Question

I’m trying to loop through a big number (6 billion to be exact), but I can’t because my computer freezes. How can I work my way around this. I’m supposed to find the largest prime factor of 600851475143.

function prime(n) {
    if (n === 1 || n === 2) return false;
    if (n % 2 === 0 || n % 3 === 0) return false;
    return true;
}
var n = 600851475143;
for (var i = 1, c = []; i < n; i++) {
    if ((n % i === 0) && prime(i)) {
        c.push(i);
    }
}

I’m done with it yet. I’m storing the primes in an array.

Answer 1

Your prime() function doesn’t do what the name says it should. There are many efficient ways of factoring primes, try this one for example:

var x = 600851475143;
var i = 2;
var sk;
while(i <= x) 
{
    while (x % i == 0)
    {
        sk = i;
        x = x / i;
    }
    i++;
}
console.log(sk);

Output: 6857

This page has another (view source) function for factoring.