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

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 PrimeNumbers = new Collection();
        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;
    }