Question

Warning: Simple homework assignment, no idea what I’m doing

So, I’m trying to make a program that finds the first prime numbers from 1 to 100 and prints them in a listbox. This is my code:

private bool IsPrime(int number)
{
    int count;
    if (number == 2)
        return true;
    for (count = 3; count < number; count = count + 2)
    {
        if (number % count == 0)
            return false;
    }
    return true;
}

private void calculateButton_Click(object sender, EventArgs e)
{
    int number;
    for (number = 1; number < 100; number = number++)
        if (IsPrime(number))
            primeList.Items.Add(number);
}

And the program isn’t catching any syntax errors, but it also freezes up every time I try to run it. Any idea why this happens? Thanks.

Answer 1

You use:

for (number = 1; number < 100; number = number++)

while you should write

 for (number = 1; number < 100; number++)

You should read these articles to understand why your original code didn’t increment the : for, ++ Operator

You can learn the behaivour of the ++ operator in some test code:

        int n = 0;
        Console.WriteLine(n); //0
        n = n++;
        Console.WriteLine(n); //0
        n = ++n;
        Console.WriteLine(n); //1 
        n = n++;
        Console.WriteLine(n); //1
        n = ++n;
        Console.WriteLine(n); //2

Another nice example would be:

        int n = 0;
        int x = n++;
        int y = ++n;
        Console.WriteLine(string.Format("x={0}", x)); //0
        Console.WriteLine(string.Format("y={0}", y)); //2
        Console.WriteLine(x + y); //n++ + ++n == 0 + 2 == 2
        n = 0;
        x = ++n;
        y = n++;
        Console.WriteLine(string.Format("x={0}", x)); //1
        Console.WriteLine(string.Format("y={0}", y)); //1
        Console.WriteLine(x + y); //++n + n++ == 1 + 1 == 2