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本文實(shí)例講述了PHP5.5新特性之yield理解與用法。分享給大家供大家參考，具體如下：

yield生成器是php5.5之后出現(xiàn)的，yield提供了一種更容易的方法來實(shí)現(xiàn)簡(jiǎn)單的迭代對(duì)象，相比較定義類實(shí)現(xiàn) Iterator 接口的方式，性能開銷和復(fù)雜性大大降低。

yield生成器允許你在 foreach 代碼塊中寫代碼來迭代一組數(shù)據(jù)而不需要在內(nèi)存中創(chuàng)建一個(gè)數(shù)組。

使用示例：

01/**
02 * 計(jì)算平方數(shù)列
03 * @param $start
04 * @param $stop
05 * @return Generator
06 */
07function squares($start, $stop) {
08  if ($start < $stop) {
09    for ($i = $start; $i <= $stop; $i++) {
10      yield $i => $i * $i;
11    }
12  }
13  else {
14    for ($i = $start; $i >= $stop; $i--) {
15      yield $i => $i * $i; //迭代生成數(shù)組： 鍵=》值
16    }
17  }
18}
19foreach (squares(3, 15) as $n => $square) {
20  echo $n . 'squared is' . $square . '<br>';
21}

輸出：

    3 squared is 9
    4 squared is 16
    5 squared is 25
    ...

示例2：

01//對(duì)某一數(shù)組進(jìn)行加權(quán)處理
02$numbers = array('nike' => 200, 'jordan' => 500, 'adiads' => 800);
03//通常方法，如果是百萬級(jí)別的訪問量，這種方法會(huì)占用極大內(nèi)存
04function rand_weight($numbers)
05{
06  $total = 0;
07  foreach ($numbers as $number => $weight) {
08    $total += $weight;
09    $distribution[$number] = $total;
10  }
11  $rand = mt_rand(0, $total-1);
12  foreach ($distribution as $num => $weight) {
13    if ($rand < $weight) return $num;
14  }
15}
16//改用yield生成器
17function mt_rand_weight($numbers) {
18  $total = 0;
19  foreach ($numbers as $number => $weight) {
20    $total += $weight;
21    yield $number => $total;
22  }
23}
24function mt_rand_generator($numbers)
25{
26  $total = array_sum($numbers);
27  $rand = mt_rand(0, $total -1);
28  foreach (mt_rand_weight($numbers) as $num => $weight) {
29    if ($rand < $weight) return $num;
30  }
31}