UK TeX FAQ -- question label labundef

### Finding if a label is undefined

People seem to want to know (at run time) if a label is undefined (I don’t actually understand why, particularly: it’s a transient state, and LaTeX deals with it quite well).

A resolved label is simply a command: `\``r@``<label-name>`; determining if the label is set is then simply a matter of detecting if the command exists. The usual LaTeX internal way of doing this is to use the command `\``@ifundefined`:

> `\``@ifundefined{`r@label-name`}{`undef-cmds`}{`def-cmds`}`

In which, <label-name> is exactly what you would use in a `\``label` command, and the remaining two arguments are command sequences to be used if the label is undefined (<undef-cmds>) or if it is defined (<def-cmds>).

Note that any command that incorporates `\``@ifundefined` is naturally fragile, so remember to create it with `\``DeclareRobustCommand` or to use it with `\``protect` in a moving argument.

If you’re into this game, you may well not care about LaTeX’s warning about undefined labels at the end of the document; however, if you are, include the command `\``G@refundefinedtrue` in <`undef-cmds`>.

And of course, remember you’re dealing in internal commands, and pay attention to the at-signs.

All the above can be avoided by using the labelcas package: it provides commands that enable you to switch according to the state of a single label, or the states of a list of labels. The package’s definition is a bit complicated, but the package itself is pretty powerful.

`labelcas.sty`
  macros/latex/contrib/labelcas (or browse the directory)

This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=labundef