This program gives me an error saying " index was outside the bounds of the array" , when i try to calculate tax when income is $10000

Question

namespace TaxCalcuator
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        double[] rateArray = { 0.10, 0.15, 0.25, 0.28, 0.33, 0.35 ,0.396 };
        double[] taxableIncomeArray = { 0.0, 9275.0, 37650.0, 91150.0, 190150.0, 413350.0, 415050.0 };
        double[] taxDueArray = { 0.0, 927.50, 5183.75, 18558.75, 46278.75, 119934.75, 120529.75 };

        private void compute_Click(object sender, EventArgs e)

        {
            double income;
            int dependents;
            const double STANDARD_DEDUCTION = 6300;
            const double PERSONAL_EXEMPTION = 4050;
            double AGI;
            double tax;
            double taxDue;
            double rate;
            income = Convert.ToDouble(incomeTextBox.Text);
            dependents = Convert.ToInt32(dependentTextBox.Text);

            AGI = income - (PERSONAL_EXEMPTION + PERSONAL_EXEMPTION* dependents) - STANDARD_DEDUCTION;
            label4.Text = "Adjusted Gross Income:" + AGI.ToString("C");

            int sub =  taxableIncomeArray.Length -1;
            while  (sub >= 0 && AGI < taxableIncomeArray[sub])
            {
                if (AGI < 0)
                    tax = 0;
                --sub;
            }; 
            taxDue = taxDueArray[sub];
                rate = rateArray[sub];

            tax = taxDueArray[sub] + (AGI - taxableIncomeArray[sub]) * rate;
            label5.Text= "Income Tax:" +tax.ToString("C");
          }

        }
    }

Answer 1

The problem probably lies here:

    while  (sub >= 0 && AGI < taxableIncomeArray[sub])
    {
        if (AGI < 0)
            tax = 0;
        --sub;
    };

The last time this loop executes, sub will start as zero and be decremented to -1. Later on you try to use it to index into the other arrays, which obviously do not have an item at index -1.