hw problem functions and sub procedures

Code:

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Public Class Frmbilling

    Dim itemamount, tax, subtotal, total As Double

    Private Sub btnCalculate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCalculate.Click

        getitem()
        calctotal()
        calctax()
        calcsubtotal()
        displaytotal()

    End Sub

    Private Sub btnClear_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnClear.Click

        Rdbcappuccino.Checked = False
        RdbEspresso.Checked = False
        rdbLatte.Checked = False
        RdbIcedLatte.Checked = False
        RdbIcedCappuccino.Checked = False
        TxtQuantity.Clear()
        TxtItemAmount.Clear()
        chkTax.Checked = False
        TxtQuantity.Focus()

    End Sub

    Private Sub btnNewOrder_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnNewOrder.Click
        Rdbcappuccino.Checked = False
        RdbEspresso.Checked = False
        rdbLatte.Checked = False
        RdbIcedLatte.Checked = False
        RdbIcedCappuccino.Checked = False

        TxtQuantity.Clear()
        TxtItemAmount.Clear()

        chkTax.Checked = False
        TxtQuantity.Focus()
        Txtsubtotal.Clear()
        TxtItemAmount.Clear()
        txtTax.Clear()
        TxtTotal.Clear()

        'CalcGrandTotal()
        'CalcAddOneCustomer()



    End Sub

    Private Sub btnExit_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnExit.Click

        End

    End Sub

    Sub getitem()

        Dim quantity As Double
        TxtQuantity.Text = quantity
        If Rdbcappuccino.Checked Then
            TxtItemAmount.Text = 2 * quantity
        ElseIf RdbEspresso.Checked Then
            TxtItemAmount.Text = 2.25 * quantity
        ElseIf rdbLatte.Checked Then
            TxtItemAmount.Text = 1.75 * quantity
        ElseIf RdbIcedLatte.Checked Then
            TxtItemAmount.Text = 2.5 * quantity
        ElseIf RdbIcedCappuccino.Checked Then
            TxtItemAmount.Text = 2.5 * quantity

        End If
    End Sub

    Private Function calcsubtotal()

        Dim sub1 As Double
        subtotal = sub1 + itemamount
        Return (subtotal)

    End Function

    Private Function calctax()

        Dim subtotal As Double
        If chkTax.Checked Then
            tax = subtotal * 0.08
        End If
        Return (tax)

    End Function

    Private Function calctotal()

        total = subtotal + tax
        Return (total)

    End Function
    Sub displaytotal()

        Txtsubtotal.Text = subtotal
        txtTax.Text = tax
        TxtTotal.Text = total

    End Sub
End Class

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i dont know exactly where to go from here, i am getting 0 in all my textboxes, i dont really understand functions and subprocedures and byval and by ref, so can someone help me to figure out what i am doing wrong.
this is a site with the same project on it heres the link if it helps you to understand what i need to accomplish
http://www.shsu.edu/~mis_gab/MIS%202...Chapter8HW.pdf

thanks-

vb Code:

TxtQuantity.Text = quantity

???

shouldn't that be:

vb Code:

quantity = TxtQuantity.Text

first of all, those functions should have a return datatype. i.e.

vb Code:

Private Function calctotal() as double

the function calls in btnCalculate_Click are incorrectly written and need to be in the right order

vb Code:

itemamount = getitem()
tax = calctax()
subtotal = calcsubtotal()
total = calctotal()
displaytotal()

getItem should be a function with a return value

vb Code:

Function getitem()as string
 
     Dim quantity As Double
     quantity = TxtQuantity.Text 
     If Rdbcappuccino.Checked Then
        TxtItemAmount.Text = 2 * quantity
     ElseIf RdbEspresso.Checked Then
        TxtItemAmount.Text = 2.25 * quantity
     ElseIf rdbLatte.Checked Then
        TxtItemAmount.Text = 1.75 * quantity
     ElseIf RdbIcedLatte.Checked Then
        TxtItemAmount.Text = 2.5 * quantity
     ElseIf RdbIcedCappuccino.Checked Then
        TxtItemAmount.Text = 2.5 * quantity
     End If
     return TxtItemAmount.Text 
End Sub

thanks for the help
the quantity =
helped alot, but know when i run my program and hit the clear button it says i cant convert from string to double,
and the subprocedure and tax and total are all still at 0

change:

vb Code:

Function getitem()as string

to:

vb Code:

Function getitem()as double

&

vb Code:

return TxtItemAmount.Text

to:

vb Code:

return cdbl(TxtItemAmount.Text)

the converting from a string to a double problem is because the string must be a number for you to convert it to a double. if the textbox is empty, it isn't a number and can't be converted to a number. one way round this is to use double tryparse:

vb Code:

Function getitem() As Double
     Dim quantity As Double
     If Double.TryParse(TxtQuantity.Text, quantity) Then
        If Rdbcappuccino.Checked Then
           TxtItemAmount.Text = 2 * quantity
        ElseIf RdbEspresso.Checked Then
           TxtItemAmount.Text = 2.25 * quantity
        ElseIf rdbLatte.Checked Then
           TxtItemAmount.Text = 1.75 * quantity
        ElseIf RdbIcedLatte.Checked Then
           TxtItemAmount.Text = 2.5 * quantity
        ElseIf RdbIcedCappuccino.Checked Then
           TxtItemAmount.Text = 2.5 * quantity
        End If
        Return CDbl(TxtItemAmount.Text)
     else
        return 0 
     End If
End Function