Question

I have run into a weird issue for problem 3 of Project Euler. The program works for other numbers that are small, like 13195, but it throws this error when I try to crunch a big number like 600851475143:

Exception in thread "main" java.lang.ArithmeticException: / by zero
    at euler3.Euler3.main(Euler3.java:16)

Here’s my code:

    //Number whose prime factors will be determined
    long num = 600851475143L;

    //Declaration of variables
    ArrayList factorsList = new ArrayList();
    ArrayList primeFactorsList = new ArrayList();

    //Generates a list of factors
    for (int i = 2; i < num; i++)
    {
        if (num % i == 0)
        {
            factorsList.add(i);
        }           
    }

    //If the integer(s) in the factorsList are divisable by any number between 1 
    //and the integer itself (non-inclusive), it gets replaced by a zero 
    for (int i = 0; i < factorsList.size(); i++)
    {
        for (int j = 2; j < (Integer) factorsList.get(i); j++)
        {
            if ((Integer) factorsList.get(i) % j == 0)
            {
                factorsList.set(i, 0);
            }
        }
    }

    //Transfers all non-zero numbers into a new list called primeFactorsList
    for (int i = 0; i < factorsList.size(); i++)
    {
        if ((Integer) factorsList.get(i) != 0)
        {
            primeFactorsList.add(factorsList.get(i));
        }
    }

Why is it only big numbers that cause this error?

Answer 1

Your code is just using Integer, which is a 32-bit type with a maximum value of 2147483647. It’s unsurprising that it’s failing when used for numbers much bigger than that. Note that your initial loop uses int as the loop variable, so would actually loop forever if it didn’t throw an exception. The value of i will go from the 2147483647 to -2147483648 and continue.

Use BigInteger to handle arbitrarily large values, or Long if you’re happy with a limited range but a larger one. (The maximum value of long / Long is 9223372036854775807L.)

However, I doubt that this is really the approach that’s expected… it’s going to take a long time for big numbers like that.