c# even number and factorial

Question

So I have problem... I need to build program with C# windows form, and calculate with texbox1 and textbox2 factorials and show even numbers in richtextbox, and use FOR cycle, I have builded this solution but it gives me only one number.....It is mind****.... If you know what I mean... Any sugestions?

private void button1_Click(object sender, EventArgs e)
    {
        int x = Convert.ToInt32(textBox1.Text);
        int y = Convert.ToInt32(textBox2.Text);
        int fact = 1;
        for (int i = x; i<=y; i++)
        {
            fact = fact * y;
        }
        richTextBox1.Text = fact.ToString();
    }

Well In mind I have build this one too, what do you think about it? And maby someone can tell me how to use it?

for (int i = X; i <= Y; i++)
    if (i % 2 == 0)
    {
        fakt = 1;

        for (int j = i; j > 0; j--)
            fakt = fakt * j;
    }

Answer 1

Let's start by defining our factorial function. You said it had to be a for loop, so:

private int Factorial(int n) {
    int result = 1;
    for (int i = 2; i <= n; i++)
        result *= i;
    return result;
}

Now, you said you wanted the textbox to have the format:

X = 1; Y = 4;
=======================|Number|Factorial|====================
=======================|2|2|=================================
=======================|4|24|================================

Let's start by making a function that will print the column headers:

private void PrintHeaders(int x, int y) {
    StringBuilder builder = new StringBuilder();
    builder.AppendFormat("X = {0}; Y = {1}", x, y);
    builder.AppendLine();
    builder.AppendLine("===============|Number|Factorial|=============");
    richTextBox1.Text = builder.ToString();
}

And, lastly, our click event handler:

private void button1_Click(object sender, EventArgs e) {
    int x = Convert.ToInt32(textBox1.Text);
    int y = Convert.ToInt32(textBox2.Text);
    PrintHeaders(x, y);
    for(int i = x; i <= y; i++) {
        if(i % 2 == 0) {
           int result = Factorial(i);   
           richTextBox1.Text += "==========|" + i + "|" + result + "|=============";
           richTextBox1.Text += Environment.NewLine; 
        }
    }
}