Article: Q59407
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,2.0,4.0,5.0,6.0
Operating System(s):
Keyword(s): kbCompiler kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600
Last Modified: 29-NOV-2001
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The information in this article applies to:
- Microsoft C for MS-DOS
- Microsoft C for OS/2
- Microsoft C/C++ for MS-DOS
- Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5
- Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 4.0, 5.0, 6.0
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SYMPTOMS
========
In Microsoft C, subtracting double values greater than or equal to 1.0E+025 may
return inaccurate results.
CAUSE
=====
This is expected behavior and is due to the imprecise nature of floating-point
math. Anytime floating-point math uses large numbers, there will be
rounding/truncation errors and errors introduced due to imprecise representation
of a result in binary format.
MORE INFORMATION
================
Because double values are only 15-digit precise, simple subtraction of two large
numbers can give unexpected results. The following sample code demonstrates this
behavior.
Double values less than 1.0E+25 may not experience the same problem.
Sample Code
-----------
#include <stdio.h>
double a = 1E+28, tmp = 9E+28;
void main (void)
{
printf ("a = %le tmp = %le\n", a, tmp);
while (tmp >= 1E+25) {
tmp -= a;
printf ("a = %le tmp = %le\n", a, tmp);
}
}
The above sample code produces the following output:
a = 1.000000e+028 tmp = 9.000000e+028
a = 1.000000e+028 tmp = 8.000000e+028
a = 1.000000e+028 tmp = 7.000000e+028
a = 1.000000e+028 tmp = 6.000000e+028
a = 1.000000e+028 tmp = 5.000000e+028
a = 1.000000e+028 tmp = 4.000000e+028
a = 1.000000e+028 tmp = 3.000000e+028
a = 1.000000e+028 tmp = 2.000000e+028
a = 1.000000e+028 tmp = 1.000000e+028
a = 1.000000e+028 tmp = 1.319414e+013
Additional query words: 8.00 8.00c 9.00
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Keywords : kbCompiler kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600
Technology : kbVCsearch kbVC400 kbAudDeveloper kbZNotKeyword8 kbvc150 kbvc100 kbCCompSearch kbZNotKeyword3 kbVC500 kbVC600 kbVC200 kbVC32bitSearch kbVC16bitSearch kbVC500Search
Version : :1.0,1.5,2.0,4.0,5.0,6.0
Issue type : kbprb
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