mirror of
https://github.com/php/doc-zh.git
synced 2026-03-24 07:02:15 +01:00
317 lines
7.6 KiB
XML
317 lines
7.6 KiB
XML
<?xml version="1.0" encoding="utf-8"?>
|
||
<!-- $Revision$ -->
|
||
<!-- EN-Revision: ba7093cf7f30dbcd301c62536ac7ef8664d891f4 Maintainer: dallas Status: ready -->
|
||
<!-- CREDITS: mowangjuanzi -->
|
||
<sect1 xml:id="control-structures.foreach" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
|
||
<title>foreach</title>
|
||
<?phpdoc print-version-for="foreach"?>
|
||
<para>
|
||
<literal>foreach</literal> 结构提供了一种遍历 <type>array</type> 和 <interfacename>Traversable</interfacename>
|
||
object 的简便方式。当用于其他数据类型的变量，或未初始化的变量时，<literal>foreach</literal> 会触发错误。
|
||
<informalexample>
|
||
<simpara>
|
||
<literal>foreach</literal> 可选择性的获取每个元素的 <literal>key</literal>：
|
||
</simpara>
|
||
<programlisting>
|
||
<![CDATA[
|
||
foreach (iterable_expression as $value) {
|
||
statement_list
|
||
}
|
||
|
||
foreach (iterable_expression as $key => $value) {
|
||
statement_list
|
||
}
|
||
]]>
|
||
</programlisting>
|
||
</informalexample>
|
||
</para>
|
||
<simpara>
|
||
第一种格式遍历给定的 <literal>iterable_expression</literal>
|
||
迭代器。每次循环中，当前单元的值被赋给 <literal>$value</literal>。
|
||
</simpara>
|
||
<simpara>
|
||
第二种格式做同样的事，只除了当前单元的键名也会在每次循环中被赋给变量
|
||
<literal>$key</literal>。
|
||
</simpara>
|
||
<simpara>
|
||
注意 <literal>foreach</literal> 不会修改类似 <function>current</function>
|
||
和 <function>key</function> 函数所使用的数组内部指针。
|
||
</simpara>
|
||
<simpara>
|
||
还能够自定义<link linkend="language.oop5.iterations">遍历对象</link>。
|
||
</simpara>
|
||
|
||
<example>
|
||
<title>常见的 <literal>foreach</literal> 用法</title>
|
||
<programlisting role="php">
|
||
<![CDATA[
|
||
<?php
|
||
|
||
/* 示例：仅有值 */
|
||
$array = [1, 2, 3, 17];
|
||
|
||
foreach ($array as $value) {
|
||
echo "Current element of \$array: $value.\n";
|
||
}
|
||
|
||
/* 示例：key 和值 */
|
||
$array = [
|
||
"one" => 1,
|
||
"two" => 2,
|
||
"three" => 3,
|
||
"seventeen" => 17
|
||
];
|
||
|
||
foreach ($array as $key => $value) {
|
||
echo "Key: $key => Value: $value\n";
|
||
}
|
||
|
||
/* 示例：多维 key-value 数组 */
|
||
$grid = [];
|
||
$grid[0][0] = "a";
|
||
$grid[0][1] = "b";
|
||
$grid[1][0] = "y";
|
||
$grid[1][1] = "z";
|
||
|
||
foreach ($grid as $y => $row) {
|
||
foreach ($row as $x => $value) {
|
||
echo "Value at position x=$x and y=$y: $value\n";
|
||
}
|
||
}
|
||
|
||
/* 示例：动态数组 */
|
||
foreach (range(1, 5) as $value) {
|
||
echo "$value\n";
|
||
}
|
||
?>
|
||
]]>
|
||
</programlisting>
|
||
</example>
|
||
<note>
|
||
<para>
|
||
<literal>foreach</literal> 不支持使用 <link
|
||
