在循环内还是循环外声明局部更好? [英] Is it better to declare a local inside or outside a loop?

问题描述

我曾经这样做:

do
    local a
    for i=1,1000000 do
        a = <some expression>
        <...> --do something with a
    end
end

代替

for i=1,1000000 do
    local a = <some expression>
    <...> --do something with a
end

我的理由是，创建1000000次局部变量的效率要比仅创建一次并在每次迭代中重用的效率低.

My reasoning is that creating a local variable 1000000 times is less efficient than creating it just once and reuse it on each iteration.

我的问题是:这是真的还是我遗漏了另一个技术细节?我之所以问是因为我没有看到任何人这样做，但是不确定原因是因为优势太小还是因为事实更糟.更好的意思是使用更少的内存并运行得更快.

My question is: is this true or there is another technical detail I am missing? I am asking because I don't see anyone doing this but not sure if the reason is because the advantage is too small or because it is in fact worse. By better I mean using less memory and running faster.

推荐答案

像任何性能问题一样，请先进行测量.在Unix系统中，您可以使用时间:

Like any performance question, measure first. In a unix system you can use time:

time lua -e 'local a; for i=1,100000000 do a = i * 3 end'
time lua -e 'for i=1,100000000 do local a = i * 3 end'

输出:

 real   0m2.320s
 user   0m2.315s
 sys    0m0.004s

 real   0m2.247s
 user   0m2.246s
 sys    0m0.000s

在Lua中，更本地的版本似乎要快一小部分，因为它不会将 a 初始化为nil.但是，这不是使用它的原因，使用最本地的范围是因为它更具可读性(这在所有语言中都是很好的样式:请参阅此问题以询问C ， C#)

The more local version appears to be a small percentage faster in Lua, since it does not initialize a to nil. However, that is no reason to use it, use the most local scope because it it is more readable (this is good style in all languages: see this question asked for C, Java, and C#)

如果要重用表而不是在循环中创建表，则可能会有更大的性能差异.无论如何，请尽可能测量并支持可读性.

If you are reusing a table instead of creating it in the loop then there is likely a more significant performance difference. In any case, measure and favour readability whenever you can.

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