Prime numbers from listbox to a textbox

Question

I want to display prime numbers through a button from a LISTBOX to a textbox. The Interface displays properly prime numbers but only from 1 to 10, after that the algorithm changes and shows non prime values as prime values such as 44.

 private void primnr()
    {
        int n = listBox1.Items.Count;
        bool prim = true;
        for (int i = 2; i < n; i++)
        {

            for (int j = 2; j <n; j++)
            {
                if (i!=j && i%j==0)
                {
                    prim = false;
                    break;
                }
            }
            if (prim)
            {
                textBox2.Text = textBox2.Text + "Numar prim: " + listBox1.Items[i].ToString() + Environment.NewLine;
            }
            prim = true;
        }



    }

Answer 1

Your algorithm is good, despite it's not necessary for j to go further than half the value of i. https://dotnetfiddle.net/ZafFsb prints:

2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47,

So I would say your issue are the values stored in listBox1.Items[i] , make sure Items[43] = 43 ? Attach a breakpoint and inspect the values.

Answer 2

Let's split the problem in two : generating prime numbers (logics), and displaying them to TextBox (UI):

  private static IEnumerable<int> Primes() {
    yield return 2;
    yield return 3;

    List<int> primes = new List<int>() {3}; 

    for (int value = 5; ; value += 2) {
      int n = (int) (Math.Sqrt(value) + 0.5); // round errors for perfect squares

      foreach (int divisor in primes) {
        if (divisor > n) {
          primes.Add(value);

          yield return value;

          break;
        }
        else if (value % divisor == 0)
          break;  
      }
    }  
  }

Now, it seems you want to get items of the list with prime indexes, ie

listBox1.Items[2], listBox1.Items[3], listBox1.Items[5],..., listBox1.Items[101], ...

You can query Primes() with a help of Linq

using System.Linq;

...

var results = Primes()
  .Take(index => index < listBox1.Count)
  .Select(index => $"Numar prim: {listBox1.Ites[index]}");

// Time to Join results into a single string
textBox2.Text = string.Join(Environment.NewLine, results);