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From: texinfo1@rusmv1.rus.uni-stuttgart.de (Rainer Schoepf)
Newsgroups: comp.text.tex
Subject: Re: (Another) puzzler for wannabe wizards
Summary: Showing off!
Keywords: Dirty Tricks, catcode changing, puzzle
Message-ID: <1991Jun10.113954.17511@rusmv1.rus.uni-stuttgart.de>
Date: 10 Jun 91 11:39:54 GMT
References: <RAICHLE.91Jun5125624@azu.informatik.uni-stuttgart.de>
Distribution: comp
Organization: Comp.Center (RUS), U of Stuttgart, FRG
Lines: 63

In article <RAICHLE.91Jun5125624@azu.informatik.uni-stuttgart.de> raichle@azu.informatik.uni-stuttgart.de writes:
>Some time ago Don Hosek posted
>> A couple puzzlers for those who think that they're TeX gurus.
>
>Here is another puzzle...
>
>% ------- CUT HERE ------
>% use plainTeX
>\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
>% ------- CUT HERE ------
>
>At the place marked with @<Insert your hack...@> there should be a
>local(!) re-definition of the active char "
>
>This re-definition should satisfy the following constraints:
>- do not assume that the catcode of " is 12
>- no side-effects (local or global) except of changing the macro
>  definition of " with catcode `active' (i.e., you can't use a
>  counter, macro, ... to save the current catcode of ")
>- with the behaviour specified in the comments
>- "small is beautiful" => TeX code with no more than 160 chars
>- do not use \aftergroup
>
>
>A solution with \aftergroup:
>
>% @<Insert your hack at this place@>=
>  \begingroup \catcode`\"=\active
>    \aftergroup\def\aftergroup"\catcode`\"=12
>    \aftergroup{\aftergroup"\aftergroup}%
>  \endgroup
>
>
>Have fun
>	-bernd
>__________________________________________________________________________
>Bernd Raichle, Student der Universit"at Stuttgart | "Le langage est source
>privat: Stettener Str. 73, D-7300 Esslingen       |  de malentendus"
>email:  raichle@azu.informatik.uni-stuttgart.de   |  (A. de Saint-Exupery)


How about this one:

  \catcode`\"\active
  \edef"{\char`\expandafter\noexpand"\expandafter}%
  \expandafter\catcode\expandafter`\expandafter\"\the\catcode`"

It relies on a few things: that " (active) is defined to expand to \"
previously, that \{} and all letters have their usual category codes,
etc.

Rainer