Question

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]);

    }
}

What is best way to solve this problem??

I’m solving this problem by,

• checking factors until num/2,
• push all into an array,
• check last element……

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

Answer 1

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).

To keep track of divisors, use a Set object.