Does VBA have Dictionary Structure?

Solution 1:

Yes.

Set a reference to MS Scripting runtime ('Microsoft Scripting Runtime'). As per @regjo's comment, go to Tools->References and tick the box for 'Microsoft Scripting Runtime'.

Create a dictionary instance using the code below:

Set dict = CreateObject("Scripting.Dictionary")

or

Dim dict As New Scripting.Dictionary 

Example of use:

If Not dict.Exists(key) Then 
    dict.Add key, value
End If 

Don't forget to set the dictionary to Nothing when you have finished using it.

Set dict = Nothing 

Solution 2:

VBA has the collection object:

    Dim c As Collection
    Set c = New Collection
    c.Add "Data1", "Key1"
    c.Add "Data2", "Key2"
    c.Add "Data3", "Key3"
    'Insert data via key into cell A1
    Range("A1").Value = c.Item("Key2")

The Collection object performs key-based lookups using a hash so it's quick.

You can use a Contains() function to check whether a particular collection contains a key:

Public Function Contains(col As Collection, key As Variant) As Boolean
    On Error Resume Next
    col(key) ' Just try it. If it fails, Err.Number will be nonzero.
    Contains = (Err.Number = 0)
    Err.Clear
End Function

Edit 24 June 2015: Shorter Contains() thanks to @TWiStErRob.

Edit 25 September 2015: Added Err.Clear() thanks to @scipilot.

Solution 3:

VBA does not have an internal implementation of a dictionary, but from VBA you can still use the dictionary object from MS Scripting Runtime Library.

Dim d
Set d = CreateObject("Scripting.Dictionary")
d.Add "a", "aaa"
d.Add "b", "bbb"
d.Add "c", "ccc"

If d.Exists("c") Then
    MsgBox d("c")
End If

Solution 4:

An additional dictionary example that is useful for containing frequency of occurence.

Outside of loop:

Dim dict As New Scripting.dictionary
Dim MyVar as String

Within a loop:

'dictionary
If dict.Exists(MyVar) Then
    dict.Item(MyVar) = dict.Item(MyVar) + 1 'increment
Else
    dict.Item(MyVar) = 1 'set as 1st occurence
End If

To check on frequency:

Dim i As Integer
For i = 0 To dict.Count - 1 ' lower index 0 (instead of 1)
    Debug.Print dict.Items(i) & " " & dict.Keys(i)
Next i

Solution 5:

Building off cjrh's answer, we can build a Contains function requiring no labels (I don't like using labels).

Public Function Contains(Col As Collection, Key As String) As Boolean
    Contains = True
    On Error Resume Next
        err.Clear
        Col (Key)
        If err.Number <> 0 Then
            Contains = False
            err.Clear
        End If
    On Error GoTo 0
End Function

For a project of mine, I wrote a set of helper functions to make a Collection behave more like a Dictionary. It still allows recursive collections. You'll notice Key always comes first because it was mandatory and made more sense in my implementation. I also used only String keys. You can change it back if you like.

Set

I renamed this to set because it will overwrite old values.

Private Sub cSet(ByRef Col As Collection, Key As String, Item As Variant)
    If (cHas(Col, Key)) Then Col.Remove Key
    Col.Add Array(Key, Item), Key
End Sub

Get

The err stuff is for objects since you would pass objects using set and variables without. I think you can just check if it's an object, but I was pressed for time.

Private Function cGet(ByRef Col As Collection, Key As String) As Variant
    If Not cHas(Col, Key) Then Exit Function
    On Error Resume Next
        err.Clear
        Set cGet = Col(Key)(1)
        If err.Number = 13 Then
            err.Clear
            cGet = Col(Key)(1)
        End If
    On Error GoTo 0
    If err.Number <> 0 Then Call err.raise(err.Number, err.Source, err.Description, err.HelpFile, err.HelpContext)
End Function

Has

The reason for this post...

Public Function cHas(Col As Collection, Key As String) As Boolean
    cHas = True
    On Error Resume Next
        err.Clear
        Col (Key)
        If err.Number <> 0 Then
            cHas = False
            err.Clear
        End If
    On Error GoTo 0
End Function

Remove

Doesn't throw if it doesn't exist. Just makes sure it's removed.

Private Sub cRemove(ByRef Col As Collection, Key As String)
    If cHas(Col, Key) Then Col.Remove Key
End Sub

Keys

Get an array of keys.

Private Function cKeys(ByRef Col As Collection) As String()
    Dim Initialized As Boolean
    Dim Keys() As String

    For Each Item In Col
        If Not Initialized Then
            ReDim Preserve Keys(0)
            Keys(UBound(Keys)) = Item(0)
            Initialized = True
        Else
            ReDim Preserve Keys(UBound(Keys) + 1)
            Keys(UBound(Keys)) = Item(0)
        End If
    Next Item

    cKeys = Keys
End Function