DOCUMENT:Q174155  24-SEP-1997  [vbwin]
TITLE   :BUG: Format Function May Return Incorrect Values
PRODUCT :Microsoft Visual Basic for Windows
PROD/VER:WINDOWS:4.0,5.0
OPER/SYS:WINDOWS
KEYWORDS:PrgOther VB4ALL VB4WIN vb5all VBKBProgramming

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The information in this article applies to:

 - 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
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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 bug in the Microsoft products listed
at the beginning of this article. We are researching this problem and will
post new information here in the Microsoft Knowledge Base as it becomes
available.

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.

To correct this problem and to keep all values consistent, do one of the
following:

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

Additional query words: Precision
======================================================================
Keywords          : PrgOther VB4ALL VB4WIN vb5all VBKBProgramming
Version           : WINDOWS:4.0,5.0
Platform          : WINDOWS
Issue type        : kbbug

=============================================================================

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Copyright Microsoft Corporation 1997.