php5.5新特性之yield理解

今天，在阅读别人代码时，其中出现了一个陌生的关键字yield，想一探究竟，于是找到：http://php.net/manual/zh/language.generators.overview.php

yield生成器是php5.5之后出现的，yield提供了一种更容易的方法来实现简单的迭代对象，相比较定义类实现 Iterator 接口的方式，性能开销和复杂性大大降低。

yield生成器允许你 在 foreach 代码块中写代码来迭代一组数据而不需要在内存中创建一个数组。

使用示例：

/** * 计算平方数列 * @param $start * @param $stop * @return Generator */function squares($start, $stop) {    if ($start < $stop) {        for ($i = $start; $i <= $stop; $i++) {            yield $i => $i * $i;        }    }    else {        for ($i = $start; $i >= $stop; $i--) {            yield $i => $i * $i; //迭代生成数组： 键=》值        }    }}foreach (squares(3, 15) as $n => $square) {    echo $n . ‘squared is‘ . $square . ‘<br>‘;}输出：    3 squared is 9    4 squared is 16    5 squared is 25    ...

示例2：

//对某一数组进行加权处理$numbers = array(‘nike‘ => 200, ‘jordan‘ => 500, ‘adiads‘ => 800);//通常方法，如果是百万级别的访问量，这种方法会占用极大内存function rand_weight($numbers){    $total = 0;    foreach ($numbers as $number => $weight) {        $total += $weight;        $distribution[$number] = $total;    }    $rand = mt_rand(0, $total-1);    foreach ($distribution as $num => $weight) {        if ($rand < $weight) return $num;    }}//改用yield生成器function mt_rand_weight($numbers) {    $total = 0;    foreach ($numbers as $number => $weight) {        $total += $weight;        yield $number => $total;    }}function mt_rand_generator($numbers){    $total = array_sum($numbers);    $rand = mt_rand(0, $total -1);    foreach (mt_rand_weight($numbers) as $num => $weight) {        if ($rand < $weight) return $num;    }}