foreach与&引用的学习

在foreach中循环时

$arr = array(1, 2, 3, 4);foreach ($arr as &$value) {    $value = $value * 2;}unset($value);  // 最后取消掉引用 var_dump($arr);  // $arr is now array(2, 4, 6, 8)

• 此时输出$arr为array(2, 4, 6, 8)；

$arr = array(1, 2, 3, 4);foreach ($arr as $value) {    $value = $value * 2;}unset($value);  // 最后取消掉引用 var_dump($arr);  // $arr is now array(1, 2, 3, 4)

• 此时输出$arr为array(1, 2, 3, 4)；

进而衍生出不用额外的空间，对索引数组进行向前或向后移动一位的操作

<?phpclass move{    public function before1(){        $arr = array('a','b','c','d','e','f');         foreach ($arr as $key => $value) {            if($key == 0){                $arr[(count($arr)-1)] = $value;            }            $value = &$arr[$key];        }        print_r($arr);    }    public function before2()    {        $arr = array('a','b','c','d','e','f');        foreach ($arr as $key => $value) {            if($key == 0){                $arr[count($arr) -1] = $arr[0];            }            if($key-1>=0){                $arr[$key-1] = $value;            }        }        print_r($arr);    }    public function ahter1(){        $arr = array('a','b','c','d','e','f');        foreach ($arr as $key => $value) {            if($key+1>=count($arr)){                $arr[0] = $value;            }else{                $arr[$key+1] = $value;            }        }        print_r($arr);    }    public function ahter2()    {        $arr = array('a','b','c','d','e','f');        foreach ($arr as $key => $value) {            if($key == 0){                $arr[0] = $arr[count($arr) -1];            }            if($key+1<count($arr)){                $arr[$key+1] = $value;            }        }        print_r($arr);    }    public function ahter3()    {        $arr = array('a','b','c','d','e','f');         foreach ($arr as $key => $value) {            if($key == 0){                $arr[1] = $arr['0'];            }            if($key + 2 < count($arr)){                $value = &$arr[$key+2];            }else{                $value = &$arr[abs(count($arr)-$key-2)];            }        }        print_r($arr);    }}$res = new move;$res->before1();echo '<hr>';$res->before2();echo '<hr>';$res->ahter1();echo '<hr>';$res->ahter2();echo '<hr>';$res->ahter3();

Array ( [0] => b [1] => c [2] => d [3] => e [4] => f [5] => a )
Array ( [0] => b [1] => c [2] => d [3] => e [4] => f [5] => a )
Array ( [0] => f [1] => a [2] => b [3] => c [4] => d [5] => e )
Array ( [0] => f [1] => a [2] => b [3] => c [4] => d [5] => e )
Array ( [0] => f [1] => a [2] => b [3] => c [4] => d [5] => e )