[2005] Manual pointers in VB

[Abrium]

Hello everyone,

Been a while but I have a question about creating manual pointers and the logic needed to stick a pointer on the head of the list and the tail of the list. I have the general idea but the logic I am trying to use just isn't working out. Please observe.

Public Class Form1
    Public PointerTracker As Nodei
    Public FrontP As Nodei
    Public TempP As Nodei

        Dim GetData As String
        GetData = txtGetData.Text

        Dim Node As New Nodei
        Node = Nothing
        Node = New Nodei


        Node.DataHolder = GetData
        Node.IndexP = PointerTracker
        PointerTracker = Node

Public Class Nodei
    Public DataHolder As String
    Public IndexP As Nodei
End Class

The simple code above will produce a linked list. I have declared the additional variables to hold the positions temp and front but in all honesty I can't find the logic that will allow me to put conditions against any of the class Nodei that I have created without receiving object null reference error.

Yep this is homework. Don't need answers as much as suggestions. Been a while since I have moved around an IDE and am new to data structures. Any ideas?

[Abrium]

I am still chopping away at this and I can't even find words to convey my frustration with trying to get pointers and linked lists to work in VB but I am solving it 1 problem at a time. I am now faced with an if statement that is being ignored and I can't figure out why because VS isn't throwing me any exceptions.

Code:

 If FrontP.Data Is Nothing Then
                    MessageBox.Show("thank god")
                    FrontP.Data = GetData
                    FrontP.NextP = Nothing
                    TempP = FrontP

When debugging the following code is simply being stepped right over. I called a new instance of my node class, being FrontP, and when I debug and step through I can not seem to make Vs evaluate FrontP as being nothing, empty, null, or 0. Any suggestions?

[bigMeUp]

i am sure i am going to end up with egg on my face, but, if you want a linked list why don't you just use LinkedList (systems.collections.generic)?

[Shaggy Hiker]

Cause it's homework, as he said.

So when you step onto the If statement, and use Shift+F9 or the tooltip to look at the value, what do you see?

Also, what is FrontP.Data? I see that you have declared FrontP to be of type Nodei in the first snippet, but there is no Data member or property to that class in the first snippet.

[bigMeUp]

so i was right about the egg, then?

dude, you gonna need explicit constructors so that class instances know who their parents are, and destructors, if you want to prevent removed items from breaking the chain.

as its your homework, i don't know whether or not the code below goes too far, but if you think it does, then DON'T read it .. .. .. innit?

'   non mea culpa...
    Public Sub DoNodes()
        Dim n As New Nodesi
        Dim nn As New Nodesi(n)
        MsgBox(n.GetData & vbNewLine & nn.GetParent.GetData)
    End Sub
 
    Public Class Nodesi
        Private _parent As Nodesi
 
        Public Sub New()
        End Sub
        Public Sub New(ByVal parent As Nodesi)
            _parent = parent
        End Sub
 
        Public Function GetData() As String
            Return CStr(Me.GetHashCode)
        End Function
 
        Public Function GetParent() As Nodesi
            If _parent Is Nothing Then Return Me Else Return _parent
        End Function
    End Class

[syntaxeater]

I'm assuming you're trying to set up a line where one object references another and can continue on into infinity. If so, look over this:

Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Dim tailObject, middleObject, leadObject As Nodei
 
        leadObject = New Nodei
        leadObject.DataHolder = "Lead"
 
        middleObject = New Nodei
        middleObject.DataHolder = "Middle"
        middleObject.Follows = leadObject
 
        tailObject = New Nodei
        tailObject.DataHolder = "Tail"
        tailObject.Follows = middleObject
    End Sub
 
    Public Class Nodei
        Public DataHolder As String
        Public Follows As Nodei
    End Class

[Shaggy Hiker]

so i was right about the egg, then?

Not even enough egg to make a reasonable breakfast, but since you asked for it, I supplied what was available

[Abrium]

Thanks for all of the help guys. I'm still rolling on this. This is what I have so far. I just need to understand the structure itself before I can use the pointers to their fullest potential. This is what I have so far. It is acting like a queue and its suppose to, sorry I omitted that in my first post:

General From wide declarations:

 Public GetData As String
    Public FrontP As Node
    Public RearP As Node

Button_click event for adding a new node:

Dim CurrP As New Node
Dim Newnode As New Node

  CurrP = Newnode
        CurrP.Data = GetData
        CurrP.NextP = Nothing
        If RearP Is Nothing Then
            RearP = CurrP
            FrontP = CurrP
        Else
            RearP.NextP = CurrP
            RearP = CurrP
        End If

Now I'm working on scanning the list for the first node, once the current data equals the front data I am deleting from the front. This section is not working thus far.

        Do Until CurrP Is FrontP
            CurrP.NextP = RearP
            CurrP = RearP
            If FrontP Is CurrP Then
                Exit Do

            End If
        Loop

I'm still working on it. Linked Lists have to be the hardest thing for me to fully grasp since I have started coding. **** is giving me a headache.

[syntaxeater]

If you want, try this approach. It's a little more digestible for what you want.

