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<h3 class="section">9.1 Defining the Comparison Function</h3>

<p><a name="index-Comparison-Function-819"></a>
In order to use the sorted array library functions, you have to describe
how to compare the elements of the array.

   <p>To do this, you supply a comparison function to compare two elements of
the array.  The library will call this function, passing as arguments
pointers to two array elements to be compared.  Your comparison function
should return a value the way <code>strcmp</code> (see <a href="String_002fArray-Comparison.html#String_002fArray-Comparison">String/Array Comparison</a>) does: negative if the first argument is &ldquo;less&rdquo; than the
second, zero if they are &ldquo;equal&rdquo;, and positive if the first argument
is &ldquo;greater&rdquo;.

   <p>Here is an example of a comparison function which works with an array of
numbers of type <code>double</code>:

<pre class="smallexample">     int
     compare_doubles (const void *a, const void *b)
     {
       const double *da = (const double *) a;
       const double *db = (const double *) b;
     
       return (*da &gt; *db) - (*da &lt; *db);
     }
</pre>
   <p>The header file <samp><span class="file">stdlib.h</span></samp> defines a name for the data type of
comparison functions.  This type is a GNU extension.

<!-- stdlib.h -->
<!-- GNU -->
   <p><a name="index-comparison_005ffn_005ft-820"></a>
<pre class="smallexample">     int comparison_fn_t (const void *, const void *);
</pre>
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