Comparison of Dictionary, Collections and Arrays

Question

I am trying to work out the relative benefits and features of dictionaries compared with collections and arrays.

I found an excellent article here but can\'t find a

Answer 1

Option Explicit

Sub CollectionsVSdictionaries() ' Requires ref to "Microsoft Scripting Runtime" Library
    Dim c As Collection         ' TypeName 1-based indexed
    Dim d As Dictionary         ' 0-based arrays
    Set c = New Collection      ' or: "Dim c As New Collection"
    Set d = New Dictionary      ' or: "Dim d As New Dictionary"

    c.Add Key:="A", Item:="AA": c.Add Key:="B", Item:="BB": c.Add Key:="C", Item:="CC"
    d.Add Key:="A", Item:="AA": d.Add Key:="B", Item:="BB": d.Add Key:="C", Item:="CC"

    Debug.Print TypeName(c)    ' -> "Collection"
    Debug.Print TypeName(d)    ' -> "Dictionary"

    Debug.Print c(3)            ' -> "CC"
    Debug.Print c("C")          ' -> "CC"
    'Debug.Print c("CC")       ' --- Invalid ---

    Debug.Print d("C")          ' -> "CC"
    Debug.Print d("CC")        ' Adds Key:="CC", Item:=""
    Debug.Print d.Items(2)      ' -> "CC"
    Debug.Print d.Keys(2)       ' -> "C"
    Debug.Print d.Keys()(0)     ' -> "A"    - Not well known ***************************
    Debug.Print d.Items()(0)    ' -> "AA"   - Not well known ***************************

    'Collection methods:
    '    .Add                   ' c.Add Item, [Key], [Before], [After] (Key is optional)
    '    .Count
    '    .Item(Index)           ' Default property;   "c.Item(Index)" same as "c(Index)"
    '    .Remove(Index)
    'Dictionary methods:
    '    .Add                   ' d.Add Key, Item (Key is required, and must be unique)
    '    .CompareMode           ' 1. BinaryCompare     - case-sensitive   ("A" < "a")
    '    .CompareMode           ' 2. DatabaseCompare   - MS Access only
    '    .CompareMode           ' 3. TextCompare       - case-insensitive ("A" = "a")
    '    .Count
    '    .Exists(Key)           ' Boolean **********************************************
    '    .Item(Key)
    '    .Items                 ' Returns full array: .Items(0)(0)
    '    .Key(Key)
    '    .Keys                  ' Returns full array: .Keys(0)(0)
    '    .Remove(Key)
    '    .RemoveAll             ' ******************************************************
End Sub

Answer 2

Please see the table below for a useful comparison of collections and dictionaries.

(The table summarises this page up to the section on "Early And late binding". FYI the page also has more detailed info about using dictionaries)

In summary it's usually best to use a dictionary or an array.

When considering using collections it may be more appropriate to use an array if the size does not change, or changes only rarely. In this case an array is likely to be more efficient than a collection as Arrays are very efficient to populate and retrieve all items at once (eg. range to array and array back to range).

Also note:

Compared to Arrays, collections offer good performance for adding and inserting items, and accessing and removing them by their Keys. However, performance is poor if items are to be accessed by index. For information about doing this efficiently see here which also discusses the inner workings of these list objects.

This cpearson page has has very useful code for working with dictionaries, collections and arrays (sorting them, and also converting them to be each other!)

Some text from cpearson's page:

The Collection object and the Dictionary object are very useful for storing groups of related data. All else being equal, I use a Dictionary object rather than a Collection object because you have access (read, write, change) to the Key property associated with an Item in the Dictionary. In a rather poor object design, the Key of an item in a Collection is write-only. You can assign a Key to an Item when you add the Item to the Collection, but you cannot retrieve the Key associated with an Item nor can you determine (directly) whether a key exists in a Collection. Dictionaries are much friendly and open with their keys. Dictionaries are also considerably faster than Collections.

Why can arrays be a bad choice. Arrays are much slower at re-sizing and inserting items in the middle as each Redim copies the entire memory block to a larger location, and if Preserve is used, all values copied over as well. This may translate to perceived slowness for every operation - in a potential application)

