Linked List Tutorial
By Tegid 23:48, 4 May 2006 (EDT)
Unfortunate Realities
Before we get started, let me address some unfortunate realities. The code I wrote earlier won't work. You can't call
set Curr.Child to Something
On a non-persistent reference. That means I can't call it on any of the objects I create in game. So that causes a serious problem. Fortunately there is a way to address it.
Creating a Real World Linked List
We create a Persistent Reference that the Nodes can talk to and pass data through. I call the base object of this Persistent Reference the DSTDataMgr. So a very basic DSTDataMgrScript will look like this
scn DSTDataMgrScript
ref Head ;This script will store the head of my list
ref ChildRef ;This is somewhere to store the data we need to know as a
;child node
long Number_of_Nodes ;It is helpful to know this information and it allows us to
;number the nodes
Now I know how to store the ChildRef that the Prev Node needs to store, but how do I tell the Prev Node that it needs to set it's Child Node? We add one more line to the DSTDataMgrScript
short setChild
Then we need to add an OnActivate block to the DSTDataMgrNode. This is an easy way to tell a node to do something. We will also need to define a new Persistent Reference DSTDataMgr. Do this by dragging a DSTDataMgr into a gameworld Cell (I like to put it by the exit to the Imperial Sewers (Tamriel->ICPrisonSewerExit01) but that's just personal taste). Then name it pDSTDataMgrRef and check the Persistent reference checkbox. Now we can access it in our DSTDataNodeScript
scn DSTDataNodeScript
ref Child
long Number
begin OnActivate
if (pDSTDataMgrRef.setChild == 1)
set Child to pDSTDataMgrRef.ChildRef
set Number to pDSTDataMgrRef.Number_of_Nodes
set pDSTDataMgrRef.Number_of_Nodes to pDSTDataMgrRef.Number_of_Nodes + 1
set pDSTDataMgrRef.ChildRef to 0
set pDSTDataMgrRef.setChild to 0
return
endif
end
With these two scripts we can set up our list like so
scn DSTTestScript1
short count
ref NewNodeRef
ref Prev
begin GameMode
if (count < 20)
set count to count + 1
set NewNodeRef to PlaceAtMe DSTDataNode 1, 0, 0
if (Prev == 0)
set pDSTDataMgrRef.Head to NewNodeRef
else
set pDSTDataMgrRef.ChildRef to NewNodeRef
set pDSTDataMgrRef.setChild to 1
Prev.Activate Prev 1
endif
set Prev to NewNodeRef
endif
end
But there are a couple of problems. Not the least of these is that even though I've created the list, I have absolutely no way to get any information out of it. Also, The last node won't have it's Number set, because the number doesn't get set until the child node get's set. That is easily fixed by using an OnLoad block added to the DSTDataNodeScript. The OnLoad block will run when the Node is created and the model loaded into memory. While we are modifying the script, let's add a Parent reference and set it.
scn DSTDataNodeScript
ref Child
ref Parent
long Number
begin OnLoad
set Number to pDSTDataMgrRef.Number_of_Nodes
set pDSTDataMgrRef.Number_of_Nodes to pDSTDataMgrRef.Number_of_Nodes + 1
set Parent to pDSTDataMgrRef.ParentRef
set pDSTDataMgrRef.ParentRef to 0
end
begin OnActivate
if (pDSTDataMgrRef.setChild == 1)
set Child to pDSTDataMgrRef.ChildRef
set pDSTDataMgrRef.ChildRef to 0
set pDSTDataMgrRef.setChild to 0
return
endif
end
To allow for this we need to add just two lines of code, one in the DSTDataMgrScript and one in DSTTestScript1 which we will now call DSTTestScript2
In DSTDataMgrScript we add
ref ParentRef
and DSTTestScript2 should now look like this
scn DSTTestScript2
short count
ref NewNodeRef
ref Prev
begin GameMode
if (count < 20)
set count to count + 1
;If there is no Prev ref, then the DataMgr should be the parent
if (Prev == 0)
set pDSTDataMgrRef.ParentRef to pDSTDataMgrRef
else
set pDSTDataMgrRef.ParentRef to Prev
endif
set NewNodeRef to PlaceAtMe DSTDataNode 1, 0, 0
if (Prev == 0)
set pDSTDataMgrRef.Head to NewNodeRef
else
set pDSTDataMgrRef.ChildRef to NewNodeRef
set pDSTDataMgrRef.setChild to 1
Prev.Activate Prev 1
endif
set Prev to NewNodeRef
endif
end
Retrieving Data
So, we have a this wonderful list, but absolutely no way to get any data from it. So how do we solve this problem? Well, first, we need to add a flag to the DSTDataMgr like the setChild flag, so that we can check for it in our DSTDataNode's OnActivate block. Then we need a place to store the contents of the Node that we are returning. It would probably also be good for the DSTDataNode to be able to set a flag that says the data has been returned. So we add
short dataRequest short dataAvail long Data
Now we need to add to the OnActivate block of the DSTDataNodeScript to handle this new request. We add this.
