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— title: Is this command defined? category: programming tags: macros permalink: /FAQ-isdef —
Macro sets from the earliest days of TeX programming may be observed to test whether commands exist by using `\ifx\<command>\undefined` ‹_stuff_› … (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 [ε-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 ε-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}`…, 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 ε-TeX `\ifcsname` and now tests for a command being undefined or `\relax` without the side effect of defining undefined commands to `\relax`.
