php之foreach遍曆數組_PHP教程

php之foreach遍曆數組

foreach

(PHP 4, PHP 5)

The foreach construct provides an easy way to iterate over arrays. foreach 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:

foreach (array_expression as $value)    statementforeach (array_expression as $key => $value)    statement

The first form loops over the array given by array_expression. On each iteration, the value of the current element is assigned to $value and the internal array pointer is advanced by one (so on the next iteration, you'll be looking at the next element).

The second form will additionally assign the current element's key to the $key variable on each iteration.

It is possible to customize object iteration.

Note:

When foreach first starts executing, the internal array pointer is automatically reset to the first element of the array. This means that you do not need to call reset() before a foreach loop.

As foreach relies on the internal array pointer, changing it within the loop may lead to unexpected behavior.

In order to be able to directly modify array elements within the loop precede $value with &. In that case the value will be assigned by reference.

$arr = array(1, 2, 3, 4);
foreach ($arr as &$value) {
$value = $value * 2;
}
// $arr is now array(2, 4, 6, 8)
unset($value); // break the reference with the last element
?>

Referencing $value is only possible if the iterated array can be referenced (i.e. if it is a variable). The following code won't work:

foreach (array(1, 2, 3, 4) as &$value) {
$value = $value * 2;
}
?>Warning

Reference of a $value and the last array element remain even after the foreach loop. It is recommended to destroy it by unset().

Note:

foreach does not support the ability to suppress error messages using '@'.

You may have noticed that the following are functionally identical:

$arr = array("one", "two", "three");
reset($arr);
while (list(, $value) = each($arr)) {
echo "Value: $value
\n";
}

foreach ($arr as $value) {
echo "Value: $value
\n";
}
?>

The following are also functionally identical:

$arr = array("one", "two", "three");
reset($arr);
while (list($key, $value) = each($arr)) {
echo "Key: $key; Value: $value
\n";
}

foreach ($arr as $key => $value) {
echo "Key: $key; Value: $value
\n";
}
?>

Some more examples to demonstrate usage:

/* 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";
}
?>

Unpacking nested arrays with list()

(PHP 5 >= 5.5.0)

PHP 5.5 added the ability to iterate over an array of arrays and unpack the nested array into loop variables by providing a list() as the value.

For example:

$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";
}
?>

The above example will output:

A: 1; B: 2A: 3; B: 4

You can provide fewer elements in the list() than there are in the nested array, in which case the leftover array values will be ignored:

$array = [
[1, 2],
[3, 4],
];

foreach ($array as list($a)) {
// Note that there is no $b here.
echo "$a\n";
}
?>

The above example will output:

13

A notice will be generated if there aren't enough array elements to fill the list():

$array = [
[1, 2],
[3, 4],
];

foreach ($array as list($a, $b, $c)) {
echo "A: $a; B: $b; C: $c\n";
}
?>

The above example will output:

Notice: Undefined offset: 2 in example.php on line 7A: 1; B: 2; C: Notice: Undefined offset: 2 in example.php on line 7A: 3; B: 4; C: 

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