INFO: fmod() Results Unexpected Due to Binary Representation

ID: Q57479

The information in this article applies to:

The C Run-Time (CRT), included with:
- Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, 6.0ax
- Microsoft C for OS/2, versions 5.1, 6.0, 6.0a
- Microsoft C/C++ for MS-DOS, version 7.0
- 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

Although there isn't a problem with the fmod() function, the following program generates incorrect results when the divisor, f2, cannot be precisely represented in binary.

Sample Code


/* Compile options needed: none
/ 

#include <stdio.h>
#include <math.h>

void main(void)
{

 double f1, f2, f3;

 scanf("%lf %lf", &f1, &f2);
 f3 = fmod(f1, f2);
 printf("f3 = %.15lf\n", f3);

}

To illustrate the problem, enter the values 2.0 and 0.2 for f1 and f2, respectively. The fmod() function should return 0 (zero), the floating-point remainder of 2.0/0.2. Instead, it returns the value 0.2 because 0.2 does not translate cleanly into binary representation. A value of 0.25 for f2 works correctly because it does translate precisely into binary.

Additional query words:

Keywords : kbcode kbCRT kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600
Version : MS-DOS:5.1,6.0,6.00a,6.00ax,7.0; OS/2:5.1,6.0,6.00a; WINDOWS:1.0,1.5; WINDOWS NT:1.0,2.0,4.0,5.0
Platform : MS-DOS NT OS/2 WINDOWS
Issue type : kbinfo