PRB: Arguments Are Evaluated from Right to Left in Some Cases

MORE INFORMATION

Version   Parameter Array   Set Number of Arguments   Switch Function
---------------------------------------------------------------------
16-bit    Left to Right     Left to Right             Right to Left
32-bit    Right to Left     Left to Right             Right to Left
				

Steps to Reproduce Behavior When Using a ParamArray Argument

1. Create a new project. On Form1 (the default form), place one command button (Command1).
2. In the Click event for the Command1 button, place this code:

      Private Sub Command1_Click()
         y = Test(inc(1), inc(2), inc(3))
         Debug.Print y
      End Sub
   						

3. In the General section for Form1, place these two code segments:

      Function Test(ParamArray z()) As Integer
         Test = z(2)
      End Function
   
      Public Function inc(y As Integer) As Integer
         Static x As Integer
         x = x + 1
         inc = x + y
      End Function
   						

4. Run the program by pressing the F5 key, and click the Command1 button.
   
   The debug window prints out the number 4. If the arguments were evaluated from left to right, the same code would print out the number 6 because x would be incremented twice before the third argument was evaluated in the inc function.
   
   If a set number of arguments are passed to the Test function, then the arguments are evaluated from left to right. This is illustrated by rewriting the function Test as follows:

      Function Test(z0, z1, z2) As Integer
         Test = z2
      End Function
   						

   In this case, the debug window prints out 6.
   
   To fix the code, and still use a parameter array, you could change the code in the Click event of Command1 to evaluate the arguments first as in this example:

      Private Sub Command1_Click()
         X1 = inc(1)
         X2 = inc(2)
         x3 = inc(3)
         y = Test(X1, X2, X3)
         Debug.Print y
      End Sub
   						

Steps to Reproduce Behavior When Using the Switch Function

1. Create a new project. On Form1 (the default form), place one command button (Command1).
2. In the Click event for the Command1 button, place this code:

      Private Sub Command1_Click()
         Test
      End Sub
   						

3. In the General section for Form1, place these two code segments:

      Public Function inc(x As Integer, ByVal y As Integer) As Integer
         x = x + 1
         inc = x
         Debug.Print "x: "; x; "evaluated at position"; y
      End Function
   
      Public Sub Test()
         Dim x As Integer
         x = -1
         y = Switch(False, inc(x, 1), False, inc(x, 2), True, inc(x, 3))
         Debug.Print y
      End Sub
   						

4. Run the program by pressing the F5 key, and click the Command1 button. From the output, you can see that inc(x,3) evaluates first, then inc(x,2) evaluates, and finally inc(x,1) evaluates.