Synchronizing the Drive, Directory, and File List Boxes Example

Savings Account - Decisions

1.   Here, we modify the Savings Account project to allow entering any three values and computing the fourth.  First, add a third command button that will clear all of the text boxes. Assign the following properties:

     

            Command3:

                        Caption                      Clear &Boxes

                        Name                         cmdClear

     

      The form should look something like this when you’re done:

     

     

     

2.   Code the cmdClear button Click event:

     

      Private Sub cmdClear_Click ()

      ‘Blank out the text boxes

      txtDeposit.Text = ""

      txtInterest.Text = ""

      txtMonths.Text = ""

      txtFinal.Text = ""

      End Sub

This code simply blanks out the four text boxes when the Clear button is clicked.

3.      Code the KeyPress event for the txtFinal object:

Private Sub txtFinal_KeyPress (KeyAscii As Integer)

      ‘Only allow number keys, decimal point, or backspace

      If (KeyAscii >= vbKey0 And KeyAscii <= vbKey9) Or KeyAscii = vbKeyDecPt Or KeyAscii = vbKeyBack Then

      Exit Sub

Else

  KeyAscii = 0

  Beep

End If

End Sub

We need this code because we can now enter information into the Final Value text box.

4.      The modified code for the Click event of the cmdCalculate button is:

Private Sub cmdCalculate_Click()

Dim IntRate As Single

Dim IntNew As Single

Dim Fcn As Single, FcnD As Single

‘Read the four text boxes

Deposit = Val(txtDeposit.Text)

Interest = Val(txtInterest.Text)

IntRate = Interest / 1200

Months = Val(txtMonths.Text)

Final = Val(txtFinal.Text)

‘Determine which box is blank

‘Compute that missing value and put in text box

If txtDeposit.Text = "" Then

‘Deposit missing

  Deposit = Final / (((1 + IntRate) ^ Months - 1) / IntRate)

  txtDeposit.Text = Format(Deposit, "#####0.00")

ElseIf txtInterest.Text = "" Then

‘Interest missing - requires iterative solution

  IntNew = (Final / (0.5* Months * Deposit) - 1) / Months

  Do

    IntRate = IntNew

    Fcn = (1 + IntRate) ^ Months - Final * IntRate / Deposit - 1

    FcnD = Months * (1 + IntRate) ^ (Months - 1) - Final / Deposit

    IntNew = IntRate - Fcn / FcnD

  Loop Until Abs(IntNew - IntRate) < 0.00001 / 12

  Interest = IntNew * 1200

  txtInterest.Text = Format(Interest, "##0.00")

ElseIf txtMonths.Text = "" Then

‘Months missing

  Months = Log(Final * IntRate / Deposit + 1) / Log(1 + IntRate)

  txtMonths.Text = Format(Months, "###.0")

ElseIf txtFinal.Text = "" Then

‘Final value missing

  Final = Deposit * ((1 + IntRate) ^ Months - 1) / IntRate

  txtFinal.Text = Format(Final, "#####0.00")

End If

End Sub       

In this code. we first read the text information from all four text boxes and based on which one is blank, compute the missing information and display it in the corresponding text box.  Solving for missing Deposit, Months, or Final information is a straightforward manipulation of the equation given in Example 2-2.

If the Interest value is missing, we have to solve an Mth-order polynomial using something called Newton-Raphson iteration - a good example of using a Do loop.  Finding the Interest value is straightforward.  What we do is guess at what the interest is, compute a better guess (using Newton-Raphson iteration), and repeat the process (loop) until the old guess and the new guess are close to each other.  You can see each step in the code.

5.      Test and save your application.  Go home and relax.

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