Compound interest calculator with compounds

[tpistol]

I am trying to complete a compound interest calculator with the ability to input the number of compounds per year. Here is what the form looks like:




I will put the code below but here are my problems:

1) When entering a number in the compound box, all the program does is spit back my initial deposit.
2) When I enter the letter 'c' in the compound box, I get zeros for each year.

As always thank you for any help you can provide.

Public Class Form1
    Dim dbldeposit As Double
    Dim intrate As Integer
    Dim intyears As Integer
    Dim strcompoundyears As String
    Dim dblyearendingbalance As Double
    Const STRFORMAT As String = "{0,-10}{1,-21:C}"

    Private Sub txtdeposit_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles txtdeposit.KeyPress
        If (e.KeyChar < "0" Or e.KeyChar > "9") And e.KeyChar <> Chr(8) And e.KeyChar <> Chr(36) And e.KeyChar <> "," Then
            e.KeyChar = Chr(0)
            MsgBox("Please enter a dollar amount.")
            'restricts keys to 0-9, backspace, dollar sign, and the comma.
        End If
    End Sub

    Private Sub txtinterest_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles txtinterest.KeyPress
        If (e.KeyChar < "0" Or e.KeyChar > "9") And e.KeyChar <> Chr(8) And e.KeyChar <> "." And e.KeyChar <> "%" Then
            e.KeyChar = Chr(0)
            MsgBox("Please enter an interest rate.")
            'restricts keys to 0-9, backspace, the period, and the percent sign.
        End If
    End Sub

    Private Sub txtyears_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles txtyears.KeyPress
        If (e.KeyChar < "0" Or e.KeyChar > "9") And e.KeyChar <> Chr(8) Then
            e.KeyChar = Chr(0)
            MsgBox("Please enter a number.")
            'restricts keys to 0-9, letter c, and the backspace.
        End If
    End Sub

    Private Sub txtcompounds_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles txtcompounds.KeyPress
        If (e.KeyChar < "0" Or e.KeyChar > "9") And e.KeyChar <> Chr(99) And e.KeyChar <> Chr(8) Then
            e.KeyChar = Chr(0)
            MsgBox("Please enter a number or the letter c for continous compounds.")
            'restricts keys to 0-9, letter c, and the backspace.
        End If
    End Sub

    Private Sub btncalculate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btncalculate.Click
        dbldeposit = txtdeposit.Text
        intrate = txtinterest.Text
        intyears = txtyears.Text
        strcompoundyears = txtcompounds.Text

        If intrate > 1 Then
            intrate = intrate / 100
        End If

        lstdisplay.Items.Add(String.Format(STRFORMAT, "Year", "Year-ending balance"))

        For i As Integer = 1 To intyears
            If IsNumeric(strcompoundyears) Then
                dblyearendingbalance = dbldeposit * ((1 + intrate / strcompoundyears) ^ (strcompoundyears * i))
            Else
                dblyearendingbalance = dbldeposit * (intrate / Math.Exp(intrate * intyears))
            End If
            lstdisplay.Items.Add(String.Format(STRFORMAT, i, dblyearendingbalance))
        Next
    End Sub

    Function IsGoodInput(ByVal input As Integer) As Boolean
        If input = "" Then
            Return False
        Else
            Return True
        End If
    End Function

    Private Sub btnclear_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnclear.Click
        txtdeposit.Clear()
        txtinterest.Clear()
        txtyears.Clear()
        txtcompounds.Clear()
        lstdisplay.Items.Clear()
        txtdeposit.Focus()
    End Sub
End Class

 

[jmcilhinney]

Have you been specifically told to handle the KeyPress events to validate input?

 

[tpistol]

I was told that was OK to do, but not absolute that it be done that way.

 

[jmcilhinney]

Your code doesn't really do what you want because it will still allow invalid values. If you're not going to write code that properly prevent invalid data then the next best option is cure. If you want to go that way then you should forget the KeyPress events and handle the Validating event. That will allow the user to enter anything they want but prevent them leaving a control if it contains invalid input. To ensure that every field is validated, even if it's not visited by the user, you call ValidateChildren before using the data.

In your event handler, you would call the TryParse method of the appropriate numeric type, e.g. Decimal.TryParse. When it comes time to use the data you know for a fact that it's all valid so you can just go ahead and convert it, e.g. using Decimal.Parse.

 

[tpistol]

OK, I will look into ValidateChildren and TryParse. We just haven't done those yet in VB class. Thank you!