foreach
(PHP 4, PHP 5, PHP 7)
foreach语法结构提供了遍历数组的简单方式。foreach仅能够应用于数组和对象，如果尝试应用于其他数据类型的变量，或者未初始化的变量将发出错误信息。有两种语法：
foreach (array_expression as $value)
statement
foreach (array_expression as $key => $value)
statement
第一种格式遍历给定的array_expression数组。每次循环中，当前单元的值被赋给$value并且数组内部的指针向前移一步（因此下一次循环中将会得到下一个单元）。
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$key。
还能够自定义遍历对象。
Note:当foreach开始执行时，数组内部的指针会自动指向第一个单元。这意味着不需要在foreach循环之前调用reset()。
由于foreach依赖内部数组指针，在循环中修改其值将可能导致意外的行为。
可以很容易地通过在$value之前加上&来修改数组的元素。此方法将以引用赋值而不是拷贝一个值。
<?php
$arr = array(1, 2, 3, 4);
foreach ($arr as &$value) {
$value = $value * 2;
}
// $arr is now array(2, 4, 6, 8)
unset($value); // 最后取消掉引用
?>
$value的引用仅在被遍历的数组可以被引用时才可用（例如是个变量）。以下代码则无法运行：
<?php
foreach (array(1, 2, 3, 4) as &$value) {
$value = $value * 2;
}
?>
Warning数组最后一个元素的$value引用在foreach循环之后仍会保留。建议使用unset()来将其销毁。
Note:foreach不支持用“@”来抑制错误信息的能力。
用户可能注意到了以下的代码功能完全相同：
<?php
$arr = array("one", "two", "three");
reset($arr);
while (list(, $value) = each($arr)) {
echo "Value: $value<br>\n";
}
foreach ($arr as $value) {
echo "Value: $value<br />\n";
}
?>
以下代码功能也完全相同：
<?php
$arr = array("one", "two", "three");
reset($arr);
while (list($key, $value) = each($arr)) {
echo "Key: $key; Value: $value<br />\n";
}
foreach ($arr as $key => $value) {
echo "Key: $key; Value: $value<br />\n";
}
?>
示范用法的更多例子：
<?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";
}
?>
用 list()给嵌套的数组解包
(PHP 5 >= 5.5.0, PHP 7)
PHP 5.5 增添了遍历一个数组的数组的功能并且把嵌套的数组解包到循环变量中，只需将list()作为值提供。
例如：
<?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";
}
?>
以上例程会输出：
A: 1; B: 2 A: 3; B: 4
list()中的单元可以少于嵌套数组的，此时多出来的数组单元将被忽略：
<?php
$array = [
[1, 2],
[3, 4],
];
foreach ($array as list($a)) {
// Note that there is no $b here.
echo "$a\n";
}
?>
以上例程会输出：
1 3
如果list()中列出的单元多于嵌套数组则会发出一条消息级别的错误信息：
<?php
$array = [
[1, 2],
[3, 4],
];
foreach ($array as list($a, $b, $c)) {
echo "A: $a; B: $b; C: $c\n";
}
?>
以上例程会输出：
Notice: Undefined offset: 2 in example.php on line 7 A: 1; B: 2; C: Notice: Undefined offset: 2 in example.php on line 7 A: 3; B: 4; C:
You can also use the alternative syntax for the foreach cycle: <?php foreach($array as $element): #do something endforeach; ?> Just thought it worth mentioning.
"Reference of a $value and the last array element remain even after the foreach loop. It is recommended to destroy it by unset()."
I cannot stress this point of the documentation enough! Here is a simple example of exactly why this must be done:
<?php
$arr1 = array("a" => 1, "b" => 2, "c" => 3);
$arr2 = array("x" => 4, "y" => 5, "z" => 6);
foreach ($arr1 as $key => &$val) {}
foreach ($arr2 as $key => $val) {}
var_dump($arr1);
var_dump($arr2);
?>
The output is:
array(3) { ["a"]=> int(1) ["b"]=> int(2) ["c"]=> &int(6) }
array(3) { ["x"]=> int(4) ["y"]=> int(5) ["z"]=> int(6) }
Notice how the last index in $arr1 is now the value from the last index in $arr2!Even though it is not mentioned in this article, you can use "break" control structure to exit from the "foreach" loop.
