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Q129931: Using For Each to Loop Through Arrays Is Not Recommended

Article: Q129931
Product(s): Microsoft Visual Basic for Windows
Version(s): 4.0
Operating System(s): 
Keyword(s): kbcode
Last Modified: 05-FEB-2002

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The information in this article applies to:

- Microsoft Visual Basic Professional Edition, 16-bit, for Windows, version 4.0 
- Microsoft Visual Basic Professional Edition, 32-bit, for Windows, version 4.0 
- Microsoft Visual Basic Enterprise Edition, 16-bit, for Windows, version 4.0 
- Microsoft Visual Basic Enterprise Edition, 32-bit, for Windows, version 4.0 
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SUMMARY
=======

The For Each statement can be used on variant type arrays and collections that
contain generic object, variant, or OLE automation variables.

You can also use the For Each statement with an array that is not a variant type,
but the result is a substantial decrease in performance. This article shows by
example how to use the For Each statement to loop through collections and
demonstrates the decrease in performance that occurs when you use the For Each
statement to loop through arrays.

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.

Additional query words: 4.00 optimum optimal speedier faster quicker vb4win better best vb4all

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Keywords          : kbcode 
Technology        : kbVBSearch kbAudDeveloper kbVB400Search kbVB400 kbVB16bitSearch
Version           : :4.0

=============================================================================

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