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 . ‘
‘;
}
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;
    }
}