Binary Tree

Hi, I wander if anyone can help me with this enquiry. I am not looking for anyone to provide me with the answer as I am a student and want to resolvemost of this myself but would appreciate it if someone could point in the direction of where I am going wrong her.

I have an assignment to store details of car registration details and cost in a binary tree. I am using visual basic and have created public type to contain the registration, cost and also a left and right pointer to show how each peice of data links together in the binary tree.

My problem is sthat I can add a car registration and cost to the array but I can't assign the required left or right pointer to the data in the array. I have copied the code I have so far below and any advice would be greatefully appreciated.

Option Explicit

Private Type CarReg
Reg As String * 7
Cost As Single
LP As Integer
RP As Integer
End Type

Dim r(0 To 20) As CarReg
Dim counter As Integer
Dim root As Integer
Dim i As Integer

Private Sub cmdAdd_Click()
'check number of characters
If txtCarReg.Text = "" Or Len(txtCarReg.Text) > 7 Then
MsgBox "Please enter a valid registration"
End If
'check if this is the first piece of data
If counter = 0 Then
r(counter).Reg = txtCarReg.Text
r(counter).Cost = Val(txtCarCost.Text)
root = counter
counter = counter + 1
Else
'find another space
r(counter).Reg = txtCarReg.Text
r(counter).Cost = Val(txtCarCost.Text)
counter = counter + 1
If r(counter).Reg > r(root).Reg And r(root).RP = -1 Then
For counter = 1 To 20
If r(counter).Reg = "qqqqqqq" Then
' we found a space
r(0).RP = counter
r(counter).Reg = txtCarReg.Text
r(counter).Cost = Val(txtCarCost.Text)
counter = 21
Else
MsgBox "The tree is full!!!"
End If
Next counter
End If

If r(counter).Reg > r(root).Reg And r(root).LP = -1 Then
For counter = 1 To 20
If r(counter).Reg = "qqqqqqq" Then
' we found a space
r(0).LP = counter
r(counter).Reg = txtCarReg.Text
r(counter).Cost = Val(txtCarCost.Text)
counter = 21
Else
MsgBox "The tree is full!!!"
End If
Next counter
End If
End If



Call printit
End Sub

Private Sub Form_Load()
For i = 0 To 20
r(i).Reg = "QQQQQQQ"
r(i).LP = -1
r(i).RP = -1
Next i
root = -1
counter = 0
Call printit


End Sub

Private Sub printit()
Cls
Print "No. Reg Cost LP RP"
For i = 0 To 20
Print i; Tab; r(i).Reg; Tab; r(i).Cost; Tab; r(i).LP; Tab; r(i).RP
Next i
Print ""
Print "Root: " & root
Print "Count: " & counter
End Sub