ACC97: Do...Loop That Uses Negative Logic Reverses Behavior of AND and OR Operators

ID: Q236370


The information in this article applies to:
  • Microsoft Access 97

Advanced: Requires expert coding, interoperability, and multiuser skills.


SYMPTOMS

When you use a Do...Loop that contains negative logic in the criteria of either the loop or in any logical test within the loop, the logical operators AND and OR reverse their behavior. For example, if you were to use either of the following code structures


Do While MyVar1 <> Val1 And MyVar2 <> Val2
   ' Perform some action.
Loop 
-or-

Do Until rs.EOF
  If MyVal1 <> Val1 And MyVal2 <> Val2 Then
   ' Perform some action.
  End If
  rs.MoveNext
Loop 
the loop would end when either condition is true, rather than when both are true. If you were to use the OR operator instead of AND in the preceding code structures, the loop would not end until both conditions were true.


RESOLUTION

Use positive logic in the criteria line. For example, instead of using logic such as


Do While MyVar1 <> Val1 And MyVar2 <> Val2
   ' Perform some action.
Loop 
use logic such as:

Do Until MyVar1 = Val1 And MyVar2 = Val2
   ' Perform some action.
Loop 


MORE INFORMATION

Steps to Reproduce Behavior

  1. Open the sample database Northwind.mdb.


  2. Open the Products table in Datasheet view, and then check to make sure you have a record with ProductName = Longlife Tofu and ProductID = 74.


  3. Add a new record with the following values:


  4. 
       ProductID = 78
       ProductName = Longlife Tofu 
  5. Insert a new module and type the following code:


  6. 
    Option Compare Database
    Option Explicit
    
    Public Sub DoWhileIf()
    
       Dim db As DAO.Database
       Dim rs As DAO.Recordset
    
       Set db = CurrentDb
       Set rs = db.OpenRecordset("Select * from Products")
    
       With rs
          Do Until .EOF
             If !ProductName <> "Longlife Tofu" And !ProductID <> 78 Then
                .MoveNext
             Else
                Debug.Print "Found it! " & !ProductName & " " & !ProductID
                Exit Sub
             End If
          Loop
       End With
    
       rs.Close
       Set rs = Nothing
       Set db = Nothing
    
    End Sub
    
    Public Sub DoWhileAnd()
    
       Dim db As DAO.Database
       Dim rs As DAO.Recordset
    
       Set db = CurrentDb
       Set rs = db.OpenRecordset("Select * from Products")
    
       With rs
          Do While !ProductName <> "Longlife Tofu" And !ProductID <> 78
             .MoveNext
          Loop
          Debug.Print "Found it! " & !ProductName & " " & !ProductID
       End With
    
       rs.Close
       Set rs = Nothing
       Set db = Nothing
    
    End Sub
    
    Public Sub DoWhileOr()
    
       Dim db As DAO.Database
       Dim rs As DAO.Recordset
    
       Set db = CurrentDb
       Set rs = db.OpenRecordset("Select * from Products")
    
       With rs
          Do While !ProductName <> "Longlife Tofu" Or !ProductID <> 78
             .MoveNext
          Loop
          Debug.Print "Found it! " & !ProductName & " " & !ProductID
       End With
    
       rs.Close
       Set rs = Nothing
       Set db = Nothing
    
    End Sub
    
    Public Sub DoWhilePos()
    
       Dim db As DAO.Database
       Dim rs As DAO.Recordset
    
       Set db = CurrentDb
       Set rs = db.OpenRecordset("Select * from Products")
    
       With rs
          Do Until !ProductName = "Longlife Tofu" And !ProductID = 78
             .MoveNext
          Loop
          Debug.Print "Found it! " & !ProductName & " " & !ProductID
       End With
    
       rs.Close
       Set rs = Nothing
       Set db = Nothing
    
    End Sub 
  7. In the Debug window, type the following line, and then press ENTER:


  8. 
    DoWhileIf 
    Note that the code unexpectedly exits the loop on the record where ProductID = 74.

  9. In the Debug window, type the following line, and then press ENTER:


  10. 
    DoWhileAnd 
    Note that the code unexpectedly exits the loop on the record where ProductID = 74.

  11. In the Debug window, type the following line, and then press ENTER:


  12. 
    DoWhileOr 
    Note that the code unexpectedly exits the loop on the record where ProductID = 78.

  13. In the Debug window, type the following line, and then press ENTER:


  14. 
    DoWhilePos 
    Note that the code exits the loop, as expected, on the record where ProductID = 78.

Additional query words: prb opposite reversed

Keywords : kbdta PgmOthr
Version : WINDOWS:97
Platform : WINDOWS
Issue type : kbprb


Last Reviewed: July 9, 1999
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