ID Number: Q59407
5.10 6.00 6.00a 6.00ax 7.00 | 5.10 6.00 6.00a
MS-DOS | OS/2
Summary:
SYMPTOMS
In Microsoft C versions 5.1, 6.0, 6.0a, 6.0ax, and C/C++ version 7.0,
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:
Since 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);
}
}
More Information:
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 reference words: 5.10 6.00 6.00a 6.00ax 7.00