<?php
$array = [ 'one', 'two', 'three', 'four', 'five' ];
foreach( $array as $value ){
if( $value == 'three' ){
echo "Number three was found!";
break;
}
}
?>
foreach and the while/list/each methods are not completely identical, and there are occasions where one way is beneficial over the other.
<?php
$arr = array(1,2,3,4,5,6,7,8,9);
foreach($arr as $key=>$value)
{
unset($arr[$key + 1]);
echo $value . PHP_EOL;
}
?>
Output:
1 2 3 4 5 6 7 8 9
<?php
$arr = array(1,2,3,4,5,6,7,8,9);
while (list($key, $value) = each($arr))
{
unset($arr[$key + 1]);
echo $value . PHP_EOL;
}
?>
Output:
1 3 5 7 9
[EDIT BY danbrown AT php DOT net: Contains a typofix by (scissor AT phplabs DOT pl) on 30-JAN-2009.]in foreach if you want to iterate through a specific column in a nested arrays for example:
$arr = array(
[1, 2, 3, 4],
[14, 6, 7, 6],
[10, 2 ,3 , 2],
);
when we want to iterate on the third column we can use:
foreach( $arr as list( , , $a)) {
echo "$a\n";
}
this will print:
3
7
3What happened to this note: "Unless the array is referenced, foreach operates on a copy of the specified array and not the array itself. foreach has some side effects on the array pointer. Don't rely on the array pointer during or after the foreach without resetting it." Is this no longer the case? It seems only to remain in the Serbian documentation: http://php.net/manual/sr/control-structures.foreach.php
WARNING: Looping through "values by reference" for "extra performance" is an old myth. It's actually WORSE!
<?php
function one($arr) {
foreach($arr as $val) { // Normal Variable
echo $val;
}
}
function two($arr) {
foreach($arr as &$val) { // Reference To Value
echo $val;
}
}
$a = array( 'a', 'b', 'c' );
one($a);
two($a);
?>
Which do you think is faster?
Lots of people think the answer is two() because it uses "reference to value, which it doesn't have to copy each value when it loops".
Well, that's totally wrong!
Here's what actually happens:
* one():
- This function takes an array as argument ($arr).
- The array function argument itself isn't passed by reference, so the function knows it isn't allowed to modify the original at all.
- Then the foreach loop happens. The array itself wasn't passed by reference to the function, so PHP knows that it isn't allowed to modify the outside array, so it therefore makes a copy of the array's internal iteration offset state (that's just a simple number which says which item you are currently at during things like foreach()), which costs almost no performance or memory at all since it's just a small number.
- Next, it uses that copied iteration offset to loop through all key/value pairs of the array (ie 0th key, 1st key, 2nd key, etc...). And the value at the current offset (a PHP "zval") is assigned to a variable called $val.
- Does $val make a COPY of the value? That's what MANY people think. But the answer is NO. It DOESN'T. It re-uses the existing value in memory. With zero performance cost. It's called "copy-on-write" and means that PHP doesn't make any copies unless you try to MODIFY the value.
- If you try to MODIFY $val, THEN it will allocate a NEW zval in memory and store $val there instead (but it still won't modify the original array, so you can rest assured).
Alright, so what's the second version doing? The beloved "iterate values by reference"?
* two():
- This function takes an array as argument ($arr).
- The array function argument itself isn't passed by reference, so the function knows it isn't allowed to modify the original at all.
- Then the foreach loop happens. The array itself wasn't passed by reference to the function, so PHP knows that it isn't allowed to modify the outside array.
- But it also sees that you want to look at all VALUES by reference (&$val), so PHP says "Uh oh, this is dangerous. If we just give them references to the original array's values, and they assign some new value to their reference, they would destroy the original array which they aren't allowed to touch!".
- So PHP makes a FULL COPY of the ENTIRE array and ALL VALUES before it starts iterating. YIKES!
Therefore: STOP using the old, mythological "&$val" iteration method! It's almost always BAD! With worse performance, and risks of bugs and quirks as is demonstrated in the manual.
