Comparison of Dictionary, Collections and Arrays
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
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
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:
