finding primes up to 65535 the third strike

SteelGolem

Jul 30th, 2020 (edited)

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using System;
using System.Collections.Generic;
namespace ConsoleApplication1
{
class Program
{
static List<int> primes;
static void Main(string[] args)
{
primes = new List<int>();
// build our primes list
int max = 65535;
for (int num = 2; num < max; num++)
{
bool notPrime = false;
for (int p = 0; p < primes.Count; p++)
if (num % primes[p] == 0) { notPrime = true; break; }
if (notPrime) continue;
primes.Add(num);
Console.WriteLine(num + " is a prime.");
}
Console.WriteLine("there were " + primes.Count + " primes up to " + max + ".");
Console.WriteLine("IsPrime(9001) == " + IsPrime(9001));
Console.ReadKey();
}
static bool IsPrime(int value)
{
// primes is sorted so we can speed up the search using BinarySearch() instead of Contains()
int index = primes.BinarySearch(value); // SO FAST
if (index < 0) return false;
return true;
}
}
}

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