Example Application: Compounded Interest

Introduction to Radio Buttons


The compounded interest is the amount of money paid as interest on a loan. This example uses some radio buttons to let the user select the frequency by which to perform the calculation.

Compound Interest Formula P = Principal
r = Annual (Interest) Rate
m = Number of Compounding Periods per Year
n = Total Number of Compounding Periods
A = Amount Earned After n periods

Practical LearningApplication: Creating the Application

  1. Start a new Windows Application named CompoundedInterest
  2. Design the form as followed:

    Compound Interest - Form Design

    Control Name Text Additional Properties
    GroupBox GroupBox   Loan Setup  
    Label Label   Principal:  
    TextBox TextBox txtPrincipal 0.00 TextAlign: Right
    Label Label   Interest Rate:  
    TextBox TextBox txtInterestRate 8.25 TextAlign: Right
    Label Label   %  
    Label Label   Number of Periods:  
    TextBox TextBox txtPeriods 1 TextAlign: Text
    Label Label   years  
    GroupBox GroupBox   Compound Frequency  
    RadioButton RadioButton rdoMonthly    
    RadioButton RadioButton rdoQuarterly    
    RadioButton RadioButton rdoSemiannually    
    RadioButton RadioButton rdoAnnually    
    GroupBox GroupBox   Results  
    Label Label   Interest Earned:  
    TextBox TextBox txtInterestEarned 0.00 TextAlign: Right
    ReadOnly: True
    Label Label   Amount Earned:  
    TextBox TextBox txtFutureValue 0.00 TextAlign: Right
    ReadOnly: True
    Button Button btnCalculate Calculate  
    Button Button btnClose Close  
  3. On the form, double-click the Close button and implement its OnClick() event as follows:
    private void btnClose_Click(object sender, EventArgs e)
  4. To test the application, press F5
  5. Close the form and return to your programming environment
  6. On the form, click Monthly
  7. In the Properties window, double-click Checked to set its value to True
  8. On the form, double-click the Calculate button and implement its Click() event as follows:
    private void btnCalculate_Click(object sender, EventArgs e)
        double principal = 0.00,
               interestRate = 0.00,
               interestEarned = 0.00;
        double futureValue = 0.00,
               ratePerPeriod = 0.00,
               periods = 0;
        int compoundType = 0;
        // Retrieve the value of the principal
        principal = double.Parse(txtPrincipal.Text);
        // Retrieve the interest rate
        interestRate = double.Parse(txtInterestRate.Text) / 100;
        // Find out what radio button was clicked to apply the compound frequency
        if (rdoMonthly.Checked)
            compoundType = 12;
        else if (rdoQuarterly.Checked)
            compoundType = 4;
        else if (rdoSemiannually.Checked)
            compoundType = 2;
            compoundType = 1;
        // Get the number of periods
        periods = double.Parse(txtPeriods.Text);
        // These values will make the calculation easier to read
        double i = interestRate / compoundType;
        double n = compoundType * periods;
        // Perform the necessary calculations
        ratePerPeriod = interestRate / periods;
        futureValue = principal * Math.Pow(1 + i, n);
        interestEarned = futureValue - principal;
        // Display the values in the appropriate text boxes
        txtInterestEarned.Text = interestEarned.ToString("C");
        txtFutureValue.Text = futureValue.ToString("C");
  9. Test the application
    Compound Interest - Results
  10. After using it, close the form and return to your programming environment

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