在循环之前或在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丢弃变量，而不是在循环内重复，是否会产生任何(性能)​​差异?一个(毫无意义) Java 中的例子:

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) 在循环内声明(重复):

b) declaration (repeatedly) inside loop:

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

a 和 b 哪个更好?

我怀疑重复的变量声明(例如b)在理论上会产生更多的开销，但是编译器足够聪明，所以这无关紧要.示例 b 的优点是更紧凑，并将变量的范围限制在使用它的地方.尽管如此，我还是倾向于根据示例a进行编码.

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.

我对 Java 案例特别感兴趣.

I am especially interested in the Java case.

推荐答案

a 和 b 哪个更好?

从性能的角度来看，您必须对其进行衡量.(在我看来，如果你能衡量一个差异，编译器就不是很好).

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.在同一个地方声明和初始化变量，在尽可能窄的范围内.不要在声明和初始化之间留下空白，也不要污染不需要的命名空间.

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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