Defining characters as macros [FAQ LaTeX]

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====== Defining characters as macros ======


Single characters can act as macros (defined commands), and both
Plain TeX and LaTeX define the character ''~'' as a "non-breakable space".
A character is made definable, or "active", by setting its
//[[2_programmation:syntaxe:catcodes:start|category code]]// (catcode) to be ''\active'' (13):
<code latex>
\catcode`\_=\active
</code>

Any character could, in principle, be activated this way and defined
as a macro:
<code latex>
\def//{\//}
</code>
which could be characterised as an over-simple answer to 
[[3_composition:texte:symboles:caracteres:tiret|using underscores]].
However, you must be wary: whereas people expect an active tilde, other
active characters may be unexpected and interact badly with other macros.
Furthermore, by defining an active character, you preclude the character's use
for other purposes, and there are few characters "free" to be subverted
in this way.

To define the character ''z'' as a command, one would say something
like:
<code latex>
\catcode`\z=\active
\def z{Yawn, I'm tired}%
</code>
and each subsequent ''z'' in the text would become a
yawn. This would be an astoundingly bad idea for most documents, but
might have special applications.  (Note that, in  ''\def  z'', 
''z'' is no longer interpreted as a letter; the space
is therefore not necessary --- ''\defz'' would do; we choose to
retain the space, for what little clarity we can manage.)
Some LaTeX packages facilitate such definitions. For example, the
[[ctanpkg>shortvrb]] package with its ''\MakeShortVerb'' command.

TeX uses category codes to interpret characters as they are read 
from the input.
//Changing a catcode value will not affect characters that have already been read//.
Therefore, it is best if characters have fixed category codes for the duration of a document.
If catcodes are changed for particular purposes (the ''\verb'' command does this), then
the altered characters will not be interpreted properly when they appear in the
argument to another command (as, for example, in
"[[3_composition:texte:paragraphes:pourquoi_verbatim_ne_fonctionne_pas_toujours|“\verb” in command arguments]]").
An exemplary case is the [[ctanpkg>doc]] package, which processes ''.dtx''
files using the [[ctanpkg>shortvrb]] package to define ''|''...''|'' as a shorthand
for ''\verb|''...''|''. But ''|'' is also used in the preambles of tabular environments,
so that tables in ''.dtx'' files can only have vertical line separation between
columns by employing special measures of some sort.

Another consequence is that catcode assignments made
in macros often don't work as expected 
(see "[[2_programmation:macros:caracteres_actifs_dans_les_arguments_d_une_macro|Active characters in command arguments]]").
For example, the definition

<code latex>
\def\mistake{%
\catcode`_=\active
\def_{\textunderscore\-}%
}
</code>

does not work because it attempts to define an ordinary ''_'' character:
When the macro is used, the category change does not apply to the 
underscore character already in the macro definition.  Instead, one may
use:

<code latex>
\begingroup
\catcode`_=\active
\gdef\works{%    note the global \gdef
  \catcode`_=\active
  \def_{\textunderscore\-}%
}
\endgroup
</code>

The alternative ("tricksy") way of creating such an isolated
definition depends on the curious properties of ''\lowercase'', which
changes characters without altering their catcodes.  Since there is
always //one// active character (''~''), we
can fool ''\lowercase'' into patching up a definition without ever
explicitly changing a catcode:
<code latex>
\begingroup
  \lccode''\~=''\_
  \lowercase{\endgroup
    \def~{\textunderscore\-}%
  }%
</code>

The two definitions have the same overall effect (the character is defined
as a command, but the character does not remain active), except that
the first defines a ''\global'' command.

For active characters to be used only in maths mode, it is much better
to leave the character having its ordinary catcode, but assign it a
special active //maths code//, as with

<code latex>
\begingroup
  \lccode''~=''x
  \lowercase{\endgroup
    \def~{\times}%
  }%
\mathcode`x="8000
</code>

The special character does not need to be redefined whenever it is
made active --- the definition of the command persists even if the
character's catcode reverts to its original value; the definition
becomes accessible again if the character once again becomes active.


-----
//Source:// [[faquk>FAQ-activechars|Defining characters as macros]]

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