3-5: Variable Declarations (continued)

There are a number of new things to note in this code:

• The two lines beginning with ' are simply comments to remind us what the code is doing. In VB these are called remark statements. They are for the human reader and are ignored by the computer. We'll discuss program comments more in later sections.
• We have not only calculated the new values interestAmount and capital, but we have also assigned those values to the Text properties of the appropriate textboxes.
• The line
  capital = capital + interestAmount
  might look unusual to you - so what is happening here? Remember that the variable name represents a memory cell in the computer. On the right hand side the expression capital + interestAmount uses the values in the memory cells named capital and interestAmount, creating a new value. This new value is stored in the memory cell named capital thereby replacing the value that was originally stored there.

The btnCalculate_Click event can be completed rather easily now. Simply repeat the last six lines twice changing txtYear1.Text, txtInterest1.Text and txtCapital1.Text to txtYear2.Text, txtInterest2.Text and txtCapital2.Text, and then to txtYear3.Text, txtInterest3.Text and txtCapital3.Text! You end up with:

Private Sub btnCalculate_Click( ... )
Dim capital As Double, interestAmount as Double
Dim interestRate As Double

' Assign values from textboxes to variables ...
capital = Convert.ToDouble(txtInitialCap.Text)
interestRate = CDbl(txtRate.Text)

' For year 1 ...
interestAmount = interestRate * capital
capital = capital + interestAmount
txtYear1.Text = 1
txtInterest1.Text = interestAmount
txtCapital1.Text = capital

' For year 2 ...
interestAmount = interestRate * capital
capital = capital + interestAmount
txtYear2.Text = 2
txtInterest2.Text = interestAmount
txtCapital2.Text = capital

' For year 3 ...
interestAmount = interestRate * capital
capital = capital + interestAmount
txtYear3.Text = 3
txtInterest3.Text = interestAmount
txtCapital3.Text = capital
End Sub

Start the program and make sure that it behaves as it should. Enter some simple values for the interest rate and initial capital so that you can check the calculations by hand. Also make sure that the Clear and Exit buttons work correctly.