php-doc-en/reference/array/functions/usort.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
<refentry xml:id="function.usort" xmlns="http://docbook.org/ns/docbook">
 <refnamediv>
  <refname>usort</refname>
  <refpurpose>Sort an array by values using a user-defined comparison function</refpurpose>
 </refnamediv>
 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis>
   <type>bool</type><methodname>usort</methodname>
   <methodparam><type>array</type><parameter role="reference">array</parameter></methodparam>
   <methodparam><type>callable</type><parameter>cmp_function</parameter></methodparam>
  </methodsynopsis>
  <para>
   This function will sort an array by its values using a user-supplied
   comparison function.  If the array you wish to sort needs to be sorted by
   some non-trivial criteria, you should use this function.
  </para>
  <note>
   <para>
    If two members compare as equal, their relative order in the sorted array is undefined.
   </para>
  </note>
  &note.no-key-association;
 </refsect1>
 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>array</parameter></term>
     <listitem>
      <para>
       The input array.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>cmp_function</parameter></term>
     <listitem>
      <para>
       &return.callbacksort;
      </para>
      &callback.cmp;
      <caution>
       <para>
        Returning <emphasis>non-integer</emphasis> values from the comparison
        function, such as <type>float</type>, will result in an internal cast to
        <type>integer</type> of the callback's return value. So values such as
        0.99 and 0.1 will both be cast to an integer value of 0, which will
        compare such values as equal.
       </para>
      </caution>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
 </refsect1>
 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   &return.success;
  </para>
 </refsect1>
 <refsect1 role="changelog">
  &reftitle.changelog;
  <para>
   <informaltable>
    <tgroup cols="2">
     <thead>
      <row>
       <entry>&Version;</entry>
       <entry>&Description;</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry>4.1.0</entry>
       <entry>
        A new sort algorithm was introduced. The <parameter>cmp_function</parameter>
        doesn't keep the original order for elements comparing as equal.
       </entry>
      </row>
     </tbody>
    </tgroup>
   </informaltable>
  </para>
 </refsect1>
 <refsect1 role="examples">
  &reftitle.examples;
  <para>
   <example xml:id="function.usort.examples.basic">
    <title><function>usort</function> example</title>
    <programlisting role="php">
<![CDATA[
<?php
function cmp($a, $b)
{
    if ($a == $b) {
        return 0;
    }
    return ($a < $b) ? -1 : 1;
}

$a = array(3, 2, 5, 6, 1);

usort($a, "cmp");

foreach ($a as $key => $value) {
    echo "$key: $value\n";
}
?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
0: 1
1: 2
2: 3
3: 5
4: 6
]]>
    </screen>
   </example>
  </para>
  <note>
   <para>
    Obviously in this trivial case the <function>sort</function>
    function would be more appropriate.
   </para>
  </note>
  <para>
   <example xml:id="function.usort.examples.multi">
    <title>
     <function>usort</function> example using multi-dimensional array
    </title>
    <programlisting role="php">
<![CDATA[
<?php
function cmp($a, $b)
{
    return strcmp($a["fruit"], $b["fruit"]);
}

$fruits[0]["fruit"] = "lemons";
$fruits[1]["fruit"] = "apples";
$fruits[2]["fruit"] = "grapes";

usort($fruits, "cmp");

while (list($key, $value) = each($fruits)) {
    echo "\$fruits[$key]: " . $value["fruit"] . "\n";
}
?>
]]>
    </programlisting>
    <para>
     When sorting a multi-dimensional array, <varname>$a</varname> and
    <varname>$b</varname> contain references to the first index of the array.
    </para>
    &example.outputs;
    <screen>
<![CDATA[
$fruits[0]: apples
$fruits[1]: grapes
$fruits[2]: lemons
]]>
    </screen>
   </example>
  </para>
  <para>
   <example xml:id="function.usort.examples.object">
    <title>
     <function>usort</function> example using a member function of an object
    </title>
    <programlisting role="php">
<![CDATA[
<?php
class TestObj {
    var $name;

    function TestObj($name)
    {
        $this->name = $name;
    }

    /* This is the static comparing function: */
    static function cmp_obj($a, $b)
    {
        $al = strtolower($a->name);
        $bl = strtolower($b->name);
        if ($al == $bl) {
            return 0;
        }
        return ($al > $bl) ? +1 : -1;
    }
}

$a[] = new TestObj("c");
$a[] = new TestObj("b");
$a[] = new TestObj("d");

usort($a, array("TestObj", "cmp_obj"));

foreach ($a as $item) {
    echo $item->name . "\n";
}
?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
b
c
d
]]>
    </screen>
   </example>
   <example xml:id="function.usort.examples.closure">
    <title>
     <function>usort</function> example using a <link linkend="functions.anonymous">closure</link>
     to sort a multi-dimensional array
    </title>
    <programlisting role="php">
<![CDATA[
<?php
$array[0] = array('key_a' => 'z', 'key_b' => 'c');
$array[1] = array('key_a' => 'x', 'key_b' => 'b');
$array[2] = array('key_a' => 'y', 'key_b' => 'a');

function build_sorter($key) {
    return function ($a, $b) use ($key) {
        return strnatcmp($a[$key], $b[$key]);
    };
}

usort($array, build_sorter('key_b'));

foreach ($array as $item) {
    echo $item['key_a'] . ', ' . $item['key_b'] . "\n";
}
?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
y, a
x, b
z, c
]]>
    </screen>
   </example>
  </para>
 </refsect1>
 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member><function>uasort</function></member>
    <member>&seealso.array.sorting;</member>
   </simplelist>
  </para>
 </refsect1>
</refentry>

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