Linked Lists

I have a project going that is a part excersize to learn the basics of classes, and partly because I would like to use linked lists in another project. I would like create a linked list class from scratch that can create a node as well as perform certain operations to be determined later. I am going by the video tutorials at http://www.youtube.com/watch?v=Ju5q1hhFCso which is a first of 9 tutorials.

Below shows the class Cnode, and below that is a subroutine located in a module that tests the class. The cNode class I copied from either here, or somewhere on the internet. So far the class seems to work, I think. The only part that does not yet seem to work is I cannot set the next of node q to be Null in the text subroutine below. Does anyone have any idea why it is not working?

Option Explicit


'A node takes in 2 things.  A data value, and a reference to the next  node


Private pData As String
Private pNext As cNode


Public Property Get aData() As String
    aData = pData
End Property
Public Property Let aData(Value As String)
    pData = Value
End Property
Public Property Get Nxt() As cNode
    Set Nxt = pNext
End Property
Public Property Set Nxt(Value As cNode)
    Set pNext = Value
End Property


Private Sub cNode(ByRef pData As String, ByRef pNext As cNode)


Me.pData = pData
Me.pNext = pNext


End Sub

Sub testnode()


Dim h As cNode
Dim q As cNode




Set h = New cNode
Set q = New cNode


Set h.Nxt = q








h.aData = "First node"


q.aData = "Second Node"


MsgBox h.aData
MsgBox h.Nxt.aData


Set q.Nxt = Null








End Sub

Re: Linked Lists

Rory,
Thanks for takign a look at that. Setting the variable to Nothing seems to work fine.

Re: Linked Lists

[FONT=&quot]I had some time to watch the videos again and took the notes below. [/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot]my notes are not necessarily correct, some of the notes are actually unanswered questions. For anyone familiar with linked lists, if you see an error or understand something a different way please post it here.[/FONT]

Re: Linked Lists

Thanks. A binary tree sounds like something that I would study in the future as I am still working on understanding linked lists. The node class writtne above seems to work ok so far. I might try to bundle it with the drawnode subroutine to see if I can draw a node on the worksheet at the same time a node is created in the class so I can visually confirm how the process is working.

Re: Linked Lists

I don't know what those terms are. My intermediate goal is to create the node class above which I think is done, and then create the code for 4 or 5 different operations on a linked list which has yet to be done.

Re: Linked Lists

Those sites are were very good sources of infomration. One of them I know I have seen it before a couple a few years ago. I am seeing alot of applications for node data structures. I did not even know about those things until I tried and failed to write a program that would store data in a tree structure. GCExcel created one for me that works well and I want to see if I can do it once I get linked lists understood. I especially liked the one that talks about the pointers and the differences between languages that have a pointer type and VBA which does not.

Re: Linked Lists

I thought i would report back in...I have not had alot of time to move forward with this project but have been reading on classes and watching some videos. Two videos I have been watching to learn some fundamentals are below. Some of the concepts I have been working on
-What is a constructor and what it is purpose? So far my understanding is a constructor occurs when the object is initialized. For example

set Obj = new cSomeClass

In VBA cSomeClass can have a special portion called "Initialize" that will run code when the class is instanciated. The class gets instanciated at the expression = New cSomeClass

Wikipedia's definition of Constructor .... Constructor = A subroutine that meets 2 conditions 1. It is located within a Class. 2. It creates an Object.

-Does the object that the constructor creates have to be an object of the class it is located within?
-Does a constructor have to be run only when the class is initialized for the first time?

The videos are below, but they are in Java. I have had to watch them a few times but they are staring to make more sense.

http://www.youtube.com/watch?v=htzJdKoEmO0
http://www.youtube.com/watch?v=-c4I3gFYe3w

Re: Linked Lists

Eventually yes. The goal for these latest posts from me is not so much the project itself, but for me is having a better understanding classes. If I can complete a project where I make the linked list class myself, then use that class to complete a project involving trees/graphs, then I will have a better understanding of classes. My goal is to use general principals that can apply to most object oriented languages. I figure if the concepts are understood, the details will take care of themselves. The "constructor" seems to be a fundamentally important concept.

