sumofprimes_2million

// An attempt to solve the sum of primes problems posted at http://projecteuler.net/problem=10
// References: http://stackoverflow.com/questions/15750574/js-prime-numbers-script-explanation

// Runs in about 89.4s on my Windows dual-core laptop, Win 7, Intel Core 2 Duo (x2GHz), 4GB RAM, using nodejs
// My output: 
//  142913828922
//  primes found 148933
//  [Finished in 89.4s]

// Verified against answer on projecteuler.net

// Summary: build up a list of primes as you go, check each number if it is divisible by the primes in that list
// A prime number is not divisible by any primes less than itself

var sum = 2;
var num = 2000000;
var starter = num; if (num%2===0) starter = num-1;
// to store cumulative list of primes
var listofprimes = [2];

var isprimeforOdd = function (x) {
    for (var i=0;i<listofprimes.length;i++) {
      var cur = listofprimes[i];
    	if (x % cur == 0) return false; // not a prime since we found a prime divisor
    }
    return true;
};

for (var i = 3; i <= starter; i+=2) {
    // even numbers out the door!
    if (isprimeforOdd(i)) { 
        listofprimes.push(i); sum+=i; 
    }
}


console.log(sum);

console.log('primes found ' + listofprimes.length)