[texhax] choice of documentstyle upon \jobname

[Reinhard Kotucha]

Hi,
I got the following mail from Susan.  Though it is usually impolite to
put private mails on mailing lists, I think that the question is
interesting enough to be discussed here.

But the main reason I answer to this list is that I'm not sure that
everything I say is correct.  On a mailing list I can make mistakes
and can be sure that people correct me if necessary.

>>>>> "Susan" == Susan Dittmar <S.Dittmar at eureca.de> writes:

  >> \expandafter\def\csname first\endcsname{\documentclass[abc]{...}}
  >> \expandafter\def\csname second\endcsname{\documentclass[xyz]{...}}

  > Is it true that, in the case where first and second are completely
  > composed of letters which TeX accepts for command name, that's
  > equivalent to 
  > \def\first{\documentclass...}
  > \def\second{\documentclass...}  ?

Maybe not equivalent, but at least similar.

\csname...\endcsname doesn't care about catcodes.  Instead of
\csname\jobname\endcsname you can better write
\csname\jobname:-)\endcsname to avoid name clashes.

You can't do it with \def so easy.  \csname...\endcsname allows you to
define "private" macros which you cannot access accidentally.

Indeed you can say \def\foo{...} or \newcommand{\foo}{...} and can
either use this macro directly (\foo) or you can say 
\csname foo\endcsname. 

In latex it's easier to use \@namedef and \@nameuse.

There is one important difference between \foo and \csname foo\endcsname
(or \@nameuse{foo}):

If \foo is undefined you get an error message if you say \foo, while 
\csname foo\endcsname doesn't complain if \foo doesn't exist.

You might wonder what this is good for.

I assume that you are working on a larger project which will not be
finished soon.  So please put this into the preamble of your
document:

   \expandafter\def\csname12:24\endcsname{\typeout{Merry Xmas!}}
   \expandafter\def\csname1:1\endcsname{\typeout{Happy new year!}}

   \csname\number\month:\number\day\endcsname

or 

   \makeatletter
   \@namedef{12:24}{\typeout{Merry Xmas!}}
   \@namedef{1:1}{\typeout{Happy new year!}}

   \@nameuse{\number\month:\number\day}
   \makeatother

It is a good exercise to find out why \expandafter is needed in the
first example.  See "TeX by Topics".

Regards,
  Reinhard

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