Using For Each to Loop Through Arrays Is Not Recommended (129931)

MORE INFORMATION

Although it is possible to use For Each on an array, there is rarely a reason to do so. You pay a substantial performance penalty on For Each statements. The For Each statement is advantageous in situations where the number of elements is unknown, but the number of elements in an array is almost always known. The only exceptions are arrays created by Visual Basic, such as ParamArray arguments (using the ParamArray keyword in a procedure creates arrays based on an indefinite number of arguments) and possibly control arrays.

For Each deals with objects rather than variables. A For Each on an array essentially does this:

   Dim MyArray(i to j) as Variant
   Dim VArray As Variant
   For i = LBound(MyArray) to UBound(MyArray)
      Set VArray = MyArray(i)
      ' Do something with VArray
   Next
				

Usually, For Each is used on arrays or collections of objects or variants. However, if you are working on an array of strings (Dim MyArray(i to j) as String in the above example) Visual Basic converts each string element to a variant object internally in order to use Set on it. That is, Visual Basic performs the equivalent of:

   Set VArray = CVar(MyArray(i))
				

Therefore, there is additional overhead when using For Each on arrays.

Steps to Demonstrate For Each versus For Next

1. Start a new project in Visual Basic. Form1 is created by default.
2. Add the following code to the Form1_Click event Procedure:

      Private Sub Form_Click()
         ForEach
      End Sub
   						

3. From the Insert Menu, choose Module to add Module1 to the project.
4. Add the following code to the general declarations section of Module1:

      #if Win32 Then
         Declare Function GetTickCount Lib "kernel32" () As Long
      #Else
         Declare Function GetTickCount Lib "User" () As Long
      #End If
   
      Public Sub ForEach()
         Dim MyArrayS(1 To 3000) As String
         Dim MyArrayI(1 To 3000) As Integer
         Dim MyArrayO(1 To 3000) As Object
         Dim MyArrayV(1 To 3000) As Variant
         Dim obj As Object
         Dim j As Integer
         Dim CurrElement, TheName, v
         Dim Date1 As Long, Date2 As Long, Date3 As Long
         Dim MyInt As New Collection
         Dim MyClasses As New Collection
   
         Date1 = GetTickCount()
         For i = 1 To 3000
            MyArrayS(i) = "dummy"
         Next
         Date2 = GetTickCount()
         For Each CurrElement In MyArrayS
            CurrElement = "dummy"
         Next
         Date3 = GetTickCount()
         Form1.Print "For i on String Array    " & CStr(Date2 - Date1)
         Form1.Print "For Each on String Array " & CStr(Date3 - Date2)
   
         Date1 = GetTickCount()
         For i = 1 To 3000
            MyArrayI(i) = 1
         Next
         Date2 = GetTickCount()
         For Each CurrElement In MyArrayI
            CurrElement = 1
         Next
         Date3 = GetTickCount()
         Form1.Print "For i on Integer Array    " & CStr(Date2 - Date1)
         Form1.Print "For Each on Integer Array " & CStr(Date3 - Date2)
   
         Date1 = GetTickCount()
         For i = 1 To 3000
            Set MyArrayO(i) = obj
         Next
         Date2 = GetTickCount()
         For Each CurrElement In MyArrayO
            Set CurrElement = obj
         Next
         Date3 = GetTickCount()
         Form1.Print "For i on Object Array    " & CStr(Date2 - Date1)
         Form1.Print "For Each on Object Array " & CStr(Date3 - Date2)
   
         Date1 = GetTickCount()
         For i = 1 To 3000
            MyArrayV(i) = v
         Next
         Date2 = GetTickCount()
         For Each CurrElement In MyArrayV
            CurrElement = v
         Next
         Date3 = GetTickCount()
         Form1.Print "For i on Variant Array    " & CStr(Date2 - Date1)
         Form1.Print "For Each on Variant Array " & CStr(Date3 - Date2)
   
         For i = 1 To 500
            MyInt.Add Item:=j, Key:=CStr(i)
         Next
         Date1 = GetTickCount()
         For i = 1 To 500
            j = MyInt.Item(i)
         Next
         Date2 = GetTickCount()
         For Each CurrElement In MyInt
            j = CurrElement
         Next
         Date3 = GetTickCount()
   
         Form1.Print "For i on Integer Collection    " & CStr(Date2 - Date1)
         Form1.Print "For Each on Integer Collection " & CStr(Date3 - Date2)
   
         For i = 1 To 500
            Dim Inst As New Class1
            TheName = "Mike" & CStr(i)
            Inst.InstanceName = TheName
            MyClasses.Add Item:=Inst, Key:=CStr(i)
            Set Inst = Nothing
         Next
         Date1 = GetTickCount()
         For i = 1 To 500
            Set a = MyClasses.Item(i)
         Next
         Date2 = GetTickCount()
         For Each CurrElement In MyClasses
            Set a = CurrElement
         Next
         Date3 = GetTickCount()
   
         Form1.Print "For i on Object Collection    " & CStr(Date2 - Date1)
         Form1.Print "For Each on Object Collection " & CStr(Date3 - Date2)
      End Sub
   						

5. From the Insert Menu, choose Class Module to add Class1 to the project.
6. Add the following code to the general declarations section of Class1:

      Public InstanceName
   
   						

7. Run the program.

In general, the results illustrate why you should use the For...Next statement on arrays and use the For Each...Next statement on collections.