You can always manually write array assignments explicitly, without references, like this:
<?php
$a = array(1, 2, 3);
foreach($a as $key => $val) {
$a[$key] = $val * 10;
}
// $a is now [10, 20, 30]
?>
The main lesson is this: DON'T blindly iterate through values by reference! Telling PHP that you want direct references will force PHP to need to copy the WHOLE array to protect its original values! So instead, just loop normally and trust the fact that PHP *is* actually smart enough to never copy your original array's values! PHP uses "copy-on-write", which means that attempting to assign something new to $val is the ONLY thing that causes a copying, and only of that SINGLE element! :-) But you never do that anyway, when iterating without reference. If you ever want to modify something, you use the "$a[$key] = 123;" method of updating the value.
Enjoy and good luck with your code! :-)If you want to use the list for multidimension arrays, you can nest several lists:
<?php
$array = [
[1, 2, array(3, 4)],
[3, 4, array(5, 6)],
];
foreach ($array as list($a, $b, list($c, $d))) {
echo "A: $a; B: $b; C: $c; D: $d;<br>";
};
?>
Will output:
A: 1; B: 2; C: 3; D: 4;
A: 3; B: 4; C: 5; D: 6;
And:
<?php
$array = [
[1, 2, array(3, array(4, 5))],
[3, 4, array(5, array(6, 7))],
];
foreach ($array as list($a, $b, list($c, list($d, $e)))) {
echo "A: $a; B: $b; C: $c; D: $d; E: $e;<br>";
};
Will output:
A: 1; B: 2; C: 3; D: 4; E: 5;
A: 3; B: 4; C: 5; D: 6; E: 7;
?>
For those who'd like to traverse an array including just added elements (within this very foreach), here's a workaround:
<?php
$values = array(1 => 'a', 2 => 'b', 3 => 'c');
while (list($key, $value) = each($values)) {
echo "$key => $value \r\n";
if ($key == 3) {
$values[4] = 'd';
}
if ($key == 4) {
$values[5] = 'e';
}
}
?>
the code above will output:
1 => a
2 => b
3 => c
4 => d
5 => eHaving to unset the reference manually completely destroys the practicality of using a referenced variable. If you make the small mistake of forgetting/deleting the unset line your code might silently mix data, the risk is too high for little gain. If you need to reassign the value, it is way better to just use: <?php $array[$key] = $newvalue; ?>
I want to add some inline comments to dtowell's piece of code about the iteration by reference:
<?php
$a = array('abe','ben','cam');
foreach ($a as $k=>&$n)
$n = strtoupper($n);
# At the end of this cycle the variable $n refers to the same memory as $a[2]
# So when the second "foreach" assigns a value to $n :
foreach ($a as $k=>$n) // notice NO reference here!
echo "$n\n";
# it is also modifying $a[2] .
# So on the three repetitions of the second "foreach" the array will look like:
# 1. ('abe','ben','abe') - assigned the value of the first element to the last element
# 2. ('abe','ben','ben') - assigned the value of the second element to the last element
# 3. ('abe','ben','ben') - assigned the value of the third element to itself
print_r($a);
?>
I want just to mention that John is not entirely true.
Simple field test:
$m = microtime(1); $array = range(1,1000000); foreach ($array as &$i) { $i = 4; } echo microtime(1) - $m;
Result: 0.21731400489807
$m = microtime(1); $array = range(1,1000000); foreach ($array as $k => $i) { $array[$k] = 4; } echo microtime(1) - $m;
Result: 0.51596283912659
PHP Version: PHP 5.6.30 (cli) (built: Jan 18 2017 19:47:36)
Conclusion: Working with reference, although a bit dangerous is >2 times faster. You just need to know well what are you doing.
