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From: eijkhout@s41.csrd.uiuc.edu (Victor Eijkhout)
Newsgroups: comp.text.tex
Subject: Re: TeX bug: optional spaces are mandatory
Message-ID: <1991Feb8.232215.3129@csrd.uiuc.edu>
Date: 8 Feb 91 23:22:15 GMT
References: <4117@cernvax.cern.ch> <GRABINER.91Feb8124329@zariski.harvard.edu>
Sender: news@csrd.uiuc.edu (news)
Organization: UIUC Center for Supercomputing Research and Development
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grabiner@math.harvard.edu (David Grabiner) writes:

>In article <4117@cernvax.cern.ch>, Alfred Nathaniel writes:

>> I discovered a bug in TeX3.0. 
>> In sequences
>>      ...\counter=<constant>\ifnum\counter...
>> and
>>      ...\counter=<constant>\fi \ifnum\counter...
>> the \ifnum test fails if <constant> is not followed by a space.

>I don't think this is a bug; a similar situation is mentioned in the
>TeXbook, p. 208.  TeX has to expand the \ifnum to see whether the
>expansion becomes part of the <constant>.  For example:

Correct. The second example is somewhat more surprising tho'...

The TeXbook is not very explicit about how conditionals are
handled, but what it boils down to is that \else and \fi
are also expandable control sequences, and with an
empty expansion. So \counter2=1\fi \ifnum1=1 2\fi
will assign \counter2=12 ! (not factorial, but as in '``"O"o"o!!''
``No, Aaaaargghh!!!'' '. Sorry. The Python in me coming out.)

And now for the people who think the understand the above:
remember that \pageno is just an abbreviation (by \countdef)
of \count0. So what's the difference between
    \count37=1\ifodd\pageno\fi2
and
    \count37=1\ifodd\count0\fi2
?

Victor.