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Home Backend Development PHP Tutorial PHP中for和foreach哪個效率高一些?

PHP中for和foreach哪個效率高一些?

Jun 13, 2016 pm 12:00 PM
for foreach php

PHP中for和foreach哪個效率高一些?

for和foreach那個效率高?原因是什么?

PHP中for和foreach效率高的是foreach

for循環(huán)遍歷方法:

public function getForTime(){
    $big_Array = range(0,1000000,1);

     /* for循環(huán)遍歷數(shù)組示例 */
    $start_For_Time = $this->microtime_float();
    //$array_Count = count($big_Array);
    for ($i=0;$i<count($big_Array);$i++) {
            $i;
    }
    $end_For_Time = $this->microtime_float();
    $for_Time = $end_For_Time - $start_For_Time;
    echo &#39;for循環(huán)遍歷耗時:&#39;.$for_Time.&#39;<br>&#39;;
}

foreach循環(huán)遍歷方法:

public function getForeachTime(){
$big_Array = range(0,1000000,1);

 /* foreach循環(huán)遍歷數(shù)組示例 */
    $start_Foreach_Time = $this->microtime_float();
    foreach ($big_Array as $key=>$val) {
            $key;
    }
    $end_Foreach_Time = $this->microtime_float();
    $foreach_Time = $end_Foreach_Time - $start_Foreach_Time;
    echo &#39;foreach循環(huán)遍歷耗時:&#39;.$foreach_Time;
}

時間計(jì)算方法:

/**
 *  時間統(tǒng)計(jì)函數(shù)
 */
private function microtime_float($time = null)
{
   list($usec, $sec) = explode(&#39; &#39;, $time ? $time : microtime());
   return ((float)$usec + (float)$sec);
}

看一下兩種方式耗時

/*
 * 輸出結(jié)果:第一種情況:先count在for循環(huán)遍歷耗時:0.028002023696899 秒
 *                         foreach循環(huán)遍歷耗時:0.003000020980835 秒
 *        第二種情況:在for循環(huán)條件中做count遍歷耗時:0.095005035400391 秒
 *                            foreach循環(huán)遍歷耗時:0.0040009021759033 秒
 * */

從上面的測試中我們可以明顯的得出兩條結(jié)論:

1、for循環(huán)遍歷的效率是低于foreach循環(huán)遍歷

2、for循環(huán)在外部做count和在條件中做count相比較,第一種效率更高

效率高的原因是什么呢?在尋找這個答案之前我們先探討原理分別是什么。

for 循環(huán):

每次從$i開始,每次循環(huán)都需要判斷$i是否小于count,這占用了很大一部分時間
小于繼續(xù),否則終止循環(huán)

foreach:

foreach 依賴 IEnumerable.

第一次 var a in GetList() 時 調(diào)用 GetEnumerator 返回第一個對象 并 賦給a,

以后每次再執(zhí)行 var a in GetList() 的時候 調(diào)用 MoveNext.直到循環(huán)結(jié)束.

期間GetList()方法只執(zhí)行一次.

從上面是分析我們明顯可以得出結(jié)論:php 的foreach循環(huán)效率是大大高于for循環(huán)。也許有很大的一個原因是for 要進(jìn)行很多次條件判斷。所以以后能用foreach的地方就用foreach,可以提高1倍的效率。

BUT:事實(shí)真的是這樣嗎?有人會說這個例子已經(jīng)很明顯了啊,結(jié)論一目了然,難道還有其他的可能嗎?

我覺得事實(shí)沒這么簡單,如果真的是這樣,for循環(huán)存在的意義是什么呢?

既然foreach效率高于for這么多倍,就直接都用foreach不就行了嗎?個人覺得我測試的這個例子有一定的局限性,并不能作為評估兩個循環(huán)方式效率高低的絕對依據(jù)。

不過,對于我們phper來說,正常工作當(dāng)中還是使用foreach循環(huán)遍歷比較好,至于編譯層是如何工作的沒必要涉及太深,如果有興趣可以深度研究一下。

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