VB 2012 Can someone help me understanding coding in this example

[Desparate_Noob]

My assignment is as follows:

• Collect and validate the number of items purchased and store the value in a variable
• Use a Do….While loop to iterate a number of times equal to the number of items purchased
• Collect the prices entered in an input box and store the values in an array
• The loop in the previous step should contain an accumulator to determine subtotal
• Create a module that applies a tax rate of 7% and calculate the total
• Report the results on a second form name results.vb (see figure 2)
• Assuming the user pays with a $100 bill, make change by using monetary denominations stored in an array. To loop through the values calculating change use a for next loop with a decreasing accumulator holding the value for change remaining
• Display change on a third form named change.vb (see fig. 3)

My subtotal is not calculating:

 Public Cost As Decimal = 0
    Public Total As Decimal = 0
    Public TotalCost As Decimal = 0


    Private Sub btnCheckOut_Click(sender As Object, e As EventArgs) Handles btnCheckOut.Click

        Dim i As Integer = 0

        Dim Prices(CInt(txtItems.Text) - 1) As Decimal

        Do While i < CInt(txtItems.Text)

            Prices(i) = CDec(InputBox("Put the value of your purchase", "Purchase Value", "0", 250, 250))

            'Call the Calculate function from the TaxCalc module (Included as comment for reference)
            'Module TaxCalc

            ' Declare constant for tax rate
            'Public Const decTAX_RATE As Decimal = 0.07D

            ' Declare a variable to hold the total cost
            'Dim TotalCost As Decimal = 0

            'Public Function Calculate(Cost, decTAX_RATE)
            'Cost += Cost * decTAX_RATE
            'Return TotalCost
            'End Sub

            'End Module


            TotalCost = Calculate(Cost, decTAX_RATE)

            TotalCost += Prices(i)
            i += 1
        Loop


        For i = 0 To Prices.Count - 1

            frmResults.lstResults.Items.Add(Prices(i))
        Next
        ' Display the results in the in the Subtotal label on form Results.vb

        frmResults.lblSubtotal.Text = TotalCost.ToString("$ " & TotalCost)

        frmResults.ShowDialog()

        ' With 2 items input at one dollar each, this results in Subtotal of $11.00 ????

    End Sub

[DataMiser]

A few things
1st off you are setting totalcost to the value returned from your function so it will never add the prices together it will instead hold the last price on the list.
Cost is defined as public in your module and local in your form so cost in the form code is a different variable than the one in the module.
Cost has not be assigned a value so the funtion will always return 0.

What you should be doing is passing price to the function that calculates the tax on the item then adding the return value to the total cost

So these two lines

TotalCost = Calculate(Cost, decTAX_RATE)
TotalCost += Prices(i)

become

TotalCost += Calculate(Prices(i), decTAX_RATE)

That said you really should not be calling the function in the loop but after the loop so that rather than calculate tax for each item it would caculate tax on the total so it would be.

    
    SubTotal += Prices(i)
     i += 1
 Loop

TotalCost = Calculate(subTotal, decTAX_RATE)

The way you are doing it does not provide a sub total as a sub total is the total before taxes

[Desparate_Noob]

Okay, so I made the following changes:

 ' Loop counter
        Dim intCount As Integer = 0

        ' Declare a variable to hold the Calculate function
        Dim Calculate As Decimal = 0

        'Declare a variable to hold the TotalCost
        Dim TotalCost As Decimal = 0

        Dim Prices(CInt(txtItems.Text) - 1) As Decimal

        Do While intCount < CInt(txtItems.Text)
            Prices(intCount) = CDec(InputBox("Put the value of your purchase", "Purchase Value", "0", 250, 250))

            'Call the Calculate function from the TaxCalc module (Included as comment for reference)
            'Module TaxCalc

            ' Declare constant for tax rate
            'Public Const decTAX_RATE As Decimal = 0.07D

            ' Declare a variable to hold the total cost
            'Dim TotalCost As Decimal = 0

            'Public Function Calculate(Subtotal, decTAX_RATE)
            'TotalCost = Subtotal + (Subtotal * decTAX_RATE)
            'Return TotalCost
            'End Sub

            'End Module


            frmResults.lblSubtotal.Text += Prices(intCount)
            intCount += 1
        Loop

        'Call the Calculate function
        TotalCost = Calculate(Subtotal, decTAX_RATE)

Now I get the following error:

'Call the Calculate function
        TotalCost = Calculate(Subtotal, decTAX_RATE)

**{Calculate} Expression is not an array or a method, and cannot have an argument list.**

This project is SOOO frustrating. I haven't even gotten to the form to display the change denominations . . . .

[DataMiser]

show the actual code rather than sticking it in as a comment.

Your function is not defined properly you need to give it a return type

Public Function Calculate(Subtotal, decTAX_RATE) As Decimal

The error kind of sounds like you have the code in the module commented out

[Desparate_Noob]

Module TaxCalc

    ' Declare constant for tax rate
    Public Const decTAX_RATE As Decimal = 0.07D

    ' Declare a variable to hold the total cost
    Dim TotalCost As Decimal = 0

    Public Function decCalculate(Subtotal, decTAX_RATE) As Decimal
        TotalCost = Subtotal + (Subtotal * decTAX_RATE)
        Return TotalCost
    End Function

End Module

    Public Subtotal As Decimal = 0

    Private Sub btnCheckOut_Click(sender As Object, e As EventArgs) Handles btnCheckOut.Click

        ' Loop counter
        Dim intCount As Integer = 0

        Dim Prices(CInt(txtItems.Text) - 1) As Decimal

        Do While intCount < CInt(txtItems.Text)
            Prices(intCount) = CDec(InputBox("Enter the value of your purchase", "Purchase Value", "0", 250, 250))

            frmResults.lblSubtotal.Text += Prices(intCount)
            intCount += 1
        Loop

        ' Declare a variable to hold the Calculate function
        Dim Calculate As Decimal = 0

        'Declare a variable to hold the TotalCost
        Dim TotalCost As Decimal = 0

        'Call the Calculate function
        TotalCost = decCalculate(Subtotal, decTAX_RATE)

        For intCount = 0 To Prices.Count - 1
            frmResults.lstResults.Items.Add(Prices(intCount))
        Next

        ' Display the results in the in the Subtotal label on form Results.vb

        frmResults.lblSubtotal.Text = TotalCost.ToString("$" & TotalCost)
        frmResults.ShowDialog()
    End Sub

Now I'm getting an error message for this line of code:

 frmResults.lblSubtotal.Text += Prices(intCount)

"Conversion from string "" to type 'Double' is not valid."

It was not giving this error message earlier.

[DataMiser]

Don't try to add directly to the text property. Instead you should use a variable that is of type decimal and add to that then when you are done with the loop you place the value in your textbox.

Textboxes are meant for display and entry. They are not meant to be used for math operations

[Desparate_Noob]

Thanks.

[GetOnMahLevel]

Did you figure it out?!?