我有一些关于在foreach()循环期间修改数组的查询.在下面的代码中,我循环遍历包含闭包/回调的三个数组,并调用每个数组.我在迭代期间将一个闭包附加到每个数组的末尾,但是有时候foreach()似乎没有认识到数组已经改变了大小,所以附加的闭包不被调用.
class Foo
{
private $a1 = array();
private $a2 = array();
public function f()
{
echo '<pre style="font-size: 20px;">';
echo 'PHP: ' . PHPversion() . '<br><br>';
$this->a1[] = function() { echo 'a1 '; };
$this->a1[] = array($this,'g');
foreach ($this->a1 as &$v)
{
// The callback added in g() never gets called.
call_user_func($v);
//echo 'count(v) = ' . count($v) . ' ';
}
echo '<br>';
// The same thing works fine with a for() loop.
$this->a2[] = function() { echo 'a2 '; };
$this->a2[] = array($this,'h');
for ($i = 0; $i < count($this->a2); ++$i)
call_user_func($this->a2[$i]);
echo '<br>';
// It also works fine using a local array as long as it
// starts off with more than one element.
$a3[] = function() { echo 'a3 '; };
//$a3[] = function() { echo 'a3 '; };
$i = 0;
foreach ($a3 as &$x)
{
call_user_func($x);
if ($i++ > 1) // prevent infinite loop
break;
// Why does this get called only if $a3 originally starts
// with more than one element?
$a3[] = function() { echo 'callback '; };
}
echo '</pre>';
}
private function g()
{
echo 'g() ';
$this->a1[] = function() { echo 'callback '; };
}
private function h()
{
echo 'h() ';
$this->a2[] = function() { echo 'callback '; };
}
}
$foo = new Foo;
$foo->f();
输出：
PHP: 5.3.14-1~dotdeb.0 a1 g() a2 h() callback a3
预期产量：
a1 g() callback a2 h() callback a3 callback
如果在循环前取消注释第二个元素,则输出$a3：
a3 a3 callback
>为什么第一个循环foreach($this-> a1& $v)实现$v有另一个元素可以迭代？
>为什么在第三个循环foreach($a3 as& $x)中修改$a3工作,但只有当数组从多个元素开始时？
1.Why doesn’t the first loop foreach ($this->a1 as &$v) realise $v has another element to iterate over?
行为看起来是由于内部指针在每个foreach迭代上在数组上前进.在数组的最后一次迭代时,将数组元素添加到数组的末尾,即当内部指针已经为空时,表示该元素不会被迭代.通过对代码的一些修改,可以看出.
class Foo
{
private $a1 = array();
private $a2 = array();
public function f()
{
echo '<pre style="font-size: 20px;">';
echo 'PHP: ' . PHPversion() . '<br><br>';
$this->a1[] = function() { echo 'a1 <br/>'; };
$this->a1[] = array($this,'g');
foreach ($this->a1 as $key => &$v)
{
//lets get the key that the internal pointer is pointing to
// before the call.
$intPtr = (key($this->a1) === null) ? 'null' : key($this->a1);
echo 'array ptr before key ',$key,' func call is ',$intPtr,'<br/>' ;
call_user_func($v);
//echo 'count(v) = ' . count($v) . ' ';
}
echo '<br><br>';
// The same thing works fine with a for() loop.
$this->a2[] = function() { echo 'a2 '; };
$this->a2[] = array($this,'h');
for ($i = 0; $i < count($this->a2); ++$i)
call_user_func($this->a2[$i]);
echo '<br><br>';
// It also works fine using a local array as long as it
// starts off with more than one element.
$a3[] = function() { echo 'a3 '; };
//$a3[] = function() { echo 'a3 '; };
$i = 0;
foreach ($a3 as &$x)
{
call_user_func($x);
if ($i++ > 1) // prevent infinite loop
break;
// Why does this get called only if $a3 originally starts
// with more than one element?
$a3[] = function() { echo 'callback '; };
}
echo '</pre>';
}
private function g()
{
echo 'g() <br>';
$this->a1[] = function() { echo 'callback '; };
}
private function h()
{
echo 'h() <br>';
$this->a2[] = function() { echo 'callback '; };
}
}
$foo = new Foo;
$foo->f();
输出：
array ptr before key 0 func call is 1 a1 array ptr before key 1 func call is null <-will not iterate over any added elements! g() a2 h() callback a3
2.Why does modifying $a3 work during the third loop foreach ($a3 as &$x),but only when the array starts off with more than one element?
当然,如果在内部指针返回null之前向数组添加元素,那么元素将被迭代.在你的情况下,如果数组有一个元素,那么在第一次迭代中,内部指针已经返回null.然而,如果最初有多个元素,则可以在第一次迭代时添加附加元素,因为内部指针此时将指向第二个初始元素.