Re: Linked Lists

[FONT=&quot]Hello Again Ozgrid,[/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot]I have not been here for awhile and I unfortunately let this project stall out. I am reading about the subject again and watching videos - I happened to discover today one of the very best video series I have found yet that explains some basic concepts of OOP. The quality of the instruction is beyond anything I have seen on youtube for quiet awhile.[/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot]https://www.youtube.com/watch?…FEs41ueBVMW0WnumNhNO7xdxg[/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot]I will post here if I make any progress and I welcome any helpful comments/tips[/FONT]

Re: Linked Lists

I thought I had better spend some time on classes / linked lists again to try to get something done with that project. One of the points of studying linked lists for me is to try to understand classes a little better.

In VBA I created a module and in that a sub routine called cTest

Option Explicit


Sub cTest()


End Sub

I then created a class module and named it cNode

Option Explicit

So I have so far a subroutine that is empty, and a class that has no code. Of course no errors. I change the code to cTest to below

Sub cTest()


Dim c As cnode


Stop




End Sub

When I run the code for cTest, it breaks at the Stop as expected. What is unexpected to me, is that it shows that there exists what the locals window calls and Expression called c. It shows the value of c is Nothing and it shows that it has a type of cNode.

Question - After the expression

Dim c as cNode

was an object created? Does there now exist an object called c with a value of nothing? I have been reading about constructors, mostly in Java but some in VBA. A Class is only a blueprint for an object and when I created the Class cNode in a class module, I was only creating a blueprint for some future object. In order for the object to be created, it is "contstructed" using a constructor. My assumption is that I would have to explicitly state in a subroutine when it is time to create an object from a class. This is explicitly done by

Set c = New cNode

The New keyword in the above means I think the same thing as "Use the cNode blueprints to build and object as specified. When that object has been built, set a pointer from the variable c to that object.

The updated code is shown below

Sub cTest()


Dim c As cnode


Set c = New cnode


Stop




End Sub

This time, the locals window shows an Expression c, but it now has a + sign to the left of it. When expending that plus sign, nothing is shown. There is no longer a value associated directly with c anymore, instead it says "No Variables" where the plus sign opened up. c Still has a type as before, it is of type cNode. I start starting to understand this and the results in the locals window a bit better now. Earlier in this post when the code was as shown below

Sub cTest()


Dim c As cnode


Stop


End Sub

The c that was shown in the locals was not an object at all, it had never been constructed. It was being shown in a similar way as a string would be shown or an integer.

Re: Linked Lists

I am pretty sure that "Nothing" is only used when referring to objects. So with the code below

Sub cTest() 
     
    Dim c As cnode 
     
    Stop  
     
End Sub

An object must have been created, because the locals window says that c has a value of Nothing, which is a value that I think only applies to objects.

Q - Does Dim c as cNode create an object? If it does, then what is the point of set c = New cNode? Any comments appreciated

Re: Linked Lists

Currently the code in my Class module is

Option Explicit


Private pData As String


Public Property Let cData(value As String)
    pData = value
End Property


Public Property Get cData() As String
    cData = pData
End Property

The name of the class is cNode

The code in the module that utilizes the class is below

Option Explicit


Sub cTest()


Dim c As cnode


Set c = New cnode


c.cData = "abc"


Stop


End Sub

When I run this code, it breaks at the Stop as expected. In the locals window, c is shown with the plus to the left. When I open the plus, it then shows both pdata and cdata as expressions and each has a value of "abc" and a type of string.

Why is pData visible in the locals window? I thought it should be hidden since I used the "Private" keyword when I declared it in the class module?

When I typed c. intellisence correctly just showed the only possibility to be c.cData as an option, and did not show c.pData as an option, which seems correct.

Re: Linked Lists

Thanks, that's a good article I have read it. One of the reasons I am having some trouble is I am learning from books based on Java, which does not always translate well to VBA I guess because VBA is not a true OOP language. One of the things I would l ike to do in this node class is to be able to pass parameters.

Option Explicit


Sub cTest


Dim c as cNode


Set c = New cNode(SomeString,SomeObject)


End Sub

..But there does not appear to be an easy way to do this. There are workarounds but I am not understanding how those workarounds are working.

Instead, I am guessing the way to do it is like so

Option Explicit


Sub cTest


Dim c as cNode


Set c = New cNode


c.Data = "Test String"


Set c.Next = SomeObject


End Sub

Re: Linked Lists

After some reasearch I am pretty sure that the Locals windows ignores scope. If you run a sub that declares and instances a class that has some variables that are declared private, those variables still show up in the locals window. Good to know.