为什么在此素数检查中得到ArrayIndexOutOfBoundsException? [英] Why do I get an ArrayIndexOutOfBoundsException in this prime number check?

问题描述

我正在找出除num之外的最高素数，如程序所示， 数组和

I was finding out highest prime factor which divides num, as shown in program, there's a issue with array and

arr[j] = i;
j++;

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 1
    at primenum.main(primenum.java:13)

//to find highest prime factor
public class primenum {
    public static void main(String[] args) {
          double num = 600851475143.0;
           int j = 1;
          int arr[] = {j};

          for(int i=2; i<=num/2; i++)
          {
              if((num%i) == 0 )
              {
                  arr[j] = i;
                  j++;
              }

          }
          // take the last item from array, coz its last big prime
          System.out.println("largest prime is "+ arr[j-1]);

    }
}

解决此问题的最佳方法是什么?

What is best way to solve this problem??

我正在解决这个问题，

• 检查因子，直到num/2，
• 将所有内容推入数组
• 检查最后一个元素...

首要的是，我需要做更多的事情，但是我仍处于起步阶段.

For prime I need to do more, but I'm stuck in initial stage.

推荐答案

似乎您正在找到num的所有除数；其中之一将是最大的主要因素.仅两个相关事实就可以使问题对于较小的数字变得易于处理:
1.如果d是除数，则num/d也是除数.
2.您不需要检查任何大于sqrt(num)的除数.

It looks like you are finding all divisors of num; one of these will be the largest prime factor. Two related facts alone should help make the problem tractable for smallish numbers:
1. If d is a divisor, then so is num/d.
2. you needn't check for any divisors greater than the sqrt(num).

要跟踪除数，请使用Set对象.

To keep track of divisors, use a Set object.

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