Java - 添加所有奇数奇怪的行为

Question

I am doing a maths challenge for project euler and i have come across a strange problem when running the program. The result should be the sum of all the odd numbers up to 10,000,000 but i get a negative number, what am i doing wrong?

package program;

import java.util.*;

public class MainClass {

/**
 * @param args
 */
public static void main(String[] args) {

    int total = 0;

    for (int counter = 1; counter < 10000000; counter++) {
        if (!((counter % 2) == 0)) {
            total+=counter;
        }

    }
    System.out.println(total);

}

}

Answer 1

Use a long instead of an int . You're getting a negative number due to integer overflow.

Answer 2

The int variable can't hold the total because the total is too big. At some point in the loop, you're getting an integer overflow and it's "rolling over" to a negative number:

You need a long .

A matter of style and efficiency, I'd change the code to iterate by 2 so that you don't need the test for oddness:

public static void main(String[] args) {
    long total = 0;
    for (int counter = 1; counter < 10000000; counter += 2) { // iterate by 2
        total += counter;
    }
    System.out.println(total);
}

Answer 3

You should use long total = 0; instead of int total = 0; int in java is 4 bytes and ranges from -2,147,483,648 to 2,147,483,647.

so 2,147,483,647 + 1 = -2,147,483,648

The total for this loop comes out to be 25,000,000,000,000 which can be accommodated by long

Answer 4

Just to throw in the more clever solution to this problem (MATH. yay).

You can solve this much easier, you just need to know that the sum of odd numbers from 1..2n-1 is equal to the square of n. It's pretty easy to prove this with induction for those who want to try.

Anyways this means that the sum from 1..X is equal to: ((X + 1) / 2)**2