在循环之前或循环中声明变量之间的区别？ [英] Difference between declaring variables before or in loop?

问题描述

我一直想知道，一般来说，在循环之前声明一个抛弃变量，而不是在循环内重复，是否会产生任何（性能）​​差异？
Java中的（毫无意义）示例：

I have always wondered if, in general, declaring a throw-away variable before a loop, as opposed to repeatedly inside the loop, makes any (performance) difference? A (quite pointless) example in Java:

a）循环前声明：

double intermediateResult;
for(int i=0; i < 1000; i++){
    intermediateResult = i;
    System.out.println(intermediateResult);
}

b）声明（重复）内循环：

b) declaration (repeatedly) inside loop:

for(int i=0; i < 1000; i++){
    double intermediateResult = i;
    System.out.println(intermediateResult);
}

哪一个更好， a 或 b'/强>？

Which one is better, a or b?

我怀疑重复变量声明（例如 b ）会在理论上产生更多的开销 ，但编译器很聪明足以让它无所谓。示例 b 具有更紧凑的优点，并将变量的范围限制为使用它的位置。不过，我倾向于根据示例进行编码 a 。

I suspect that repeated variable declaration (example b) creates more overhead in theory, but that compilers are smart enough so that it doesn't matter. Example b has the advantage of being more compact and limiting the scope of the variable to where it is used. Still, I tend to code according example a.

编辑： 我是特别感兴趣的是Java案例。

I am especially interested in the Java case.

推荐答案

哪个更好， a 或 b ？

从性能角度来看，您必须对其进行衡量。 （在我看来，如果你可以测量差异，编译器就不太好了。）

From a performance perspective, you'd have to measure it. (And in my opinion, if you can measure a difference, the compiler isn't very good).

从维护的角度来看， b 更好。在尽可能最窄的范围内，在同一位置声明和初始化变量。不要在声明和初始化之间留下空洞，也不要污染你不需要的命名空间。

From a maintenance perspective, b is better. Declare and initialize variables in the same place, in the narrowest scope possible. Don't leave a gaping hole between the declaration and the initialization, and don't pollute namespaces you don't need to.

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