Collections vs. Dictionaries in VBA

Feature                 | COLLECTION | DICTIONARY | Remark
------------------------+------------+------------+--------------------------------
Usually faster          |            |     X      | 
------------------------+------------+------------+--------------------------------
Supported by VB Script  |            |     X      | Collections do not exist in VBS.
------------------------+------------+------------+--------------------------------
                        |            |            | Dicts: Add ref to Miscrosoft 
Native to VBA           |     X      |            | Scripting Library. Usage:
                        |            |            | Dim MyDict As Scripting.Dictionary
                        |            |            | Set MyDict = New Scripting.Dictionary
------------------------+------------+------------+--------------------------------
Can change Keys and     |            |            | Dict properties are writable.
Items                   |            |     X      | For collections, remove the item
                        |            |            | and add a new item.
------------------------+------------+------------+--------------------------------
                        |            |            | A collection enumerates its items:
                        |            |            |  For Each x In MyCollection
                        |            |            |      Debug.Print x
Enumerated              |     X      |     X      |  Next x
                        |            |            | A dict enumerates its keys:
                        |            |            |  For Each x In MyDictionary
                        |            |            |      Debug.Print MyDictionary.Item(x)
                        |            |            |  Next x
------------------------+------------+------------+--------------------------------
                        |            |            | A 1-d array of keys 
Directly output to      |            |            | and items can be returned by 
array                   |            |     X      | dict methods .Keys and .Items.
                        |            |            | (The array is zero-based even 
                        |            |            |  with Option Base 1.)
------------------------+------------+------------+--------------------------------
Retrieve and access     |     X      |     X      |
items                   |            |            |  
------------------------+------------+------------+--------------------------------
Add items               |     X      |     X      |
------------------------+------------+------------+--------------------------------
Implicitly add items    |            |     X      | Dicts can implicitly add items 
                        |            |            | using .Item property.
------------------------+------------+------------+--------------------------------
Remove items            |     X      |     X      |
------------------------+------------+------------+--------------------------------
Remove all items in     |            |            | With collections, each item must
one step                |            |     X      | be removed in turn, or the 
                        |            |            | collection destroyed and recreated.
------------------------+------------+------------+--------------------------------
Count items             |     X      |     X      |
------------------------+------------+------------+--------------------------------
Return item using key   |     X      |     X      |
as lookup value         |            |            |
------------------------+------------+------------+--------------------------------
Return item using       |            |            |
ordinal position        |     X      |   (Slow)   |
as lookup value         |            |            |
------------------------+------------+------------+--------------------------------
Return ordinal          |            |            |
position using item     |     X      |     ??     |
as lookup value         |            |            |
------------------------+------------+------------+--------------------------------
Retrieve and access     |            |     X      | Collection keys only used to
keys                    |            |            | look up data, not retrievable.
------------------------+------------+------------+--------------------------------
Keys optional           |     X      |            | Big + of collections, assuming keys
                        |            |            | are not needed. (Access via index.)
------------------------+------------+------------+--------------------------------
Case sensitivity        |            |     X      |
optional                |            |            |  
------------------------+------------+------------+--------------------------------
                        |            |            | Collection keys must be strings.
Keys can be any type    |            |     X      | Dict keys can have any type
                        |            |            | (except arrays), incl. mixed types.
------------------------+------------+------------+--------------------------------
Keys must be unique     |     X      |     X      |
------------------------+------------+------------+--------------------------------
                        |            |            | * For collections, add code:
                        |            |            |  Public Function _
                        |            |            |     Contains(col As Collection, _
Supports .Exists method |  Remark*   |     X      |     key As Variant) As Boolean
                        |            |            |     On Error Resume Next
                        |            |            |     col(key)
                        |            |            |     Contains = (Err.Number = 0)
------------------------+------------+------------+--------------------------------
Preserve key order when |            |     X      | This is because collection keys 
sorting by item value   |            |            | are write-only, not read. Poor design!

The original image, which has more information and is more clearly arranged:

Answer 3

Option Explicit

Sub UpdateSummary()

    Dim varData
    Dim objDicCountry As Object
    Dim objDicCity As Object
    Dim objDicData As Object
    Dim lngR As Long
    Dim varResult
    Dim lngC As Long
    Dim strKey As String
    Dim varUnique

    varData = Sheet12.Range("A1").CurrentRegion
    Set objDicCity = CreateObject("Scripting.Dictionary")
    Set objDicCountry = CreateObject("Scripting.Dictionary")
    Set objDicData = CreateObject("Scripting.Dictionary")

    For lngR = LBound(varData) + 1 To UBound(varData)

        strKey = varData(lngR, 1) '--Country
        objDicCountry.Item(strKey) = ""

        strKey = varData(lngR, 2) '--City
        objDicCity.Item(strKey) = ""

        strKey = varData(lngR, 1) & "|" & varData(lngR, 2) '--Country and City
        objDicData.Item(strKey) = objDicData.Item(strKey) + varData(lngR, 3)

