PRB: Format Function May Return Incorrect Values

• Microsoft Visual Basic Control Creation, Learning, Professional, and Enterprise Editions for Windows, version 5.0
• Microsoft Visual Basic Standard, Professional, and Enterprise Editions, 16-bit and 32-bit, for Windows, version 4.0

SYMPTOMS

Decimal values rounded by the Format function may be incorrect. For a specific decimal number, the rounded value returned by the Format function may differ for different data types and between different versions of Microsoft Visual Basic.

CAUSE

When a variable is passed into the Format function, an algorithm is used to determine the most appropriate numeric data-type to convert the value to, and how to format it. This algorithm may not evaluate certain decimal values as expected.

RESOLUTION

To avoid these unexpected conversions, explicitly convert the value to the desired data type before using the Format function. For instance, use the CCur function to convert the decimal value to a currency data-type that maintains more accuracy in rounding.

STATUS

Microsoft has confirmed this to be a problem in the Microsoft products listed at the beginning of this article. This problem does not occur in Visual Basic 6.0.

MORE INFORMATION

Decimal values stored in single and double data-types are binary representations of the number. Certain decimal values cannot be exactly represented in binary and are approximately represented. When evaluated by the Format function, these values may not be rounded as expected.

There are two methods recommended to minimize these errors:

1. Use the Currency data type rather than singles or doubles. Currency values are actually stored as integers with the decimal point scaled four places.



2. Convert the values to Currency before evaluation by the Format function.

Steps to Reproduce Behavior

1. In Microsoft Visual Basic, create a new Standard EXE project. Form1 is created by default.



2. On Form1, add a textbox (Text1) and three labels (Label1, Label2, and Label3).



3. Set the Text property of Text1 to a zero and the Caption property of each of the labels to nothing.



4. Add the following code to the Change event of the textbox:




      Dim dblValue As Double
      Dim sngValue As Single
      Dim curValue As Currency

      dblValue = Text1.Text
      sngValue = Text1.Text
      curValue = Text1.Text

      Label1.Caption = Format(dblValue, "#.000")
      Label2.Caption = Format(sngValue, "#.000")
      Label3.Caption = Format(curValue, "#.000") 

5. Run the project, and enter some decimal values in the textbox, such as:
   
   0.9125
   0.7125
   0.0915
   
   Note that the values displayed in the labels are different. The Format function rounds some values up and some down based upon the data-type. For instance, 0.9125 may be displayed as .912 and .913.

1. Use the CCur() function to convert the values to a Currency data type before using the Format() function: (This should not be used if the values will have more than 13 digits.)
   
   TextDblCur = Format(CCur(dblValue), "#.000")
   TextSngCur = Format(CCur(sngValue), "#.000")
   



2. Declare the variables as Currency.

REFERENCES

For additional information, please see the following articles in the Microsoft Knowledge Base:

(Complete) Tutorial to Understand IEEE Floating-Point Errors

HOWTO: Work Around Floating-Point Accuracy/Comparison Problems

Additional query words: Precision

Keywords : kbVBp400 kbVBp500 kbVBp600fix PrgOther VBKBProgramming
Version : WINDOWS:4.0,5.0
Platform : WINDOWS
Issue type : kbbug