后增量运算符不在 for 循环中递增 [英] Post increment operator not incrementing in for loop

问题描述

我正在做一些关于 Java 的研究，发现这很令人困惑:

I'm doing some research about Java and find this very confusing:

for (int i = 0; i < 10; i = i++) {
  System.err.print("hoo... ");
}

这是永无止境的循环！

有人能很好地解释为什么会发生这样的事情吗?

Anybody has good explanation why such thing happens?

推荐答案

for (int i = 0; i < 10; i = i++) {

上面的循环本质上是一样的:-

The above loop is essentially the same as: -

for (int i = 0; i < 10; i = i) {

for 语句的第 3^rd 部分 - i = i++，被评估为: -

the 3^rd part of your for statement - i = i++, is evaluated as: -

int oldValue = i; 
i = i + 1;
i = oldValue;  // 3rd Step 

<小时>

您需要从那里删除分配，以使其工作:-

---

You need to remove the assignment from there, to make it work: -

for (int i = 0; i < 10; i++) {

<小时>

(根据评论的 OP 请求)

---

(On OP request from Comments)

就您在评论中指定的问题而言，以下表达式的结果:-

As far as your issue as specified in the comment is concerned, the result of the following expression: -

x = 1; 
x = x++ + x++;

获得如下:-

让我们标记第二个语句的不同部分:-

Let's mark different parts of the second statement: -

x = x++ + x++;
R    A     B

现在，首先将评估 RHS 部分 (A + B)，然后将最终结果分配给 x.所以，让我们继续前进.

Now, first the RHS part (A + B) will be evaluated, and then the final result will be assignmed to x. So, let's move ahead.

第一个 A 被评估:-

old1 = x;  // `old1 becomes 1`
x = x + 1; // Increment `x`. `x becomes 2`
//x = old1; // This will not be done. As the value has not been assigned back yet.

现在，由于 A 到 R 的赋值没有在这里完成，所以不执行第 3 步.

Now, since the assignment of A to R is not done here, the 3rd step is not performed.

现在，转到 B 评估:-

Now, move to B evaluation: -

old2 = x;  // old2 becomes 2. (Since `x` is 2, from the evaluation of `A`)
x = x + 1; // increment `x`. `x becomes 3`.
// x = old2; // This will again not be done here.

现在，为了获得x++ + x++的值，我们需要做我们在A和B，因为现在是在 x 中分配的值.为此，我们需要替换:-

Now, to get the value of x++ + x++, we need to do the last assignment that we left in the evaluation of A and B, because now is the value being assigned in x. For that, we need to replace: -

A --> old1
B --> old2   // The last assignment of both the evaluation. (A and B)

/** See Break up `x = old1;` towards the end, to understand how it's equivalent to `A = old1; in case of `x = x++`, considering `x++ <==> A` in this case. **/

所以，x = x++ + x++，变成:-

x = old1 + old2;
  = 1 + 2;
  = 3;  // Hence the answer

<小时>

分解x = x++的第三部分，看看它在x = x++ + x++情况下是如何工作的:-

想知道为什么替换为 A -->old1 而不是 x -->old1，如 x = x++ 的情况.

---

Break up of 3rd part of x = x++, to see how it works in x = x++ + x++ case: -

Wonder why the replacement is done as A --> old1 and not x --> old1, as in case of x = x++.

深入查看x = x++部分，特别是最后一个赋值:-

Take a deep look at x = x++ part, specially the last assignment: -

x = oldValue;

如果你认为这里的 x++ 是 A，那么上面的赋值可以分解为以下步骤:-

if you consider x++ to be A here, then the above assignment can be broken into these steps: -

A = oldValue;
x = A;

现在，对于当前的问题，它是一样的:-

Now, for the current problem, it is same as: -

A = old1;
B = old2;
x = A + B;

我希望这能说清楚.

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