    Next lngR

    ReDim varResult(1 To objDicCountry.Count + 1, 1 To objDicCity.Count + 1)

    varUnique = objDicCountry.keys '--get Unique Country

    For lngR = LBound(varUnique) To UBound(varUnique)
        varResult(lngR + 2, 1) = varUnique(lngR)
    Next lngR

    varUnique = objDicCity.keys '--get Unique City

    For lngC = LBound(varUnique) To UBound(varUnique)
        varResult(1, lngC + 2) = varUnique(lngC)
    Next lngC


    For lngR = LBound(varResult) + 1 To UBound(varResult)
        For lngC = LBound(varResult) + 1 To UBound(varResult, 2)
            strKey = varResult(lngR, 1) & "|" & varResult(1, lngC) '--Country & "|" & City
            varResult(lngR, lngC) = objDicData.Item(strKey)
        Next lngC
    Next lngR

    Sheet12.Range("F6").Resize(UBound(varResult), UBound(varResult, 2)).Value = varResult
    MsgBox "Done", vbInformation

End Sub

Answer 4

With regards to the performance of collections versus dictionaries, I am finding that writing to dictionaries performs similarly to writing to collections, reading from a dictionary takes about twice as long as reading from a collection. Creating a dictionary in the first place is way slower than creating a collection.

These are results I got for doing 100,000 iterations of reading from, writing to, and creating dictionaries/collections:

Creating Multiple Dictionaries:   731ms
Writing To Dictionary:            494ms
Reading From Dictionary:           65ms

Creating Multiple Collections:     29ms
Writing To Collection:            459ms
Reading From Collection:           26ms

Note adding a reference to the Microsoft Scripting Runtine improves the speed of creating multiple dictionaries (to 495ms here).

This is the code I used for testing this:

Option Explicit

Private p_lngTestCount As Long

Sub SetUp()
  p_lngTestCount = 100000
End Sub

Sub TestAll()
  CreatingMultipleDictionaries
  WritingToDictionary
  ReadingFromDictionary

  CreatingMultipleCollections
  WritingToCollection
  ReadingFromCollection
End Sub

Sub CreatingMultipleDictionaries()

  Const sSOURCE As String = "CreatingMultipleDictionaries"

  Dim oPerfMon As CDevPerformanceMonitor
  Set oPerfMon = New CDevPerformanceMonitor
  Dim i As Long
  Dim dcTest As Dictionary
  SetUp

  Dim dblTimeElapsed As Double
  oPerfMon.StartCounter

  For i = 0 To p_lngTestCount
    'Set dcTest = CreateObject("Scripting.Dictionary")
    Set dcTest = New Dictionary
  Next i

  dblTimeElapsed = oPerfMon.TimeElapsed

  Debug.Print sSOURCE & ": " & p_lngTestCount & " iterations. " & vbCrLf & _
              "Time elapsed: " & Round(dblTimeElapsed, 0) & "ms" & vbCrLf
End Sub

Sub CreatingMultipleCollections()

  Const sSOURCE As String = "CreatingMultipleCollections"

  Dim oPerfMon As CDevPerformanceMonitor
  Set oPerfMon = New CDevPerformanceMonitor
  Dim i As Long
  Dim colTest As Collection
  SetUp

  Dim dblTimeElapsed As Double
  oPerfMon.StartCounter

  For i = 0 To p_lngTestCount
    Set colTest = New Collection
  Next i

  dblTimeElapsed = oPerfMon.TimeElapsed

  Debug.Print sSOURCE & ": " & p_lngTestCount & " iterations. " & vbCrLf & _
              "Time elapsed: " & Round(dblTimeElapsed, 0) & "ms" & vbCrLf
End Sub

Sub WritingToDictionary()

  Const sSOURCE As String = "WritingToDictionary"

  Dim oPerfMon As CDevPerformanceMonitor
  Set oPerfMon = New CDevPerformanceMonitor
  Dim i As Long
  Dim dcTest
  SetUp

  Set dcTest = CreateObject("Scripting.Dictionary")
  'Set dcTest = New Dictionary

  Dim dblTimeElapsed As Double
  oPerfMon.StartCounter

  For i = 0 To p_lngTestCount
   ' Performance about the same for both ways:
    dcTest.Item(CStr(i)) = "test"
    'dcTest.Add CStr(i), "test"
  Next i

  dblTimeElapsed = oPerfMon.TimeElapsed

  Debug.Print sSOURCE & ": " & p_lngTestCount & " iterations. " & vbCrLf & _
              "Time elapsed: " & Round(dblTimeElapsed, 0) & "ms" & vbCrLf
End Sub

Sub WritingToCollection()

  Const sSOURCE As String = "WritingToCollection"

  Dim oPerfMon As CDevPerformanceMonitor
  Set oPerfMon = New CDevPerformanceMonitor
  Dim i As Long
  Dim colTest As Collection
  SetUp