Best of luck and happy coding allforeach by reference internally deleted and created a new reference in each iteration, so it is not possible to directly use this value as a variable parameter values, look at the following example where the problem is observed and a possible solution:
<?php
class test
{
private $a = false;
private $r = null;
public function show(&$v)
{
if(!$this->a)
{
$this->a = true;
$this->r = &$v;
}
var_dump($this->r);
}
public function reset()
{
$this->a = false;
}
}
$t = new test();
$a = array(array(1,2),array(3,4),array(5,6));
foreach($a as &$p)
$t->show($p);
/* Output obtain:
array (size=2)
0 => int 1
1 => int 2
array (size=2)
0 => int 1
1 => int 2
array (size=2)
0 => int 1
1 => int 2
*/
$t->reset();
foreach($a as $p)
{
$b = &$p;
$t->show($b);
}
/* Output obtain:
array (size=2)
0 => int 1
1 => int 2
array (size=2)
0 => int 3
1 => int 4
array (size=2)
0 => int 5
1 => int 6
*/modifying array while foreach'ing it(yeah, such slime code;-) if elements were added on last iteration or into array with 1 element, then added elements wont be iterated as foreach checks for pointer before iteration cycle so it just quit and added elements wont be treated
String keys of associative arrays, for which is_numeric() is true and which can be type-juggled to an int will be cast to an int! If the key is on the other hand a string that can be type-juggled into a float, it will stay a string. (Observed on PHP 7.0.0RC8)
<?php
$arr = array();
$arr[0] = "zero"; // will stay an int
$arr["1"] = "one"; // will be cast to an int !
$arr["two"] = "2"; // will stay a string
$arr["3.5"] = "threeandahalf"; // will stay a string
foreach($arr as $key => $value) {
var_dump($key);
}
?>
The output will be
int(0)
int(1)
string(3) "two"
string(3) "3.5"foreach retains the state of internal defined variable:
<?php
/**
Result for this array is:
Hello World
Hello World
Hello World
*/
$arr = ["a", "b", "c"];
$title = "";
foreach ($arr as $r) {
if ($r == "a") {
$title = "Hello World";
}
echo $title."<br>";
}
?>
in this case, all we need to do is to add an else statement:
<?php
$arr = ["a", "b", "c"];
$title = "";
/**
This prints:
Hello World
*/
foreach ($arr as $r) {
if ($r == "a") {
$title = "Hello World";
} else {
$title = "";
}
echo $title."<br>";
}
?>
<?php
$d3 = array('a'=>array('b'=>'c'));
foreach($d3['a'] as &$v4){}
foreach($d3 as $v4){}
var_dump($d3);
?>
will get something look like this:
array(1) {
["a"]=>
array(1) {
["b"]=>
&array(1) {
["b"]=>
*RECURSION*
}
}
}
then you try to walk some data with this array.
the script run out of memory and connect reset by peer
the document says:
Warning
Reference of a $value and the last array element remain even after the foreach loop. It is recommended to destroy it by unset().
so what I learn is that NEVER ignore """Warning""" in document....Just a simple strange behavior I have ran into:
If you accidentally put a semicolon after the foreach statement, you get no errors, but the loop will only run on the last element of the array:
<?php
$array = array(1,2,3);
foreach ($array as $key);
{
echo $key;
}
// output: 3
?>
Correctly:
<?php
$array = array(1,2,3);
foreach ($array as $key)
{
echo $key;
}
// output: 123
?>
It took me a while to find that semicolon.Here is an example of how to print a 2 dimension array.
$agenda =array("fulanito"=> array("nombre" =>"Gsancho" ,
"direccion" => "sierra olvira" ,
"fuerza" => "Over 9000") ,
"jonki" => array("nombre" => "jonki",
"direccion" => "valdemin",
"fuerza" => "0"));
foreach($agenda as $clave_agenda =>$persona){
echo "$clave_agenda ";
foreach($persona as $clave_agenda2 => $datos){
echo "$clave_agenda2 : $datos <br>";
}If you wondered how to create a list of all possible combinations of variable amount of arrays (multiple foreach), you might use this:
<?php
$a[0] = array('a1','a2');
$a[1] = array('b1','b2','b3');
$a[2] = array('c1','c2');
function getAllCombinations($a,$i,$s)
{
foreach ($a[$i] as $v)
{
if (!isset($a[$i+1]))
{
echo $s.$v."\n";
} else {
getAllCombinations($a,$i+1,$s.$v);
}
}
return $s;
}
echo getAllCombinations($a,0,'');
?>
the result:
a1b1c1
a1b1c2
a1b2c1
a1b2c2
a1b3c1
a1b3c2
a2b1c1
a2b1c2
a2b2c1
a2b2c2
a2b3c1
a2b3c2This is a decent, simple, and easy way to reference other values of an associative array when using foreach. (effective "next", "prev", etc.)
