Incorrect rounding results occur when you use some functions in Visual C++

RESOLUTION

The following code demonstrates this behavior. The results are shown for both Visual C++ 5.0, Visual C++ 6.0, Visual C++ 2005 (32-bit compiler), and Visual C++ .NET (32-bit compiler) and Visual C++ 1.52 (16-bit compiler).

Sample Code

#include <stdio.h>
#include <stdlib.h>

void main( void )

{
double Value;
   int    Decimal;
   int    Sign;

Value = 6.6975;
(void) printf( "1)  %.7f  -->  %.3f  -->  %s\n", Value, Value,
_fcvt( Value, 3, &Decimal, &Sign ) );
Value = 6.06975;
(void) printf( "2)  %.7f  -->  %.4f  -->  %s\n", Value, Value,
_fcvt( Value, 4, &Decimal, &Sign ) );
Value = 6.006975;
(void) printf( "3)  %.7f  -->  %.5f  -->  %s\n", Value, Value,
_fcvt( Value, 5, &Decimal, &Sign ) );
Value = 1.2345;
(void) printf( "4)  %.7f  -->  %.3f  -->  %s\n", Value, Value,
_fcvt( Value, 3, &Decimal, &Sign ) );
Value = 1.02345;
(void) printf( "5)  %.7f  -->  %.4f  -->  %s\n", Value, Value,
_fcvt( Value, 4, &Decimal, &Sign ) );
Value = 1.002345;
(void) printf( "6)  %.7f  -->  %.5f  -->  %s\n", Value, Value,
_fcvt( Value, 5, &Decimal, &Sign ) );
}
				

VC++ 1.52c (16-bit compiler) results:

   1)    6.6975000  -->  6.698  -->  6698
   2)    6.0697500  -->  6.0698  -->  60698
   3)    6.0069750  -->  6.00698  -->  600698
   4)    1.2345000  -->  1.235  -->  1235
   5)    1.0234500  -->  1.0235  -->  10235
   6)    1.0023450  -->  1.00235  -->  100235
				

VC++ 5.0 (32-bit compiler) results:

   1)    6.6975000  -->  6.697  -->  6697
   2)    6.0697500  -->  6.0698  -->  60698
   3)    6.0069750  -->  6.00697  -->  600697
   4)    1.2345000  -->  1.234  -->  1234
   5)    1.0234500  -->  1.0235  -->  10235
   6)    1.0023450  -->  1.00235  -->  100235
				

With Visual C++ 5.0, test cases 2, 5, and 6 are correct, while 1, 3, and 4 do not round as expected.

To work around this behavior, add a very small number to the variable used. In the example above, add 1e-10 to Value. Modify each assignment, as shown in the following example:

   Value = 6.06975+1e-10;