  Dim dblTimeElapsed As Double
  Set colTest = New Collection

  oPerfMon.StartCounter

  For i = 0 To p_lngTestCount
    colTest.Add "test", CStr(i)
  Next i

  dblTimeElapsed = oPerfMon.TimeElapsed

  Debug.Print sSOURCE & ": " & p_lngTestCount & " iterations. " & vbCrLf & _
              "Time elapsed: " & Round(dblTimeElapsed, 0) & "ms" & vbCrLf
End Sub

Sub ReadingFromDictionary()

  Const sSOURCE As String = "ReadingFromDictionary"

  Dim oPerfMon As CDevPerformanceMonitor
  Set oPerfMon = New CDevPerformanceMonitor
  Dim i As Long
  Dim dcTest
  SetUp

  Set dcTest = CreateObject("Scripting.Dictionary")
  'Set dcTest = New Dictionary
  dcTest.Add "key", "test"

  Dim stTest As String
  Dim dblTimeElapsed As Double

  oPerfMon.StartCounter

  For i = 0 To p_lngTestCount
    stTest = dcTest.Item("key")
  Next i

  dblTimeElapsed = oPerfMon.TimeElapsed

  Debug.Print sSOURCE & ": " & p_lngTestCount & " iterations. " & vbCrLf & _
              "Time elapsed: " & Round(dblTimeElapsed, 0) & "ms" & vbCrLf
End Sub

Sub ReadingFromCollection()

  Const sSOURCE As String = "ReadingFromCollection"

  Dim oPerfMon As CDevPerformanceMonitor
  Set oPerfMon = New CDevPerformanceMonitor
  Dim i As Long
  Dim colTest As Collection
  SetUp

  Dim stTest As String
  Dim dblTimeElapsed As Double
  Set colTest = New Collection
  colTest.Add "test", "key"

  oPerfMon.StartCounter

  For i = 0 To p_lngTestCount
    stTest = colTest.Item("key")
  Next i

  dblTimeElapsed = oPerfMon.TimeElapsed

  Debug.Print sSOURCE & ": " & p_lngTestCount & " iterations. " & vbCrLf & _
              "Time elapsed: " & Round(dblTimeElapsed, 0) & "ms" & vbCrLf
End Sub

Performance monitor class (CDevPerformanceMonitor):

Option Explicit

' Performance monitoring used in logging
' See: https://stackoverflow.com/questions/198409/how-do-you-test-running-time-of-vba-code

Private Type LARGE_INTEGER
  lowpart As Long
  highpart As Long
End Type

#If VBA7 Then
  Private Declare PtrSafe Function QueryPerformanceCounter Lib "kernel32" (lpPerformanceCount As LARGE_INTEGER) As Long
#Else
  Private Declare Function QueryPerformanceCounter Lib "kernel32" (lpPerformanceCount As LARGE_INTEGER) As Long
#End If

#If VBA7 Then
  Private Declare PtrSafe Function QueryPerformanceFrequency Lib "kernel32" (lpFrequency As LARGE_INTEGER) As Long
#Else
  Private Declare Function QueryPerformanceFrequency Lib "kernel32" (lpFrequency As LARGE_INTEGER) As Long
#End If

Private m_CounterStart As LARGE_INTEGER
Private m_CounterEnd As LARGE_INTEGER
Private m_crFrequency As Double

Private Const TWO_32 = 4294967296#               ' = 256# * 256# * 256# * 256#

Private Function LI2Double(LI As LARGE_INTEGER) As Double
  Dim Low As Double
  Low = LI.lowpart
  If Low < 0 Then
    Low = Low + TWO_32
  End If
  LI2Double = LI.highpart * TWO_32 + Low
End Function

Private Sub Class_Initialize()
  Dim PerfFrequency As LARGE_INTEGER
  QueryPerformanceFrequency PerfFrequency
  m_crFrequency = LI2Double(PerfFrequency)
End Sub

Public Sub StartCounter()
  QueryPerformanceCounter m_CounterStart
End Sub

Public Function PerformanceCount() As Double
  Dim liPerformanceCount As LARGE_INTEGER
  QueryPerformanceCounter liPerformanceCount
  PerformanceCount = LI2Double(liPerformanceCount)
End Function

Public Function MicroTime() As Double
  MicroTime = Me.PerformanceCount * 1000000# / m_crFrequency
End Function

Public Property Get TimeElapsed() As Double
  Dim crStart As Double
  Dim crStop As Double
  QueryPerformanceCounter m_CounterEnd
  crStart = LI2Double(m_CounterStart)
  crStop = LI2Double(m_CounterEnd)
  TimeElapsed = 1000# * (crStop - crStart) / m_crFrequency
End Property