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

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