Question

        Dim index As Integer
        Dim choice As String
        Dim total As Integer

        total = 0

        index = NumericUpDown1.Value


        Dim arr(4) As Integer
        arr(0) = 10
        arr(1) = 5
        arr(2) = 21
        arr(3) = 33




        If index > 0 Then
            choice = (Combobox1.SelectedItem.ToString + " x " + NumericUpDown1.Value.ToString)
            ListBox1.Items.Add(choice)
            CheckedListBox1.Items.Add(choice)
            total += arr(Combobox1.SelectedIndex) * index
            TotalLabel.Text = total.ToString()


      Else
            MsgBox("error.")

        End If

I can calculate the total of single choice, but fail to accumulate to sum.
What’s wrong of the code?

Current Situation:
Step 1:
choose arr(0), index = 2
total = 20

Step 2:
choose arr(2), index = 1
total = 21

Correct Situation:
Step 1:
choose arr(0), index = 2
total = 20

Step 2:
choose arr(2), index = 1
total = 41

Answer 1

You’ll need a either a global variable or a class with a public variable. You should create a Transaction class to store the data about the transaction and probably a Product class to store the data about the product. What you put in it is up to you, but I’d start out with something like this:

Public Class Transaction
    Private _productsList As List(of Product)
    Private _transationNumber As Integer
    '...more stuff...

    'you'll want to remember what products are in your "cart" for the transaction
    Public Property ProductsList As List(of Product)
        'your get/set accessors
    End Property
    Public Property TransactionNumber As Integer
        'your get/set accessors
    End Property
    Public Property TotalTransactionCost() As Double
        Get 
            'this will sum of the prices of all of the products you have stored in your 
            'list of products for this transaction
            Return _productsList.Sum(product => product.Price)
        End Get
    End Property

    Public Sub New()
        '...constructor stuff
    End Sub
    Public Sub AddProductToTransaction(byval product)
        _productsList.Add(product)
    End Sub

End Class

Public Class Product
    Private _price As Double
    Private _productName As String
    Private _UPC As String

    Public Property Price() As Double
        'your get/set accessors
    End Property
    Public Property ProductName() As String
        'your get/set accessors
    End Property
    Public UPC As String () As String
        'your get/set accessors
    End Property

    Public Sub New()
       'constructor stuff
    End Sub
End Class

These are a couple class shells to get you started. If you’re serious about making a product, this is a step in the right direction. If you’re going to write code, write it the right way.

If you’re just looking for a quick and dirty solution, you can declare a global variable and just keep a running sum. Just don’t forget to clear it out before you start a new transaction.

You’ll want to do something like:

Private TransactionCost As Double in your form outside of all your methods.

Again, I would recommend the first way of going about things. You’ll need at least those two classes and they’ll definitely be more fleshed out for a real product.

I hope this helps and answers your question. If it does, hit me with an upvote and accept the answer. Welcome to SO.