TeX/def

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< TeX

[edit] Synopsis

\def <command> <parameter-text> {<replacement-text>}

[edit] Description

\def allows you to define new macros. The command must begin with an escape character (category code 0) and be composed of the longest series of letters (category code 11) or one single non-letter. (For instance \delta or \{.) Alternatively, <command> can be an active character (category code 13). Then, the parameter text tells TeX the valid syntax of the command being defined; furthermore, in the parameter text, one can include arguments (denoted by #1, etc., up to #9) which can be reused in the replacement text. For example, in

\def \foo [#1][#2]{The first argument is ``#1'', the second one is ``#2''}

The first argument must be delimited by two square brackets while the second may be a single character (strictly spoken: a single token having a category code distinct from 1 or 2) or any balanced text surrounded by braces {...}. Finally, the replacement text tells TeX what the command is supposed to expand to and how to deal with the arguments if any (see example above).

Contrarily to LaTeX (with \newcommand), TeX doesn't check whether the command to be defined is important or not. You might well say \def \def etc., except that the most probable is that you encounter some problems quite quickly. One way to be safe in this respect is to write

\ifx <command> \undefined
  \def <command> . . .
\fi

\def can be preceded by \global (the redefinition isn't confined to the current group, also controlled by \globaldefs), \long (the command admits long arguments), \outer (the command is not tolerated in parts of the code which are read at high speed).

See also the command \let.

[edit] Examples

This defines a simple shortcut:

\def \NAS {National Academy of Science}
\def \author {William {\sc Smith}}

This defines a quotation environment

\def \beginquote {\par \begingroup \narrower}
\def \endquote {\par \endgroup}