循环是否真的更快？ [英] Are loops really faster in reverse?

问题描述

我已经听过好几遍了。向后计数JavaScript循环真的快吗？如果是这样，为什么？我见过一些测试套件的例子，显示反向循环更快，但我找不到任何解释为什么！

我假设这是因为循环不再需要评估一个属性，每次检查它是否完成，它只是检查最终的数值。

Ie

  for（var i = count  -  1; i> = 0; i--）
 {
 // count只计算一次，然后比较总是在0. 
} 
  

我 - 比 i ++ 更快。实际上，它们都是同样快的。

每个 i 数组的大小。在这个循环中：

  for（var i = array.length; i--;）
  

当您声明时，只评估一次 .length （$ i $ / code>），而对于这个循环

  for（var i = 1; i <= array。 length; i ++）
  

您评估 .length 每次你增加 i ，当你检查 i <= array.length 。

在大多数情况下，您甚至不必担心这种优化。

I've heard this quite a few times. Are JavaScript loops really faster when counting backward? If so, why? I've seen a few test suite examples showing that reversed loops are quicker, but I can't find any explanation as to why!

I'm assuming it's because the loop no longer has to evaluate a property each time it checks to see if it's finished and it just checks against the final numeric value.

I.e.

for (var i = count - 1; i >= 0; i--)
{
  // count is only evaluated once and then the comparison is always on 0.
}

解决方案

It's not that i-- is faster than i++. Actually, they're both equally fast.

What takes time in ascending loops is evaluating, for each i, the size of your array. In this loop:

for(var i = array.length; i--;)

You evaluate .length only once, when you declare i, whereas for this loop

for(var i = 1; i <= array.length; i++)

you evaluate .length each time you increment i, when you check if i <= array.length.

In most cases you shouldn't even worry about this kind of optimization.

这篇关于循环是否真的更快？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IT屋！