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  • 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;
        }
    }
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  • 原文地址:https://www.cnblogs.com/cqingt/p/5438221.html
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