为什么循环会忽略 5 的倍数？

Question

I've just started learning Java, so my question is probably incredibly stupid.

I'm trying to solve one of the simpliest problems.

Modify the previous program such that only multiples of three or five are considered in the sum, eg 3, 5, 6, 9, 10, 12, 15 for n=17

Here is my code:

import java.util.Scanner;

public class NewProblem {
    public static void main (String[] args) {

        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();

        for (int i = 0; i < n; i++) {
            if (i % 5 == 0 || i % 3 == 0) {
                System.out.println(i);
            }
            else {
                i++;
            }
        }

    }
}

When I run it, it's showing only multiples of three. For example, here is the output for n = 17:

0 3 6 9 10 15

So it just ignores all multiples of 5.

What's wrong? Sorry again if the question is really stupid.

Answer 1

The problem here is the else statement, which will add one to "i" if it's not divisble by 3 or 5 . Now, let's see what your method does if n = 5 . First, the for loop runs with 0 , which is divisble by 5 and 3 , so the for loop runs again and now i = 1 . 1 is not divisible by 5 or 3 , so the else statement is called, adding 1 to i , but since you have already set the for loop to do this, it will effectively add 2 to i , skipping i = 2 , and you don't notice, because 2 shouldn't be printed out in the first place. Then i = 3 , the if statement fires printing out like normal, the for loop fires again and this time i = 4 . And 4 is not divisible by 3 or 5 , so the else statement fires again effectively adding 2 to i since the for loop adds one and the else statement one, resulting in i = 6 , skipping i = 5 . This is where the problem lies. Since the for loop already adds one to i every time it loops (the last section of the for statement says i++ ) resulting in some numbers being skipped. Simply remove the else statement and your code should work fine.

Answer 2

您不需要在 else 块中增加 i ，它会在 for 循环的每次迭代中增加。