Frequently Asked Question List for TeX

# Is this command defined?

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

``````\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)

``````{\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 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:

``````\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 2018 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`.

FAQ ID: Q-isdef
Tags: macros