Loading user input into an array

[Inigo Montoya]

I am learning about arrays, and I need to figure out how to enter data from a form into the array. The user enters a subtotal, then the form calculates a discount percentage and amount, and give a final total. I need to have that total loaded into the array each time. The array needs to store 5 totals. Eventually I need to create a display box that will show the last five totals. Any general guidance would be greatly appreciated. I have read and re-read this in my book so many times and just not getting it. I'm thinking I would use a foreach loop here but not sure.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace InvoiceTotal
{
    // This is the starting point for exercise 8-1 from
    // "Murach's C# 2010" by Joel Murach
    // (c) 2010 by Mike Murach & Associates, Inc. 
    // www.murach.com
 
    public partial class frmInvoiceTotal : Form
	{
		public frmInvoiceTotal()
		{
			InitializeComponent();
		}

        // TODO: declare class variables for array and list here
        decimal[] totals = new decimal[5];

        private void btnCalculate_Click(object sender, EventArgs e)
		{
            totals[0] = 0.0m;
            totals[1] = 0.0m;
            totals[3] = 0.0m;
            totals[4] = 0.0m;

            try
            {

                if (txtSubtotal.Text == "")
                {
                    MessageBox.Show(
                        "Subtotal is a required field.", "Entry Error");
                }
                else
                {
			        decimal subtotal = Decimal.Parse(txtSubtotal.Text);
                    if (subtotal > 0 && subtotal < 10000)
                    {
                        decimal discountPercent = 0m;
                        if (subtotal >= 500)
                            discountPercent = .2m;
                        else if (subtotal >= 250 & subtotal < 500)
                            discountPercent = .15m;
                        else if (subtotal >= 100 & subtotal < 250)
                            discountPercent = .1m;
                        decimal discountAmount = subtotal * discountPercent;
			            decimal invoiceTotal = subtotal - discountAmount;

                        discountAmount = Math.Round(discountAmount, 2);
                        invoiceTotal = Math.Round(invoiceTotal, 2);

                        txtDiscountPercent.Text = discountPercent.ToString("p1");
                        txtDiscountAmount.Text = discountAmount.ToString();
                        txtTotal.Text = invoiceTotal.ToString();

                    }
                    else
                    {
                        MessageBox.Show(
                            "Subtotal must be greater than 0 and less than 10,000.", 
                            "Entry Error");
                    }
                }
            }
            catch (FormatException)
            {
                MessageBox.Show(
                    "Please enter a valid number for the Subtotal field.", 
                    "Entry Error");
            }
            txtSubtotal.Focus();
        }

		private void btnExit_Click(object sender, EventArgs e)
		{
            // TODO: add code that displays dialog boxes here

            this.Close();
		}

	}
}