The only care that needs to be taken is if the array is HUGE in size, so you don't run into memory use problems. (and potential speed issues)
This example uses the 'primary' array, $aPublishSeq, which is ksort-ed to put the array in order according to the associative keys. The array is then copied using a foreach loop to make a duplicate array where the key and value order correspond to the first array, but the keys are sequential numeric starting at zero.
ksort ($aPublishSeq, SORT_STRING); // put them all in the right order keeping array keys
foreach ($aPublishSeq as $aValue)
$aPublishIdx[] = $aValue; // duplicate array using corresponding sequential numeric keys
Now, in the usage foreach loop, an index variable is used to keep in sync with the associative array.
$PubIdx = -1; // start at -1 to start at 0 below
foreach ($aPublishSeq as $sKey => $sValue) {
++$PubIdx; // index into $aPublishIdx array of corresponding element in $aPublishSeq array (for "next" element check, etc.)
echo $aPublishIdx[$PubIdx - 1] // previous array value
echo $aPublishIdx[$PubIdx] // current array value
echo $aPublishIdx[$PubIdx + 1] // next array value
....
It's simple, but it works, and without much muss or fuss.You can even iterate through "dynamic" arrays that do not physically exist, but are objects that implement Iterator interface. They don't need to be stored in memory when foreach starts.
Consider the array that contains some values (I called it $allValues in the example below) and we want to have only some of them (eg. the ones that are dividable by 2). I create an object that would serve as dynamic array, that means it would "dynamically update" its values together with $allValues. The main advantage is that I store only one array, and it's the only array I serialize.
An object of MyIter class will not contain any values itself:
<?php
class MyIter implements Iterator { // you can implement ArrayAccess and Countable interfaces too, this will make class MyIter behave like a "real" array
private $position = 0; // an internal position of the current element
// please note that $position has nothing common with $allValues!
private function getTable(){ // prepare a temporary "static" table of all objects in the class
global $allValues;
$result=array(); // temporary variable
foreach($allValues as $obj){
if($obj % 2 == 0) // check if the value is even
$result[]=$obj; // if yes, I want it
}
return $result;
}
// the all below declared methods are public and belong to the Iterator interface
function rewind() { // a method to start iterating
$this->position = 0; // just move to the beginning
}
function current() { // retrieves the current element
$table=$this->getTable(); // let us prepare a table
return $table[$this->position]; // and return the current element
}
function key() { // retrieves the current element's key
return $this->position; // this is used by foreach(... as $key=>$value), not important here
}
function next() { // move to next element
++$this->position;
}
function valid() { // check if the current element is valid (ie. if it exists)
return array_key_exists($this->position, $this->getTable());
}
} // end of class
// now prepare the array of 12 elements
$allValues=array(0,1,2,3,4,5,6,7,8,9,10,11);
//we would like to have a dynamic array of all even values
$iterator=new MyIter();
foreach($iterator as $value){
echo $value."<br />";
}
?>
This will result in:
0
2
4
6
8
10
(You may also like to see what var_dump($iterator) produces).
Another great advantage is that you can modify the main table "on-the-fly" and it has its impact. Let us modify the last foreach loop:
<?php
// ...all above shall stay as it was
foreach($iterator as $value){
echo $value."<br />";
if($value==6){
$allValues=array(2,3);
echo "I modified source array!<br />";
}
}
?>
This produces now:
0
2
4
6
I modified source array!
However, if you feel it is rather a catastrophic disadvantage (maybe for example, it shows the values 0, 4, and 6 which were removed when we reached 6), and wish to have a "static" array that will iterate even in modified objects, just call getTable() in rewind() method and save it in temporary (private perhaps) field. In my example getTable() is called every iteration, and it calls another foreach through $allValues, which together might be time-consuming. Consider what you need.