INFO: fmod() Results Unexpected Due to Binary Representation

Last reviewed: September 2, 1997
Article 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, versions 1.0, 1.5 - Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 4.0, 5.0

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.

Keywords          : CRTIss kbcode kbfasttip
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

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Last reviewed: September 2, 1997
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