Javascript for..in vs for loop performance [英] Javascript for..in vs for loop performance

问题描述

我使用kmean算法聚集了大约40000点。在程序的第一个版本中，我写了这样的欧几里德距离函数

I was clustering around 40000 points using kmean algorithm. In the first version of the program I wrote the euclidean distance function like this

var euclideanDistance = function( p1, p2 ) { // p1.length === p2.length == 3
    var sum = 0;
    for( var i in p1 ){
        sum += Math.pow( p1[i] - p2[i], 2 );
    }
    return Math.sqrt( sum );
};

总体程序非常缓慢，平均需要7秒才能执行。经过一些分析后，我重写了上述函数，如下所示

The overall program was quite slow taking on average 7sec to execute. After some profiling I rewrote the above function like this

var euclideanDistance = function( p1, p2 ) { // p1.length === p2.length == 3
    var sum = 0;
    for( var i = 0; i < p1.length; i++ ) {
        sum += Math.pow( p1[i] - p2[i], 2 );
    }
    return Math.sqrt( sum );
};

现在程序平均需要400毫秒左右。由于我编写for循环的方式，这是一个巨大的时间差异。我通常不会为数组使用 for..in 循环，但出于某种原因我在编写此函数时使用它。

Now the programs on average take around 400ms. That's a huge time difference just because of the way I wrote the for loop. I normally don't use for..in loop for arrays but for some reason I used it while writing this function.

有人可以解释为什么这两种风格之间的性能存在巨大差异吗？

Can someone explain why there is this huge difference in performance between these 2 styles?

推荐答案

看看发生了什么不同的事情每次迭代：

Look at what's happening differently in each iteration:

for( var i = 0; i < p1.length; i++ ) 

1. 检查 i< p1.length


2. 增加我一个

1. Check if i < p1.length
2. Increment i by one

非常简单快速。

现在看看每次迭代中发生的事情：

Now look at what's happening in each iteration for this:

for （var i in p1）

重复

Repeat

1. 令P为obj的下一个属性的名称，其[[Enumerable]]属性为true。如果没有这样的属性，请返回（正常，V，
   为空）。

它必须在可枚举的对象中找到下一个属性。使用你的数组，你知道这可以通过一个简单的整数增量来实现，其中找到下一个可枚举的算法很可能不那么简单，因为它必须处理任意对象及其原型链键。

It has to find next property in the object that is enumerable. With your array you know that this can be achieved by a simple integer increment, where as the algorithm to find next enumerable is most likely not that simple because it has to work on arbitrary object and its prototype chain keys.

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