php-doc-en/language/control-structures/foreach.xml
2021-03-31 16:09:52 +02:00

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<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
<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>
The <literal>foreach</literal> construct provides an easy way to
iterate over arrays. <literal>foreach</literal> works only on arrays
and objects, and will issue an error when you try to use it on a variable
with a different data type or an uninitialized variable. There are two
syntaxes:
<informalexample>
<programlisting>
<![CDATA[
foreach (iterable_expression as $value)
statement
foreach (iterable_expression as $key => $value)
statement
]]>
</programlisting>
</informalexample>
</para>
<simpara>
The first form traverses the iterable given by
<literal>iterable_expression</literal>. On each iteration, the value of
the current element is assigned to <literal>$value</literal>.
</simpara>
<simpara>
The second form will additionally assign the current element's key to
the <literal>$key</literal> variable on each iteration.
</simpara>
<simpara>
Note that <literal>foreach</literal> does not modify the internal array
pointer, which is used by functions such as <function>current</function>
and <function>key</function>.
</simpara>
<simpara>
It is possible to
<link linkend="language.oop5.iterations">customize object iteration</link>.
</simpara>
<para>
In order to be able to directly modify array elements within the loop precede
<literal>$value</literal> with &amp;. In that case the value will be assigned by
<link linkend="language.references">reference</link>.
<informalexample>
<programlisting role="php">
<![CDATA[
<?php
$arr = array(1, 2, 3, 4);
foreach ($arr as &$value) {
$value = $value * 2;
}
// $arr is now array(2, 4, 6, 8)
unset($value); // break the reference with the last element
?>
]]>
</programlisting>
</informalexample>
</para>
<warning>
<para>
Reference of a <literal>$value</literal> and the last array element
remain even after the <literal>foreach</literal> loop. It is recommended
to destroy it by <function>unset</function>.
Otherwise you will experience the following behavior:
</para>
<informalexample>
<programlisting role="php">
<![CDATA[
<?php
$arr = array(1, 2, 3, 4);
foreach ($arr as &$value) {
$value = $value * 2;
}
// $arr is now array(2, 4, 6, 8)
// without an unset($value), $value is still a reference to the last item: $arr[3]
foreach ($arr as $key => $value) {
// $arr[3] will be updated with each value from $arr...
echo "{$key} => {$value} ";
print_r($arr);
}
// ...until ultimately the second-to-last value is copied onto the last value
// output:
// 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 )
?>
]]>
</programlisting>
</informalexample>
</warning>
<para>
It is possible to iterate a constant array's value by reference:
<informalexample>
<programlisting role="php">
<![CDATA[
<?php
foreach (array(1, 2, 3, 4) as &$value) {
$value = $value * 2;
}
?>
]]>
</programlisting>
</informalexample>
</para>
<para>
<note>
<para>
<literal>foreach</literal> does not support the ability to
suppress error messages using
<literal linkend="language.operators.errorcontrol">@</literal>.
</para>
</note>
</para>
<para>
Some more examples to demonstrate usage:
<informalexample>
<programlisting role="php">
<![CDATA[
<?php
/* foreach example 1: value only */
$a = array(1, 2, 3, 17);
foreach ($a as $v) {
echo "Current value of \$a: $v.\n";
}
/* foreach example 2: value (with its manual access notation printed for illustration) */
$a = array(1, 2, 3, 17);
$i = 0; /* for illustrative purposes only */
foreach ($a as $v) {
echo "\$a[$i] => $v.\n";
$i++;
}
/* foreach example 3: key and value */
$a = array(
"one" => 1,
"two" => 2,
"three" => 3,
"seventeen" => 17
);
foreach ($a as $k => $v) {
echo "\$a[$k] => $v.\n";
}
/* foreach example 4: multi-dimensional arrays */
$a = array();
$a[0][0] = "a";
$a[0][1] = "b";
$a[1][0] = "y";
$a[1][1] = "z";
foreach ($a as $v1) {
foreach ($v1 as $v2) {
echo "$v2\n";
}
}
/* foreach example 5: dynamic arrays */
foreach (array(1, 2, 3, 4, 5) as $v) {
echo "$v\n";
}
?>
]]>
</programlisting>
</informalexample>
</para>
<sect2 xml:id="control-structures.foreach.list">
<title>Unpacking nested arrays with list()</title>
<?phpdoc print-version-for="foreach.list"?>
<para>
It is possible to iterate over an array of arrays and unpack the
nested array into loop variables by providing a <function>list</function>
as the value.
</para>
<para>
For example:
<informalexample>
<programlisting role="php">
<![CDATA[
<?php
$array = [
[1, 2],
[3, 4],
];
foreach ($array as list($a, $b)) {
// $a contains the first element of the nested array,
// and $b contains the second element.
echo "A: $a; B: $b\n";
}
?>
]]>
</programlisting>
&example.outputs;
<screen>
<![CDATA[
A: 1; B: 2
A: 3; B: 4
]]>
</screen>
</informalexample>
</para>
<para>
You can provide fewer elements in the <function>list</function> than there
are in the nested array, in which case the leftover array values will be
ignored:
<informalexample>
<programlisting role="php">
<![CDATA[
<?php
$array = [
[1, 2],
[3, 4],
];
foreach ($array as list($a)) {
// Note that there is no $b here.
echo "$a\n";
}
?>
]]>
</programlisting>
&example.outputs;
<screen>
<![CDATA[
1
3
]]>
</screen>
</informalexample>
</para>
<para>
A notice will be generated if there aren't enough array elements to fill
the <function>list</function>:
<informalexample>
<programlisting role="php">
<![CDATA[
<?php
$array = [
[1, 2],
[3, 4],
];
foreach ($array as list($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>
</sect1>
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