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<a name="Define"></a>
<div class="header">
<p>
Next: <a href="Arguments.html#Arguments" accesskey="n" rel="next">Arguments</a>, Up: <a href="Definitions.html#Definitions" accesskey="u" rel="up">Definitions</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Defining-a-macro"></a>
<h3 class="section">5.1 Defining a macro</h3>

<p>The normal way to define or redefine macros is to use the builtin
<code>define</code>:
</p>
<dl>
<dt><a name="index-define"></a>Builtin: <strong>define</strong> <em>(<var>name</var>, <span class="roman">[</span><var>expansion</var><span class="roman">]</span>)</em></dt>
<dd><p>Defines <var>name</var> to expand to <var>expansion</var>.  If
<var>expansion</var> is not given, it is taken to be empty.
</p>
<p>The expansion of <code>define</code> is void.
The macro <code>define</code> is recognized only with parameters.
</p></dd></dl>

<p>The following example defines the macro <var>foo</var> to expand to the text
&lsquo;<samp>Hello World.</samp>&rsquo;.
</p>
<div class="example">
<pre class="example">define(`foo', `Hello world.')
&rArr;
foo
&rArr;Hello world.
</pre></div>

<p>The empty line in the output is there because the newline is not
a part of the macro definition, and it is consequently copied to
the output.  This can be avoided by use of the macro <code>dnl</code>.
See <a href="Dnl.html#Dnl">Dnl</a>, for details.
</p>
<p>The first argument to <code>define</code> should be quoted; otherwise, if the
macro is already defined, you will be defining a different macro.  This
example shows the problems with underquoting, since we did not want to
redefine <code>one</code>:
</p>
<div class="example">
<pre class="example">define(foo, one)
&rArr;
define(foo, two)
&rArr;
one
&rArr;two
</pre></div>

<a name="index-GNU-extensions-1"></a>
<p>GNU <code>m4</code> normally replaces only the <em>topmost</em>
definition of a macro if it has several definitions from <code>pushdef</code>
(see <a href="Pushdef.html#Pushdef">Pushdef</a>).  Some other implementations of <code>m4</code> replace all
definitions of a macro with <code>define</code>.  See <a href="Incompatibilities.html#Incompatibilities">Incompatibilities</a>,
for more details.
</p>
<p>As a GNU extension, the first argument to <code>define</code> does
not have to be a simple word.
It can be any text string, even the empty string.  A macro with a
non-standard name cannot be invoked in the normal way, as the name is
not recognized.  It can only be referenced by the builtins <code>indir</code>
(see <a href="Indir.html#Indir">Indir</a>) and <code>defn</code> (see <a href="Defn.html#Defn">Defn</a>).
</p>
<a name="index-arrays"></a>
<p>Arrays and associative arrays can be simulated by using non-standard
macro names.
</p>
<dl>
<dt><a name="index-array"></a>Composite: <strong>array</strong> <em>(<var>index</var>)</em></dt>
<dt><a name="index-array_005fset"></a>Composite: <strong>array_set</strong> <em>(<var>index</var>, <span class="roman">[</span><var>value</var><span class="roman">]</span>)</em></dt>
<dd><p>Provide access to entries within an array.  <code>array</code> reads the entry
at location <var>index</var>, and <code>array_set</code> assigns <var>value</var> to
location <var>index</var>.
</p></dd></dl>

<div class="example">
<pre class="example">define(`array', `defn(format(``array[%d]'', `$1'))')
&rArr;
define(`array_set', `define(format(``array[%d]'', `$1'), `$2')')
&rArr;
array_set(`4', `array element no. 4')
&rArr;
array_set(`17', `array element no. 17')
&rArr;
array(`4')
&rArr;array element no. 4
array(eval(`10 + 7'))
&rArr;array element no. 17
</pre></div>

<p>Change the &lsquo;<samp>%d</samp>&rsquo; to &lsquo;<samp>%s</samp>&rsquo; and it is an associative array.
</p>
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<p>
Next: <a href="Arguments.html#Arguments" accesskey="n" rel="next">Arguments</a>, Up: <a href="Definitions.html#Definitions" accesskey="u" rel="up">Definitions</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p>
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