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.

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

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

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

3. 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")
   

4. 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;

         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:

   ARTICLE-ID: Q42980
   TITLE     : (Complete) Tutorial to Understand IEEE Floating-Point Errors

   ARTICLE-ID: Q69333
   TITLE     : HOWTO: Work Around Floating-Point Accuracy/Comparison
               Problems