Project Euler #7

problem7.cs

// By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13
// What is the 10 001st prime number?
// https://projecteuler.net/problem=7
void Main()
{
	var knownPrimes = new List<int>(new int[] { 2 });
	
	int N = 3;
	while (knownPrimes.Count() < 10001)
	{
		if (isPrime(N, knownPrimes))
		{
			knownPrimes.Add(N);
		}
		N++;
	}

	knownPrimes.Last().Dump("Prime #" + knownPrimes.Count());
}

bool isPrime(int n, IEnumerable<int> knownPrimes)
{
	if (Math.Sqrt(n) <= knownPrimes.Last())
	{
		return false == knownPrimes.Any(p=> 0 == n%p);
	}
	else
	{
		throw new ApplicationException("Something wrong with knownPrimes");
	}	
}