Public Class Node
        Public Data As String
        Public NextP As Node
    End Class
    Private myNodes As Generic.List(Of Node)
    Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        myNodes = New Generic.List(Of Node)
    End Sub
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim newNode As New Node
        If myNodes.Count > 0 Then
            myNodes(myNodes.Count - 1).NextP = newNode
        End If
        newNode.Data = GetData()
        myNodes.Add(newNode)
    End Sub

To test, use the following:

Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        Dim display As New System.Text.StringBuilder
        For Each hold As Node In myNodes
            If hold.NextP IsNot Nothing Then  'LastOn won't have a reference to append.
                display.AppendLine(hold.Data & " ref: " & hold.NextP.Data)
            Else
                display.AppendLine(hold.Data & " ref: N/A")
            End If
        Next
        MessageBox.Show(display.ToString)
    End Sub

The difference is the code above will help you keep track of your order by using a collection. When a new item is added, the previous item's "NextP" is assigned to it.

[syntaxeater]

And just for your reference... Taking into account the same concept as above, but building on the object; you could do something like the following:

Private myNodes As Generic.SortedList(Of Int32, Node)
    Public Class Node
        Private m_data As String
        Private m_node As Node
#Region "Constructors"
        Public Sub New()
            Me.New(String.Empty, Nothing)
        End Sub
        Public Sub New(ByVal data As String)
            Me.New(data, Nothing)
        End Sub
        Public Sub New(ByVal node As Node)
            Me.New(String.Empty, node)
        End Sub
        Public Sub New(ByVal data As String, ByVal node As Node)
            m_data = data
            m_node = node
 
            m_currentInstance = Me
            If node Is Nothing Then node = m_currentInstance _
                Else node.Node = m_currentInstance
        End Sub
#End Region
#Region "Members"
        Public Property Data() As String
            Get
                Return m_data
            End Get
            Set(ByVal value As String)
                m_data = value
            End Set
        End Property
        Public Property Node() As Node
            Get
                If Not Object.ReferenceEquals(m_currentInstance, Me) Then Return m_node
                Return Nothing
            End Get
            Set(ByVal value As Node)
                m_node = value
            End Set
        End Property
#End Region
#Region "Singleton Members"
        Private Shared m_currentInstance As Node
        Protected Friend Shared ReadOnly Property CurrentInstance() As Node
            Get
                Return m_currentInstance
            End Get
        End Property
#End Region
    End Class
    Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        myNodes = New Generic.SortedList(Of Int32, Node)
    End Sub
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        myNodes.Add(myNodes.Count, New Node(GetData, Node.CurrentInstance))
    End Sub

Testing is more or less the same code:

Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        Dim display As New System.Text.StringBuilder
        For Each hold As KeyValuePair(Of Int32, Node) In myNodes
            If hold.Value.Node IsNot Nothing Then
                display.AppendLine(hold.Value.Data & " ref: " & hold.Value.Node.Data)
            Else
                display.AppendLine(hold.Value.Data & " ref: N/A")
            End If
        Next
        MessageBox.Show(display.ToString)
    End Sub

I would deffinitely encourage you to look at both. Even if you're not completely comfortable with the above - ask questions and learn right. .Net and all that it encompasses is an OOP language. It's not until you learn how to work with, manipulate and enhance these objects that the framework starts to show its real potential.

[Seraph]

The whole point of VB is so you DON'T have to do this kind of stuff right?
I can understand in C++ you're teacher making you make this stuff and then telling you about the STL, but in VB? What's the point?
Unless you wanna make a custom one? But I thought Linked Lists were Linked Lists.

[syntaxeater]

The whole point of VB is so you DON'T have to do this kind of stuff right?
I can understand in C++ you're teacher making you make this stuff and then telling you about the STL, but in VB? What's the point?
Unless you wanna make a custom one? But I thought Linked Lists were Linked Lists.

The point of VB is for RAD-type projects. While historically, VB could have been referred to as a lazy professional's language - this has deffinitely not been the case with VB.Net. Can you slof off with VB.Net and only utilize what is immediately available? Yeah - very easily.

But keep in mind these two things:
A) You're working with a framework. Code isn't being rewritten for each application and those types of projects will start to become exceedingly rare. The IDE's functionality with design time drag/drop has become so easy to use - most companies/projects could teach people who are not IT savy to build their own layouts and basic functionality (say... the design department).
B) Your world has collided with people who don't use lazy languages. J#, C#, C++, the linux community via Mono - all of them are players and contributers now. While you may create your object or application with the idea "it's good enough for VB," these people not only live for the details... They revel in finding ways make eachother's code look sloppy, inadequate or slow.

As for the LinkedList being a LinkedList - yes; that's true. But when you get into professional n-tier applications; you'll hear alot about BLL. These are objects that describe a business model.

In a simple example... You have employees and they belong to a department. The department is an object that contains a collection with references to all of these employees. Now, say you want to find all of the Employees who are currently in the office for this department. You could take the collection, do a for loop and check the status of each employee... But every application and process that required this functionality would have to duplicate the code. If you had an object called Department that housed the collection - you could place the for/loop inside of it and all the developer requesting it would need to do is call the function that returned a filtered collection.

That demonstrates the convenience of this model, but there is also a practicality side to it. Say your boss finds out that all of these lowly employees know his status. This infuriates him and he only wants people who are employeeType 2 or greater to be able to see this information. Using just a LinkedList, each and every place people copied that for/loop would need to be updated. But if you had a department object in place providing a way to query on that precise data - you only need to modify it with this new business rule. The changes would scale out to all applications/processes and no more effort would be needed.

[Seraph]

I see. Thanks for taking the time to explain that.
I've been doing stuff like this since HighSchool, getting my start doing HTML at 13 and then PHP in HS then before doing VB6 in 12th grade for a class and then going beyond that on myown. Now I'm in college for programming and I may have taught myself some stuff over the past 10 or so years but I don't know much about the realities of the business world just yet.
And thanks for the wiki links too.