Processing the data

To process the data first we need to set up some variables to hold the values and to convert them into suitable types. The main new instruction is the use of Single which is short for single precision numbers - as you can probably guess as there is a Single there is also a Double. In this case a single precision calculation is sufficiently accurate:

Dim amt As Single=ComboBox1.SelectedItem
Dim months As Single = 
        ComboBox2.SelectedItem * 12
Dim mrate As Single = 
        ComboBox3.SelectedItem / 100 / 12
Dim mpay As Single
Dim tpay As Single

The number of years is converted nto months and the interest rate is converted from a percentage to a decimal fraction e.g. 8% is 0.08 and then divided by 12 to give a monthly interest rate. The variables mpay and tpay will be used to store the monthly and total payments respectively.

Next we have to compute the interest payments either using a repayment mortgage or interest only. All we have to do is use an If statement to test if the appropriate RadioButton is pressed:

If RadioButton1.Checked Then
  mpay = -Pmt(mrate, months, amt, 0, 0)
  tpay = mpay * months
 End If

 If RadioButton2.Checked Then
  mpay = mrate * amt
  tpay = mpay * months
 End If

The two different calculations are performed according to which button is pressed. Finally we can update the Labels to show the result:

 Label5.Text = Format(mpay, "Currency")
 Label6.Text = Format(tpay, "Currency")
End Sub

The only new feature here is the use of Format to create a correctly formatted currency display.

Now we have the calculation routine - but when should it be use or  called.

We could put a button on the form and allow the user to recalculate by clicking it but as the calculation is so quick we could try to recalculate every time the user made a change.

To do this we have to define event handlers for every event that implies a change has or might have happened.

One tip worth knowing before moving on is that all of the event handlers described and used below can be added to the code by simply double clicking on the control in question in the designer. This speed things up a lot and means you only have to enter a small amount of text.

For the RadioButtons the only events that can cause a change are the CheckedChanged events

Private Sub RadioButton1_CheckedChanged(
   ByVal sender As System.Object, 
   ByVal e As System.EventArgs) 
    Handles RadioButton1.CheckedChanged
 Call Calculate()
End Sub

Private Sub RadioButton2_CheckedChanged(
   ByVal sender As System.Object, 
   ByVal e As System.EventArgs) 
    Handles RadioButton2.CheckedChanged
 Call Calculate()
End Sub

Notice that all the event handler has to do is call the calculate subroutine - this is of course the reason why it was defined as a separate subroutine!

For the ComboBoxes there is a useful looking event called "SelectedIndexChanged". This event occurs when ever the user changes the value in the ComboBox and so the ComboBox event handlers should be defined as

Private Sub ComboBox1_SelectedIndexChanged(
   ByVal sender As System.Object, 
   ByVal e As System.EventArgs) 
    Handles ComboBox1.SelectedIndexChanged
 Call Calculate()
End Sub

Private Sub ComboBox2_SelectedIndexChanged(
   ByVal sender As System.Object, 
   ByVal e As System.EventArgs) 
    Handles ComboBox2.SelectedIndexChanged
 Call Calculate()
End Sub

Private Sub ComboBox3_SelectedIndexChanged(
   ByVal sender As System.Object, 
   ByVal e As System.EventArgs) 
    Handles ComboBox3.SelectedIndexChanged 
 Call Calculate()
End Sub

To access the complete project including the graphics once you have registered,  click on CodeBin.

This is the fourth chapter of an eBook introducing Visual Basic 2010 - more soon!

See also:

Chapter 1 - Hello VB

Chapter 2 - Mastering VB Properties

Chapter 3 - Mastering VB Events

Chapter 5 - Graphics and Animation

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