if (pDSTDataMgrRef.dataRequest)
set pDSTDataMgrRef.Data to Number
set pDSTDataMgrRef.dataAvail to 1
set pDSTDataMgrRef.dataRequest to 0
return
endif
So, when a node receives a request for data, it will put it's Number into the Persistent References Data field so that an external script can read it. Then it will inform that script that the data is there by setting the dataAvail to 1.
There is still one problem. We have no way to traverse the list for information. We could ask the Head to return it's data by using the following code
set pDSTDataMgrRef.dataRequest to 1
ref lHead
set lHead to pDSTDataMgrRef.Head
if (lHead != 0)
lHead.Activate lHead 1
endif
Please note that the activating reference (the value after Activate and before the 1) doesn't mean anything and is unimportant. (The 1 means use the default activation, or run the OnActivate script, don't for instance pick the object up. More details can be found on the Activate page).
But we have no way to get to nodes further down the list and to do that, we are going to have to decide what kind of list we are.
Types of Lists
Lists
We have been talking about linked lists this whole time surely. Yes, but there are other types of Linked Lists. To traverse a List, you have what is called an Iterator that moves down the list and points at each node in turn. In this case it is as simple as a reference variable and a little new code. In DSTDataMgrScript we add
short getNext ref Current
in DSTDataNodeScript, we modify the dataRequest if statement like this
if (pDSTDataMgrRef.dataRequest)
set pDSTDataMgrRef.Data to Number
set pDSTDataMgrRef.dataAvail to 1
set pDSTDataMgrRef.dataRequest to 0
if (pDSTDataMgrRef.getNext)
set pDSTDataMgrRef.Current to Child
set pDSTDataMgrRef.getNext to 0 ;This is not NECESSARY, but I like to
;clean up after myself
endif
return
endif
;We also need to add this block in case we just want to move down the list
;but don't want the data in this node
if (pDSTDataMgrRef.getNext)
set pDSTDataMgrRef.Current to Child
set pDSTDataMgrRef.getNext to 0
return
endif
So, in this model to retrieve the numbers of each node, we write a new DSTTestScript3. Assume that DSTTestScript2 has already been run.
scn DSTTestScript3
ref Curr
short count
long Num
begin GameMode
if (pDSTDataMgrRef.dataAvail == 1)
set Num to pDSTDataMgrRef.Data
set pDSTDataMgrRef.dataAvail to 0
Message "I have received the data for Node #%.0f", Num, 1
endif
if (count < 20)
set count to count + 1
;This code checks to see if this is the first iteration
if (pDSTDataMgrRef.Current == 0)
set pDSTDataMgrRef.Current to pDSTDataMgrRef.Head
if (pDSTDataMgrRef.Current == 0) ;We need to make sure there are elements in
;the list
Message "There are no nodes in the list", 1
return
endif
endif
set Curr to pDSTDataMgrRef.Current
set pDSTDataMgrRef.getNext to 1
set pDSTDataMgrRef.dataRequest to 1
Curr.Activate Curr 1
endif
end
Now we should print out a message for each of the nodes in order as we traverse down the list. Of course these Nodes don't contain any real data, but we could store a Reference or 6 references or a reference and a location or anything inside of a node as long as the equivalent variable ares on our Persistent Reference so that when a node is asked for it's data, it can put them somewhere that other scripts can reference them.
Queues
Coming Soon
Stacks
Coming Soon
Sorted Lists
Coming Soon
Cleaning Up
Coming Soon