linkend="language.operators.errorcontrol"><literal>@</literal></link> 运算符来抑制错误消息。
|
||
</para>
|
||
</note>
|
||
|
||
<sect2 xml:id="control-structures.foreach.list">
|
||
<title>解包嵌套数组</title>
|
||
<?phpdoc print-version-for="foreach.list"?>
|
||
<para>
|
||
可以通过遍历数组中的数组，在 value 的位置使用<link
|
||
linkend="language.types.array.syntax.destructuring">数组解构</link>（<literal>[]</literal>）或
|
||
<function>list</function> 语言结构将嵌套数组解包到循环变量中。
|
||
|
||
<note>
|
||
<simpara>
|
||
请注意，通过 <literal>[]</literal> 进行<link
|
||
linkend="language.types.array.syntax.destructuring">数组解构</link>仅在 PHP 7.1.0 及以上版本中可用。
|
||
</simpara>
|
||
</note>
|
||
</para>
|
||
|
||
<para>
|
||
<informalexample>
|
||
<simpara>
|
||
在以下两个示例中，将设 <literal>$a</literal> 为嵌套数组的第一个元素，<literal>$b</literal> 将包含第二个元素：
|
||
</simpara>
|
||
<programlisting role="php">
|
||
<![CDATA[
|
||
<?php
|
||
$array = [
|
||
[1, 2],
|
||
[3, 4],
|
||
];
|
||
|
||
foreach ($array as [$a, $b]) {
|
||
echo "A: $a; B: $b\n";
|
||
}
|
||
|
||
foreach ($array as list($a, $b)) {
|
||
echo "A: $a; B: $b\n";
|
||
}
|
||
?>
|
||
]]>
|
||
</programlisting>
|
||
&example.outputs;
|
||
<screen>
|
||
<![CDATA[
|
||
A: 1; B: 2
|
||
A: 3; B: 4
|
||
]]>
|
||
</screen>
|
||
</informalexample>
|
||
</para>
|
||
|
||
<para>
|
||
当提供的变量数量少于数组中的元素数量时，将会忽略多余的元素。类似地，可通过使用逗号跳过某些元素：
|
||
<informalexample>
|
||
<programlisting role="php">
|
||
<![CDATA[
|
||
<?php
|
||
$array = [
|
||
[1, 2, 5],
|
||
[3, 4, 6],
|
||
];
|
||
|
||
foreach ($array as [$a, $b]) {
|
||
// 注意此处没有 $c
|
||
echo "$a $b\n";
|
||
}
|
||
|
||
foreach ($array as [, , $c]) {
|
||
// 跳过 $a 和 $b
|
||
echo "$c\n";
|
||
}
|
||
?>
|
||
]]>
|
||
</programlisting>
|
||
&example.outputs;
|
||
<screen>
|
||
<![CDATA[
|
||
1 2
|
||
3 4
|
||
5
|
||
6
|
||
]]>
|
||
</screen>
|
||
</informalexample>
|
||
</para>
|
||
|
||
<para>
|
||
如果数组元素数量不足以填充 <function>list</function>，将生成一条 notice 级别的错误消息。
|
||
|
||
<informalexample>
|
||
<programlisting role="php">
|
||
<![CDATA[
|
||
<?php
|
||
$array = [
|
||
[1, 2],
|
||
[3, 4],
|
||
];
|
||
|
||
foreach ($array as [$a, $b, $c]) {
|
||
echo "A: $a; B: $b; C: $c\n";
|
||
}
|
||
?>
|
||
]]>
|
||
</programlisting>
|
||
&example.outputs;
|
||
<screen>
|
||
<![CDATA[
|
||
Notice: Undefined offset: 2 in example.php on line 7
|
||
A: 1; B: 2; C:
|
||
|
||
Notice: Undefined offset: 2 in example.php on line 7
|
||
A: 3; B: 4; C:
|
||
]]>
|
||
</screen>
|
||
</informalexample>
|
||
</para>
|
||
</sect2>
|
||
|
||
<sect2 xml:id="control-structures.foreach.reference">
|
||
<title>foreach 和引用</title>
|
||
<para>
|
||
可以通过在 <literal>$value</literal> 前加上
|
||
<literal>&</literal>，就可以在循环中直接修改数组元素。此时，值将以<link
