Question

I declare my 2D lists:

Dim _invoiceitems As New List(Of List(Of String))
Dim _dbitems As New List(Of List(Of String))

Each List is filled like this:

Example Code To fill:

_invoiceitems.Add(New List(Of String))
_invoiceitems(0).Add("Code #")
_invoiceitems(0).Add("Quantity")

Well, now i need a third list called (_changesitems) Note that this result with the differences:
be the result of subtraction of quantities if this is found (dbitems – invoiceitems).

How i can get this result?

Answer 1

The following code will generate the results you are looking for:

Private Function getChangesItems(ByVal invoiceItems As Dictionary(Of String, Integer), ByVal dbItems As Dictionary(Of String, Integer)) As Dictionary(Of String, Integer)
    Dim changesItems As Dictionary(Of String, Integer) = New Dictionary(Of String, Integer)()
    Dim allCodes As List(Of String) = New List(Of String)()
    allCodes.AddRange(invoiceItems.Keys)
    allCodes.AddRange(dbItems.Keys)
    For Each code As String In allCodes
        If Not changesItems.ContainsKey(code) Then
            Dim dbQuantity As Integer = 0
            Dim invoiceQuantity As Integer = 0
            If dbItems.ContainsKey(code) Then
                dbQuantity = dbItems(code)
            End If
            If invoiceItems.ContainsKey(code) Then
                invoiceQuantity = invoiceItems(code)
            End If
            Dim changeQuantity As Integer = dbQuantity - invoiceQuantity
            If changeQuantity <> 0 Then
                changesItems.Add(code, changeQuantity)
            End If
        End If
    Next
    Return changesItems
End Function

I used dictionaries instead of lists as was recommended by others. As long as your data only contains a code and a value, the dictionary is a better fit. If you have more columns, I would suggest creating a class that contains properties for each column and then make a list of that class type, rather than a simple 2D list of strings. Doing so would be more type-safe and easier to read.