i want to create a dynamic list. For example, i want to read a file and fill this dyanmic list. but i should be able to delete for this list too. I know i can use dynamic array but how do i delete item from the array and resize it.
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i want to create a dynamic list. For example, i want to read a file and fill this dyanmic list. but i should be able to delete for this list too. I know i can use dynamic array but how do i delete item from the array and resize it.
you can only delete elements from the back end. i.e., if you want to delete the third from the back, you'd have to delete the last three
you just redim the array with the Preserve keyword. A Collection is much better for a single element list
Look up collections in MSDN
Code:'this is rough but it should give you the idea
'you need.
Option Explicit
Private Sub Quit_Click()
'load your new array and save it to your file
'on next open you have your adjusted list
Dim myArr()
Dim i As Integer
Dim intNum As Integer
intNum = FreeFile
Dim myFile As String
myFile = "C:\my documents\myfile.txt"
Open myFile For Output As intNum
For i = 0 To List1.ListCount - 1
List1.ListIndex = i
ReDim Preserve myArr(i)
myArr(i) = List1.Text
Next i
For i = LBound(myArr) To UBound(myArr)
Print #intNum, myArr(i)
Next i
Close intNum
Unload Me
End Sub
Private Sub Form_Load()
'open your file and load your array and listbox
Dim myArr()
Dim i As Integer
Dim intNum As Integer
intNum = FreeFile
Dim myFile As String
myFile = "C:\my documents\myfile.txt"
Open myFile For Input As intNum
Do While Not EOF(intNum)
ReDim Preserve myArr(i)
Line Input #intNum, myArr(i)
i = i + 1
Loop
Close intNum
For i = LBound(myArr) To UBound(myArr)
List1.AddItem myArr(i)
Next i
End Sub
Private Sub List1_DblClick()
'delete the item from the list
List1.RemoveItem (List1.ListIndex)
End Sub
What you really want here is a linked list. A Linked list
means you do not have the overhead of shifting array
elements each time one in the middle is removed.
You can implement a linked list any number of ways. Using
classes is a good way because it allows you to totally
encapsulate all the code for accessing the list. I've put
a sample in here.
The clsElement class provides the payload of the list.
That is, the mData element is what we are trying to store.
The mNext and mPrev just allow us to know our place in the
list.
The clsList class keeps track of the head and tail of the
list, and makes sure that any adds or removes are handled
properly.
You would need to add an accessor to the class to make it
useful of course. Generally, you would provide an accessor
to get you to a specific element (an index) or if the list
was a queue or stack, the accessor would provide you the
next element (the head or tail depending on what it is).
It is a wise move to go to a structure like this (even
though my implementation written in 20 minutes late on
Sunday night might not be the best for you) because it is a
very very common structure in OO programming.
Code:'clsElement
Option Explicit
Dim mNext As clsElement
Dim mPrev As clsElement
Dim mData As Variant
Public Property Set NextElement(anElement As clsElement)
Set mNext = anElement
End Property
Public Property Set PrevElement(anElement As clsElement)
Set mPrev = anElement
End Property
Public Property Get NextElement() As clsElement
Set NextElement = mNext
End Property
Public Property Get PrevElement() As clsElement
Set PrevElement = mPrev
End Property
Public Property Let Data(vdata As Variant)
' this could be anything you want. If it is an object,
' then change this to a set method and change the Get as well
' you can also have more members to depict more data
mData = vdata
End Property
Public Property Get Data() As Variant
Data = mData
End Property
Code:'clsList
Option Explicit
Dim mHead As clsElement
Dim mTail As clsElement
Public Function Add(vdata As Variant) As clsElement
' add method to create and insert a new element
' to implement a sorted list, simply find the correct place
' in the list to place the new item
Dim anElement As clsElement
Dim tmp As clsElement
Set anElement = New clsElement
anElement.Data = vdata
If mHead Is Nothing Then
' must be the first element
Set mHead = anElement
Set mTail = anElement
Else
' not the first, so we'll add it to the tail end
Set mTail.NextElement = anElement
Set anElement.PrevElement = mTail
Set mTail = anElement
End If
Set Add = anElement
End Function
Public Function Remove(anElement As clsElement)
' simple remove method to remove an existing element
Dim tmp As clsElement
If anElement.PrevElement Is Nothing Then
' no previous element - must be the head
Set mHead = anElement.NextElement
Set mHead.PrevElement = Nothing
ElseIf anElement.NextElement Is Nothing Then
' no next element = must be the tail
Set mTail = anElement.PrevElement
Set mTail.NextElement = Nothing
Else
' somewhere in the middle
With anElement
Set tmp = .PrevElement
Set .PrevElement.NextElement = .NextElement
Set .NextElement.PrevElement = .PrevElement
Set .NextElement = Nothing
Set .PrevElement = Nothing
End With
End If
End Function
Private Sub Class_Terminate()
Dim tmp As clsElement
Do
Set tmp = mTail.PrevElement
Set mTail.PrevElement = Nothing
Set mTail = tmp
Loop Until mTail Is Nothing
End Sub
Public Sub PrintList()
Dim tmp As clsElement
Set tmp = mHead
Do
Debug.Print tmp.Data
Set tmp = tmp.NextElement
Loop Until tmp Is Nothing
End Sub
The example form is pretty weak in that it allows you toCode:' form1 with text box and 3 command buttons.
Option Explicit
Dim myList As clsList
Dim lastElement As clsElement
Dim keepElement As clsElement
Private Sub Command1_Click()
Set lastElement = myList.Add(Text1)
myList.PrintList
End Sub
Private Sub Command2_Click()
myList.Remove keepElement
End Sub
Private Sub Command3_Click()
Set keepElement = lastElement
End Sub
Private Sub Form_Load()
Set myList = New clsList
End Sub
Private Sub Form_Unload(Cancel As Integer)
Set lastElement = nothng
Set keepElement = Nothing
Set myList = Nothing
End Sub
easily add items, but to remove an item, you need to pass a
clsElement object. To do this, I use button 3 to
"remember" the item we want tot delete later. Then button
2 tries to delete the item.
If you need more info, post a message because I am off to
sleep..
Cheers