Check prime numbers

Question

I need to make a GUI application that checks prime numbers when the user input two numbers. The criteria is that I make two methods, one that checks if a number is prime and that is my isPrime(int n) and another that checks which of the two inputs is lower/higher so that the program knows where to start.

I can't manage to do this. When I run the script I don't have any output in Listbox.

My code so far:

private void button1_Click(object sender, EventArgs e)
{
    int num1, num2;
    num1 = Int32.Parse(textBox1.Text);
    num2 = Int32.Parse(textBox2.Text);

    for (int i = num1; i <= num2; i++) //Method that checks which of the two inputs is lower/hihger
    {
        if (isPrime(i))
        {
            for (int j = 2; j <= 1 / 2; j++)
            {
                if (i % j == 0)
                {
                    break;
                }
            }
            if (i == 0 && i != 1)
            {
                listBox1.Items.Add(i);
            }
        }
    }
}

private static bool isPrime(int n) //Method to check prime numbers
{
    for (int i = 2; i < n; i++)
    {
        if (n % i == 0)
        {
            return false;
        }
    }
    return true;
}

Answer 1

Let's start from isPrime. Since it takes int n as an argument, n can be up to int.MaxValue ~ 2_000_000_000; 2 billions iterations is a bit too many when we can easily implement a routine with sqrt(2_000_000_000) / 2 ~ 30_000 loops only:

    private static bool isPrime(int n) {
      if (n <= 1)
        return false;
      else if (n % 2 == 0)
        return n == 2;

      int max = (int)(Math.Sqrt(n) + 0.5);

      for (int divisor = 3; divisor <= max; divisor += 2)
        if (n % divisor == 0)
          return false;

      return true;   
    }     

Now, it's time to get the primes between num1 and num2:

    for (int i = num1; i <= num2; ++i)
      if (isPrime(i))
        listBox1.Items.Add(i);

please, note, that since we've optimized isPrime we can keep for loop being very simple.

Answer 2

Your isPrime function is correct, just minor change you need is that update condition to i <= Math.Sqrt(n) as for (int i = 2; i <= Math.Sqrt(n); i++). That would be more efficient. Sieve of Eristhostenes.

You can also refer below links to find better and efficient way to check isPrime. As suggested by @Fildor in comments.

• Program to find prime numbers
• https://stackoverflow.com/a/1072205/982149

To find min or max value from two inputs use Math.Min & Math.Max like shown below. And update your for loop as below.

private void button1_Click(object sender, EventArgs e)
{
    int num1, num2;
    num1 = Math.Min(Int32.Parse(textBox1.Text), Int32.Parse(textBox2.Text));
    num2 = Math.Max(Int32.Parse(textBox1.Text), Int32.Parse(textBox2.Text));

    for (int i = num1; i <= num2; i++) //Method that checks which of the two inputs is lower/hihger
    {
        if (i != 0 && i != 1 && isPrime(i))
        {               
            listBox1.Items.Add(i);
        }
    }
}

private static bool isPrime(int n) //Method to check prime numbers
{
    for (int i = 2; i <= Math.Sqrt(n); i++)
    {
        if (n % i == 0)
        {
            return false;
        }
    }
    return true;
}

Answer 3

Just to add another view to the problem:

I wouldn't check ( as in "compute" ) primes here at all.

Int32 contains a pretty finite number of primes, which allows us to either precompute or even just download a list from the internet if you can find one.

As soon as we have it, all you need to do is a lookup of those that happen to be in the desired range.