|
||
linkend="language.references">引用</link>的方式赋值。
|
||
<informalexample>
|
||
<programlisting role="php">
|
||
<![CDATA[
|
||
<?php
|
||
$arr = [1, 2, 3, 4];
|
||
foreach ($arr as &$value) {
|
||
$value = $value * 2;
|
||
}
|
||
// $arr is now [2, 4, 6, 8]
|
||
unset($value); // 断开与最后一个元素的引用
|
||
?>
|
||
]]>
|
||
</programlisting>
|
||
</informalexample>
|
||
</para>
|
||
<warning>
|
||
<simpara>
|
||
对数组最后一个元素的 <literal>$value</literal> 的引用在 <literal>foreach</literal>
|
||
循环结束后仍然存在。建议使用 <function>unset</function> 将其销毁，否则将出现以下行为：
|
||
</simpara>
|
||
<informalexample>
|
||
<programlisting role="php">
|
||
<![CDATA[
|
||
<?php
|
||
$arr = [1, 2, 3, 4];
|
||
foreach ($arr as &$value) {
|
||
$value = $value * 2;
|
||
}
|
||
// $arr 现在是 [2, 4, 6, 8]
|
||
|
||
// 如果没有使用 unset(value)，$value 仍会引用最后一个元素：$arr[3]
|
||
|
||
foreach ($arr as $key => $value) {
|
||
// $arr[3] 会随着 $arr 中的每个值而更新……
|
||
echo "{$key} => {$value} ";
|
||
print_r($arr);
|
||
}
|
||
// ……最终会复制倒数第二个值到最后一个值上
|
||
?>
|
||
]]>
|
||
</programlisting>
|
||
&example.outputs;
|
||
<screen>
|
||
<![CDATA[
|
||
0 => 2 Array ( [0] => 2, [1] => 4, [2] => 6, [3] => 2 )
|
||
1 => 4 Array ( [0] => 2, [1] => 4, [2] => 6, [3] => 4 )
|
||
2 => 6 Array ( [0] => 2, [1] => 4, [2] => 6, [3] => 6 )
|
||
3 => 6 Array ( [0] => 2, [1] => 4, [2] => 6, [3] => 6 )
|
||
]]>
|
||
</screen>
|
||
</informalexample>
|
||
</warning>
|
||
<example>
|
||
<title>通过引用遍历常量数组的值</title>
|
||
<programlisting role="php">
|
||
<![CDATA[
|
||
<?php
|
||
foreach ([1, 2, 3, 4] as &$value) {
|
||
$value = $value * 2;
|
||
}
|
||
?>
|
||
]]>
|
||
</programlisting>
|
||
</example>
|
||
</sect2>
|
||
|
||
<sect2 role="seealso">
|
||
&reftitle.seealso;
|
||
<simplelist>
|
||
<member><link linkend="language.types.array">array</link></member>
|
||
<member><interfacename>Traversable</interfacename></member>
|
||
<member><link linkend="language.types.iterable">iterable</link></member>
|
||
<member><function>list</function></member>
|
||
</simplelist>
|
||
</sect2>
|
||
|
||
</sect1>
|
||
|
||
<!-- Keep this comment at the end of the file
|
||
Local variables:
|
||
mode: sgml
|
||
sgml-omittag:t
|
||
sgml-shorttag:t
|
||
sgml-minimize-attributes:nil
|
||
sgml-always-quote-attributes:t
|
||
sgml-indent-step:1
|
||
sgml-indent-data:t
|
||
indent-tabs-mode:nil
|
||
sgml-parent-document:nil
|
||
sgml-default-dtd-file:"~/.phpdoc/manual.ced"
|
||
sgml-exposed-tags:nil
|
||
sgml-local-catalogs:nil
|
||
sgml-local-ecat-files:nil
|
||
End:
|
||
vim600: syn=xml fen fdm=syntax fdl=2 si
|
||
vim: et tw=78 syn=sgml
|
||
vi: ts=1 sw=1
|
||
-->
|