Java-根据条件将for循环计数器调回 [英] Java - Turning the for-loop counter back based on a conditional

问题描述

以下是我的大学作业代码的一部分.

The following is part of the code for my college assignment.

else if (!codeList.contains(userCode)) {
                    i--; // i is the counter for the for-loop
                }
else if (userQuantity[i]==0) {
                    i--;
                }

第一部分确保如果用户输入错误的代码，则计数器i不会递增1，或者说，它会从最近递增的计数器中减去1.这部分工作正常.

The first part makes sure that if the user enters the wrong code, the counter i does not increment 1, or rather, it subtracts 1 from the recently incremented counter. This part works fine.

但是第二部分似乎是我遇到的问题.userQuantity []是一个int数组，它必须是一个数组.这似乎对代码没有任何作用.即使为该数量输入了0，它仍会补偿不必要的计数器.

The second part however is what I seem to be having problems with. userQuantity[] is an int array and it has to be an array. This does not seem to do anything to the code. Even if 0 is entered for the quantity, it still incrememnts the counter which is not desireable.

为了避免混淆，我应该解释这是一个无限的for循环(带有break语句).我执行for循环的原因是因为我必须这样做.是因为我的for循环条件不起作用，还是我做错了什么?

这是我的大学作业，因此我希望获得带说明的答案，而不仅仅是快速解决方案.如果您需要我解释，请告诉我.

This is for my college assignment so I would appreciate answers with explanation and not just quick-fixes. If you need me to explain, let me know please.

推荐答案

尽管在Java中并非严格违法，但从循环内部更改for循环控制变量的值并不是一个好主意.(这种修改 在其他一些语言中是非法的.)

Although it's not strictly illegal in Java, it's not a good idea to change the value of the for loop control variable from within the loop. (Such modification is illegal in some other languages.)

通过更改循环中的循环迭代变量，您会弄乱使用 for 循环所提供的隐式假设.例如，如果读者看到以下内容:

By changing the loop iteration variable within the loop, you're messing with the implicit assumptions offered by your use of a for loop. For example, if a reader sees:

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

读者将正确地认为循环旨在执行10次(如果其中有 break ，则循环不超过10次).但是，如果您要在循环中更改 i 的值，则该假设不再有效.

the reader will rightfully assume that the loop is intended to execute exactly 10 times (or, no more than 10 if there is a break in there). However, if you go changing the value of i within the loop, this assumption is no longer valid.

如果必须更改计数器的值，我建议将其写为 while 循环:

If you must change the value of the counter, I would suggest writing this as a while loop instead:

int i = 0;
while (i < 10) {
    // ...
    i++;
}

以及一条注释，解释了为什么要在循环中更改 i 以及这样做的含义.

along with a comment that explains why you are changing i within the loop and what it means to do so.

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