Prime Number Formula

Question

I am trying to write a prime number function in C# and I am wondering if the follow code will work. It \"appears\" to work with the first 50 numbers or so. I just want to ma

Answer 1

You are apparently writing from a contrafactual dimension where 9 is a prime number, so I guess that our answers might not work for you. Two things though:

1. Prime number generating functions are a non-trivial but exiting matter, the Wikipedia page is a good starter (http://en.wikipedia.org/wiki/Formula_for_primes)

2. from (number%2!=0) it follows (number%4!=0). If you can't divide by 10, then you can't divide by 100 either.

Answer 2

https://www.khanacademy.org/computing/computer-science/cryptography/comp-number-theory/a/trial-division

    public static bool isPrime(int number)
    {
        for (int k = 2; k <= Math.Ceiling(Math.Sqrt(number)); k++)
        {
            if (number > k && number % k == 0)
                break;
            if (k >= Math.Ceiling(Math.Sqrt(number)) || number == k)
            {
                return true;
            }
        }
        return false;
    }

Answer 3

ExchangeCore Forums have a good bit of code that will pretty much let you generate any ulong number for primes. But basically here's the gist:

int primesToFind = 1000;
int[] primes = new int[primesToFind];
int primesFound = 1;
primes[0] = 2;
for(int i = 3; i < int.MaxValue() && primesFound < primesToFind; i++)
{
   bool isPrime = true;
   double sqrt = Math.sqrt(i);
   for(int j = 0; j<primesFound && primes[j] <= sqrt; j++)
   {
      if(i%primes[j] == 0)
      {
         isPrime = false;
         break;
      }
   }
   if(isPrime)
      primes[primesFound++] = i;
}

Once this code has finished running your primes will all be found in the primes array variable.

Answer 4

It had to be done...

public static bool IsPrime(this int number)
{
    return (Enumerable.Range(1,number).Where(x => number % x == 0).Count() == 2);
}

Answer 5

This is a simple code for find prime number depend on your input.

  static void Main(string[] args)
    {
        String input = Console.ReadLine();
        long num = Convert.ToInt32(input);
        long a, b, c;
        c = 2;
        for(long i=3; i<=num; i++){
            b = 0;
            for (long j = 2; j < i ; j++) {
                a = i % j;
                if (a != 0) {
                    b = b+1;
                }
                else {
                    break;
                }
            }

            if(b == i-2){
                Console.WriteLine("{0}",i);
            }
        }
        Console.ReadLine();
    }

Answer 6

Primality testing is the way to go, but in case you want a quick and dirty hack, here's something.

If it's not working fast enough, you can build a class around it and store the PrimeNumbers collection from call to call, rather than repopulating it for each call.

    public bool IsPrime(int val)
    {
        Collection<int> PrimeNumbers = new Collection<int>();
        int CheckNumber = 5;
        bool divisible = true;
        PrimeNumbers.Add(2);
        PrimeNumbers.Add(3);

        // Populating the Prime Number Collection
        while (CheckNumber < val)
        {
            foreach (int i in PrimeNumbers)
            {
                if (CheckNumber % i == 0)
                {
                    divisible = false;
                    break;
                }
                if (i * i > CheckNumber) { break; }
            }
            if (divisible == true) { PrimeNumbers.Add(CheckNumber); }
            else { divisible = true; }
            CheckNumber += 2;
        }
        foreach (int i in PrimeNumbers)
        {
            if (CheckNumber % i == 0)
            {
                divisible = false;
                break;
            }
            if (i * i > CheckNumber) { break; }
        }
        if (divisible == true) { PrimeNumbers.Add(CheckNumber); }
        else { divisible = true; }

        // Use the Prime Number Collection to determine if val is prime
        foreach (int i in PrimeNumbers)
        {
            if (val % i == 0) { return false; }
            if (i * i > val) { return true; }
        }
        // Shouldn't ever get here, but needed to build properly.
        return true;
    }