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Q114136: BUG: FoxPro Math-Precision Problems with SET DECIMALS

Article: Q114136
Product(s): Microsoft FoxPro
Version(s): MS-DOS:2.0,2.5x,2.6x; WINDOWS:2.5x,2.6x,3.0,3.0b,5.0,5.0a,6.0
Operating System(s): 
Keyword(s): kbvfp500aBUG kbvfp250bug kbvfp250aBUG kbvfp250bBUGkbbuglist
Last Modified: 09-FEB-2000

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The information in this article applies to:

- Microsoft Visual FoxPro for Windows, versions 3.0, 3.0b, 5.0, 5.0a, 6.0 
- Microsoft FoxPro for Windows, versions 2.5x, 2.6x 
- Microsoft FoxPro for MS-DOS, versions 2.0, 2.5x, 2.6x 
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SYMPTOMS
========

SET DECIMALS TO 2 changes the result when a numeric expression that evaluates to
a decimal value is used in a comparison. For example, type the following in the
Command window:

     SET DECIMAL TO 18
     ? 12/8888888 = 0     && FoxPro returns FALSE
   
     SET DECIMALS TO 2
     ? 12/8888888 = 0     && FoxPro returns TRUE

Under dBASE IV, both of these expressions will return FALSE.

In FoxPro version 2.6a, a similar decimal-precision problem occurs when any whole
number is used in math problems. For example:

     SET DECIMALS TO 18
     ?12/8888888       && dBASE returns 0.00000135... but FoxPro returns 0
   
     ?12/88888888      && dBASE returns 0.0000000000... but FoxPro returns 0
   
     ?12.0/8888888     && dBASE returns 0.00000135... but FoxPro returns 0

However the behavior of FoxPro versions 2.5x, 2.6 and Visual FoxPro is closer to
that of dBase. For example:

     SET DECIMALS TO 18
     ?12/8888888       && FoxPro returns 0.000001350000135000
   
     ?12/88888888      && FoxPro returns 0
   
     ?12.0/8888888     && FoxPro returns 0.000001350000135000

RESOLUTION
==========

If you need greater precision in FoxPro, set the SET DECIMALS command to a
higher value. For greater precision with constants, replace the constants with
memory variables, so that FoxPro performs the calculation at run time.

If you must use constants, express them in the precision that is desired. For
example, use this expression

     ?12.0000000/88888888  && FoxPro returns 0.00000135...

rather than this one:

     ?12/88888888  && FoxPro returns 0.0000000000...

STATUS
======

Microsoft has confirmed this to be a problem in the Microsoft products listed at
the beginning of this article.

MORE INFORMATION
================

NOTE: FoxPro returns a zero when the first significant digit is in the seventh
position to the right of the decimal; for example, 0.0000001.


Additional query words: math set decimal kbFP250 kbFP260 kbvfp300 kbvfp500 kbvfp600

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Keywords          : kbvfp500aBUG kbvfp250bug kbvfp250aBUG kbvfp250bBUG kbbuglist
Technology        : kbVFPsearch kbAudDeveloper kbFoxproSearch kbZNotKeyword3 kbFoxPro200DOS kbVFP300 kbVFP300b kbVFP500 kbVFP600 kbVFP500a
Version           : MS-DOS:2.0,2.5x,2.6x; WINDOWS:2.5x,2.6x,3.0,3.0b,5.0,5.0a,6.0
Issue type        : kbbug

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