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
category code (catcode) to be \active (13):
\catcode'_=\active.
Any character could, in principle, be activated this way and defined
as a macro ( To define the character " 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 Another consequence is that catcode assignments made
in macros often don't work as expected
(see "Active characters in command arguments").
For example, the definition
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
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=activechars
\def_{\} - the simple answer to
using underscores), but 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.
z" as a command, one would say something
like:
and each subsequent "
\catcode`\z=\active
\def z{Yawn, I'm tired}%
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
shortvrb package with its \MakeShortVerb command.
\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
"\verb in command arguments").
An exemplary case is the doc package, which processes .dtx
files using the 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.
does not work because it attemts to define an ordinary
\def\mistake{%
\catcode`_=\active
\def_{\textunderscore\-}%
}
_ 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:
The alternative ("tricksy") way of creating such an isolated
definition depends on the curious properties of
\begingroup
\catcode`_=\active
\gdef\works{% note the global \gdef
\catcode`_=\active
\def_{\textunderscore\-}%
}
\endgroup
\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:
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
\begingroup
\lccode`\~=`\_
\lowercase{\endgroup
\def~{\textunderscore\-}%
}%
\global command.
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.
\begingroup
\lccode`~=`x
\lowercase{\endgroup
\def~{\times}%
}%
\mathcode`x="8000