PRB: Type Conversion Functions Can Return Unexpected Results

ID: Q195657

The information in this article applies to:

Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, versions 5.0, 6.0
Microsoft Visual Basic Standard, Professional, and Enterprise Editions, 16-bit and 32-bit, for Windows, version 4.0

SYMPTOMS

Type conversion functions, such as CInt, may sometimes return unexpected results. For example, CInt(2.5) should return 2 according to the documentation. However, CInt(25.0 * 0.1) will sometimes result in 3.

CAUSE

This is caused by the way floating point values are handled in computers. Because most floating point values cannot be accurately represented with fixed length binary values, the internal result of the floating point calculation may differ slightly. For example, 25.0 * 0.1 may be 02.5000... 001 or 02.4999...999 depending on many factors. This rounding error causes the CInt function to return 2 or 3 in different situations.

RESOLUTION

This is expected behavior and there is no fix for this. However, you can make these functions return the expected values more often by using wrapping functions. For example, you can use the following function instead of using CInt:


   Private Function MyCInt(ByVal dTemp As Double) As Integer
      MyCInt = CInt(dTemp)
   End Function

This works by forcing the expression to be evaluated and stored into an intermediate variable. This forces rounding, which often discards the small difference between the correct value and the real value in the computer. This method can also be used for CLng, etc.

STATUS

This behavior is by design.

MORE INFORMATION

Steps to Reproduce Behavior

Create a Standard EXE project in Visual Basic. Form1 is created by default.

Add two CommandButtons to Form1 and keep the default names, Command1 and Command2.

Add the following code to the code window of Form1:


      Option Explicit

      Private Function MyCInt(Byval dTemp As Double) As Integer
         MyCInt = CInt(dTemp)
      End Function

      Private Sub Command1_Click()
         Dim t As Single, s As Single
         Dim z As Integer, h As Integer
         Dim Msg As String

         Msg = "Using CInt for these calculations:" & vbCrLf
         t = 25#
         s = t * 0.1
         Msg = Msg & "The result of 25.0 * 0.1 = " & s & vbCrLf

         h = CInt(s)
         Msg = Msg & "Where t = 25#, and s = t * 0.1, the result of " _
         & "CInt(t * 0.1) = " & h & vbCrLf

         z = CInt(t * 0.1)
         Msg = Msg & "And the result of CInt(t * 0.1) = " & z

         MsgBox Msg
      End Sub

      Private Sub Command2_Click()
         Dim t As Single, s As Single
         Dim z As Integer, h As Integer
         Dim Msg As String

         Msg = "Using MyCInt for these calculations:" & vbCrLf
         t = 25#
         s = t * 0.1
         Msg = Msg & "The result of 25.0 * 0.1 = " & s & vbCrLf

         h = MyCInt(s)
         Msg = Msg & "Where t = 25#, and s = t * 0.1, the result of " _
         & "MyCInt(t * 0.1) = " & h & vbCrLf

         z = MyCInt(t * 0.1)
         Msg = Msg & "And the result of MyCInt(t * 0.1) = " & z

         MsgBox Msg
      End Sub

Press the F5 key to run the application. Click Command1 and the result of CInt(2.5 * 0.1) is 3 (incorrect). Click Command2 and the result of MyCInt(2.5 * 0.1) is 2 (correct).

REFERENCES

For more information regarding precision and accuracy in floating point calculations, please see the following articles in the Microsoft Knowledge Base:

Q125056 INFO: Precision and Accuracy in Floating-Point Calculations

Q145889 INFO: Why Floating Point Numbers May Lose Precision

Additional query words:

Keywords : kbFloatPoint kbVBp kbVBp400 kbVBp500 kbVBp600 kbGrpVB kbCodeSam
Version : WINDOWS:4.0,5.0,6.0
Platform : WINDOWS
Issue type : kbprb