为什么使用 while 循环填充新数组的速度如此之快?

问题描述

限时删除！！

我正在考虑使用本机方法创建包含默认值的数组的方法，结果是

I was considering ways to create arrays containing a default value using native methods and ended up with

function pushMap(length, fill){
    var a = [], b = [];
    a.length = length;
    b.push.apply(b,a);
    return b.map(function(){return fill;});
}

期望它比 while 循环慢 2 或 3 倍，因为本机方法必须循环两次而 while 只循环一次，所以我比较了它jsperf反对

Expecting it to be 2 or 3 times slower than a while loop, as the native methods have to loop twice whereas while loops only once, so I compared it on jsperf against

function whileLengthNew(len, val) {
    var rv = new Array(len);
    while (--len >= 0) {
        rv[len] = val;
    }
    return rv;
}

它实际上慢了 18 到 27 倍(在 Ubuntu 上用 Google Chrome 测试，欢迎浏览器/操作系统).

and it is actually 18 to 27 times slower (tested with Google Chrome on Ubuntu, browsers/OSs welcome).

发生了什么导致如此大的差异?

What is happening that causes such a big difference?

推荐答案

我认为这是由于两个主要因素:

I would expect that this is due to two major factors:

1. 内存分配 -- whileLengthNew 先创建一个正确大小的数组，然后对其进行操作，pushMap 一次创建一个元素的最终数组使用 map.这可能会导致多次分配，尤其是在源数组很大的情况下.(创建初始 a 和 b 数组的方式基本上无关紧要，因为 map 正在构建一个新数组，无论如何都要返回——它实际上并没有改变 b)

1. Memory allocation -- whileLengthNew creates an array of the correct size first, and then operates on it, pushMap creates the final array one element at a time with map. This may cause multiple allocations, especially if the source array is large. (The way that you create the initial a and b arrays is basically irrelevant, since map is building up a new array to return anyway -- it doesn't actually change anything in b)

函数调用开销 -- 在调用 map 时，您正在为数组的每个元素调用一个函数.这涉及相当多的开销；设置激活记录和作用域链，堆栈操作，并将返回值传回.-- 所有这些都是为了访问函数内的常量变量.最重要的是，您已经设置了一个闭包，因此即使访问fill 变量也比whileLengthNew 版本中的要慢.

Function call overhead -- in your call to map, you are calling a function for every element of the array. This involves quite a lot of overhead; setting up activation records and scope chains, stack manipulation, and passing the return value back. -- all of this to access a variable which is constant within the function. On top of that, you have set up a closure, so even accessing the fill variable is slower than it is in the whileLengthNew version.

这篇关于为什么使用 while 循环填充新数组的速度如此之快?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！

1403页，肝出来的..