为什么foreach这么慢？ [英] Why is foreach so slow?

问题描述

PHPBench.com 在每个页面上运行快速的基准测试脚本。在foreach测试中，当我加载时，foreach比第三个例子需要4到10倍的时间。

为什么是一个本地语言结构显然比执行自己的逻辑慢？

解决方案

也许这与事实，foreach工作的副本数组？

或者，也许这是因为在循环foreach时，每次迭代都会改变内部数组指针，指向下一个元素？

引用 foreach 的手册页：

：除非数组被引用，否则
foreach对
指定数组的副本进行操作，而对阵列
本身不作操作。 foreach在数组指针上有一些副作用
。

据我所知，你链接的第三个测试不会做这两件事中的任何一件 - 这意味着两个测试不会做同样的事情 - 这意味着你没有比较两种编写相同代码的方式。 （我也会说这种微观优化在实际应用中根本就不重要 - 但是我想你已经知道了，只是出于好奇而问了）

在这个测试中还有一件事情是不正确的：它只做一次测试;对于一个更好的测试来说，对所有这些不止一次的测试可能是有用的 - 在100微秒的时间内进行测试并不需要做太大的改变。

（考虑到第一次测试的刷新率在300％到500％之间不等...）

<
对于那些不想点击的人来说，这是第一个测试（我得到了3xx％，443％和529％）：

  foreach（$ aHash as $ key => $ val）{
 $ aHash [$ key]。=a; 
 
  

第三个（100％）：

  $ key = array_keys（$ aHash）; 
 $ size = sizeOf（$ key）; （$ i = 0; $ i <$ size; $ i ++）{
 $ aHash [$ key [$ i]]。=a; 
。 
} 
  

PHPBench.com runs quick benchmark scripts on each pageload. On the foreach test, when I load it, foreach takes anywhere from 4 to 10 times as long to run than the third example.

Why is it that a native language construct is apparently slower than performing the logic oneself?

解决方案

Maybe it has to do with the fact that foreach works on a copy of the array ?

Or maybe it has to do with the fact that, when looping with foreach, on each iteration, the internal array pointer is changed, to point to the next element ?

Quoting the relevant portion of foreach's manual page :

Note: Unless the array is referenced, foreach operates on a copy of the specified array and not the array itself. foreach has some side effects on the array pointer.

As far as I can tell, the third test you linked to doesn't do any of those two things -- which means both tests don't do the same thing -- which means you are not comparing two way of writing the same code.

(I would also say that this kind of micro-optimization will not matter at all in a real application -- but I guess you already know that, and just asked out of curiosity)

There is also one thing that doesn't feel right in this test : it only does the test one time ;; for a "better" test, it might be useful to test all of those more than once -- with timings in the order of 100 micro-seconds, not much is required to make a huge difference.
(Considering the first test varies between 300% and 500% on a few refreshes...)

For those who don't want to click, here's the first test (I've gotten 3xx%, 443%, and 529%) :

foreach($aHash as $key=>$val) {
    $aHash[$key] .= "a";
}

And the third one (100%) :

$key = array_keys($aHash);
$size = sizeOf($key);
for ($i=0; $i<$size; $i++) {
    $aHash[$key[$i]] .= "a";
}

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