Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!uupsi!sunic!chalmers.se!cs.chalmers.se!jeffrey
From: jeffrey@cs.chalmers.se (Alan Jeffrey)
Newsgroups: comp.text.tex
Subject: Re: (Another) puzzler for wannabe wizards
Keywords: Dirty Tricks, catcode changing, puzzle
Message-ID: <4651@undis.cs.chalmers.se>
Date: 6 Jun 91 09:14:43 GMT
References: <RAICHLE.91Jun5125624@azu.informatik.uni-stuttgart.de>
Sender: news@cs.chalmers.se
Distribution: comp
Organization: Dept. of CS, Chalmers, Sweden
Lines: 38

In article <RAICHLE.91Jun5125624@azu.informatik.uni-stuttgart.de> 
Bernd Raichle <raichle@informatik.uni-stuttgart.de> posed a horrible 
catcoding problem...  
 
> \catcode`\"=\active\gdef"{\"}
> T"ater                     % output: T\"ater
> \begingroup\catcode`\"=12
>   \showthe\catcode`\"      % print:  12
>   % @<Insert your hack at this place@>
>   \showthe\catcode`\"      % print:  12
>   T"ater                   % output: T''ater
>   \catcode`\"=\active
>   T"ater                   % output: T''ater
> \endgroup
> T"ater                     % output: T\"ater
> \bye
 
> At the place marked with @<Insert your hack...@> there should be a
> local(!) re-definition of the active char "
 
Well, here's my solution---the only cheat is that it uses an extra 
auxilary definition, to swap the order of evaluation (I can't see any way 
to do this without something similar, as you need to give TeX's stomach 
the tokens "_13 then "_12 despite having read them in the 
order "_12 then "_13):
 
   \def\Swap#1#2{#2{#1}}
 
And the contents of the gold envelope:
 
   \expandafter\Swap\expandafter{\the\catcode`\"\def#1{"}}
      {\catcode`\"\active\Swap}{\def"#1}\catcode`\"""
 
Anyone got a solution that doesn't use \Swap or something similar?
 
Alan.
Alan Jeffrey         Tel: +46 31 72 10 98         jeffrey@cs.chalmers.se
Department of Computer Sciences, Chalmers University, Gothenburg, Sweden