Q57479: INFO: fmod() Results Unexpected Due to Binary Representation

Article: Q57479
Product(s): Microsoft C Compiler
Version(s): 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.
Operating System(s): 
Keyword(s): kbcode kbCRT kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600
Last Modified: 25-JUL-2001

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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 
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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:

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Keywords          : kbcode kbCRT kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600 
Technology        : kbVCsearch kbAudDeveloper kbCRT
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
Issue type        : kbinfo

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