> \if\firstmark\firstmark, and \if\botmark\botmark are rarely true.
> (What?!?!  Something doesn't equal itself?!?!) ...

The answer is actually simpler than the question suggests, no trickery
with marks is involved. As an experiment, try to run the following
through TeX:

    \def\a{xxyz}
    \def\b{pqr}
    \message{*\if\a\b\else false\fi*}

You'll see that the message that is printed is "*yzpqr*", although
certainly \a and \b are different. The reason is that \if expands
tokens until two unexpandable tokens are found. In this case, \a is
expanded which gives the two unexpandable tokens "x" and "x", which
compare equal. The <true text> of the \if is then "yz\b" (TeX book,
p. 209). \topmark etc. expand just like macros (p. 213), so if you
use \if to compare them you are actually comparing something entirely
different.

To test things without expanding them, you usually use \ifx instead
of \if. That would work for the above example with \a and \b, but
not with \botmark etc., because the latter are primitives, not macros
(hence, \ifx\firstmark\botmark is always false, regardless of the marks).
The solution is to first assign the expansion to a macro, using \edef:

    \edef\first{\firstmark}
    \edef\bot{\botmark}
    \ifx\first\bot
        % \firstmark and \botmark have equal values
    \else
        % \firstmark and \botmark have different values
    \fi


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