### 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@`lab-name`}{`undef-cmds`}`{def-cmds}

In which, <lab-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`>.

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