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<head>
<title>UK TeX FAQ -- question label labundef</title>
</head><body>
<h3>Finding if a label is undefined</h3>
<p>People seem to want to know (at run time) if a label is undefined (I
don’t actually understand <em>why</em>, particularly: it’s a transient
state, and LaTeX deals with it quite well).
<p>A resolved label is simply a command:
<code>\</code><code>r@</code><code><<i>label-name</i>></code>; 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
<code>\</code><code>@ifundefined</code>:
<blockquote>
<code>\</code><code>@ifundefined{</code><em></em>r@<em>label-name</em><code>}{</code><em>undef-cmds</em><code>}{</code><em>def-cmds</em><code>}</code>
</blockquote><p>
In which, <<i>label-name</i>> is exactly what you would use in
a <code>\</code><code>label</code> command, and the remaining two arguments are command
sequences to be used if the label is undefined
(<<i>undef-cmds</i>>) or if it is defined
(<<i>def-cmds</i>>).
<p>Note that any command that incorporates <code>\</code><code>@ifundefined</code> is naturally
fragile, so remember to create it with <code>\</code><code>DeclareRobustCommand</code> or to
use it with <code>\</code><code>protect</code> in a moving argument.
<p>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 <code>\</code><code>G@refundefinedtrue</code> in
<<i><code>undef-cmds</i>></code>.
<p>And of course, remember you’re dealing in internal commands, and pay
attention to the <a href="FAQ-atsigns.html">at-signs</a>.
<p>All the above can be avoided by using the <i>labelcas</i> 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.
<dl>
<dt><tt><i>labelcas.sty</i></tt><dd><a href="ftp://cam.ctan.org/tex-archive/macros/latex/contrib/labelcas.zip">macros/latex/contrib/labelcas</a> (<a href="ftp://cam.ctan.org/tex-archive/macros/latex/contrib/labelcas.tar.gz">gzipped tar</a>, <a href="http://www.tex.ac.uk/tex-archive/macros/latex/contrib/labelcas/">browse</a>)
</dl>
<p><p>This question on the Web: <a href="http://www.tex.ac.uk/cgi-bin/texfaq2html?label=labundef">http://www.tex.ac.uk/cgi-bin/texfaq2html?label=labundef</a>
</body>
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