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— title: Is this command defined? category: programming tags: macros permalink: /FAQ-isdef date: 2014-06-10 —

Macro sets from the earliest days of TeX programming may be observed to test whether commands exist by using `\ifx\<command>\undefined` &lsaquo;_stuff_&rsaquo; &hellip; (which of course actually tests that the command _doesn't_ exist). LaTeX programmers can make use of the internal command

`\@ifundefined{cmd name}{action1}{action2}`

which executes `action1` if the command is undefined, and `action2` if it is defined (_cmd name_ is the command name only, omitting the `\` character).

The `\@ifundefined` command is based on the sequence ```latex \expandafter \ifx \csname cmd name\endcsname \relax ``` which relies on the way `\csname` works: if the command doesn't exist, it simply creates it as an alias for `\relax`.

So: what is wrong with these techniques?

Using `\undefined` blithely assumes that the command is indeed not defined. This isn't entirely safe; one could make the name more improbable, but that may simply make it more difficult to spot a problem when things go wrong. LaTeX programmers who use the technique will typically employ `\@undefined`, adding a single level of obscurity.

The original `\@ifundefined` mechanism had the unfortunate property of polluting the name space: each test that turns out undefined adds a name to the set TeX is holding, and often all those `\relax` names serve no purpose whatever.

David Kastrup offers the (rather tricky) ```latex {\expandafter}\expandafter\ifx \csname cmd name\endcsname\relax … ``` which “creates” the `\relax`-command inside the group of the first `\expandafter`, therefore forgets it again once the test is done. The test is about as good as you can do with macros.

The [&epsilon;-TeX system](FAQ-etex) system comes to our help here: it defines two new primitives:

- `\ifdefined`, which tests whether a thing is defined (the

  negative of comparing with `\undefined`, as it were), and

- `\ifcsname cmd name\endcsname`, which does the

  negative of `\@ifundefined` without the `\relax`-command
  side-effect.

So, in an &epsilon;-TeX-based system, the following two conditional clauses do the same thing: ```latex \ifdefined\foo

\message{\string\foo\space is defined}%

\else

\message{no command \string\foo}%

\fi % \ifcsname foo\endcsname

\message{\string\foo\space is defined}%

\else

\message{no command \string\foo}%

\fi ``` However, after using the original LaTeX `\@ifundefined{foo}`&hellip;, the conditionals will detect the command as “existing” (since it has been `\let` to `\relax`); so it is important not to mix mechanisms for detecting the state of a command.

In the 2016 LaTeX release, the definition of `\@ifundefined` was adapted to use the &epsilon;-TeX `\ifcsname` and now tests for a command being undefined or `\relax` without the side effect of defining undefined commands to `\relax`.

2_programmation/macros/cette_commande_est_elle_definie.1528035422.txt.gz · Dernière modification: 2018/06/03 16:17 par samcarter
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