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+<head>
+<title>UK TeX FAQ -- question label actinarg</title>
+</head><body>
+<h3>Active characters in command arguments</h3>
+<p>Occasionally, it's nice to make one or two characters active in the
+argument of a command, to make it easier for authors to code the
+arguments.
+<p>Active characters <em>can</em> be used safely in such situations; but
+care is needed.
+<p>An example arose while this answer was being considered: an aspirant
+macro writer posted to <i>comp.text.tex</i> asking for help to
+make <code>#</code> and <code>b</code> produce musical sharp and flat signs, respectively,
+in a macro for specifying chords.
+<p>The first problem is that both <code>#</code> and <code>b</code> have rather important uses
+elsewhere in TeX (to say the least!), so that the characters can
+only be made active while the command is executing.
+<p>Using the techniques discussed in 
+"<a href="FAQ-activechars.html">characters as commands</a>",
+we can define:
+<blockquote>
+<pre>
+\begingroup
+  \catcode`\#=\active
+  \gdef#{$\sharp$}
+\endgroup
+</pre>
+</blockquote>
+and:
+<blockquote>
+<pre>
+\begingroup
+  \lccode`\~=`\b
+  \lowercase{\endgroup
+    \def~{$\flat$}%
+  }
+</pre>
+</blockquote>
+The second problem is one of timing: the command has to make each
+character active <em>before</em> its arguments are read: this means that
+the command can't actually "have" arguments itself, but must be
+split in two.  So we write:
+<blockquote>
+<pre>
+\def\chord{%
+  \begingroup
+    \catcode`\#=\active
+    \catcode`\b=\active
+    \Xchord
+}
+\def\Xchord#1{%
+    \chordfont#1%
+  \endgroup
+}
+</pre>
+</blockquote>
+and we can use the command as <code>\</code><code>chord{F#}</code> or
+<code>\</code><code>chord{Bb minor}</code>.
+<p>Two features of the coding are important:
+<ul>
+<li> <code>\</code><code>begingroup</code> in <code>\</code><code>chord</code> opens a group that is closed by
+  <code>\</code><code>endgroup</code> in <code>\</code><code>Xchord</code>; this group limits the change of
+  category codes, which is the <em>raison d'&ecirc;tre</em> of the whole
+  exercise.
+<li> Although <code>#</code> is active while <code>\</code><code>Xchord</code> is executed, it's
+  <em>not</em> active when it's being defined, so that the use of <code>#1</code>
+  doesn't require any special attention.
+</ul>
+<p>Note that the technique used in such macros as <code>\</code><code>chord</code>, here, is
+analagous to that used in such commands as <code>\</code><code>verb</code>; and, in just the
+same way as <code>\</code><code>verb</code> (see
+
+"<a href="FAQ-verbwithin.html"><code>\</code><code>verb</code> doesn't work in arguments</a>"),
+<code>\</code><code>chord</code> won't work inside the argument of another command (the
+error messages, if they appear at all, will probably be rather odd).
+<p><p><p>This question on the Web: <a href="http://www.tex.ac.uk/cgi-bin/texfaq2html?label=actinarg">http://www.tex.ac.uk/cgi-bin/texfaq2html?label=actinarg</a>
+</body>