反向循环真的更快吗? [英] Are loops really faster in reverse?
问题描述
我已经听过很多次了.向后计数时 JavaScript 循环真的更快吗?如果是这样,为什么?我已经看到一些测试套件示例表明反向循环更快,但我找不到任何解释为什么!
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!
我假设这是因为循环不再需要在每次检查属性是否完成时都评估它,它只检查最终的数值.
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.
即
for (var i = count - 1; i >= 0; i--)
{
// count is only evaluated once and then the comparison is always on 0.
}
推荐答案
并不是 i--
比 i++
快.实际上,它们的速度都一样快.
It's not that i--
is faster than i++
. Actually, they're both equally fast.
在升序循环中需要时间的是为每个 i
评估数组的大小.在这个循环中:
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--;)
当你声明 i
时,你只计算一次 .length
,而对于这个循环
You evaluate .length
only once, when you declare i
, whereas for this loop
for(var i = 1; i <= array.length; i++)
每次增加 i
时都会评估 .length
,当您检查 i <= array.length
时.
you evaluate .length
each time you increment i
, when you check if i <= array.length
.
在大多数情况下,您甚至不必担心这种优化.
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