PRODUCT  :  Borland C++                           NUMBER  :  1735
  VERSION  :  All
       OS  :  All
     DATE  :  October 25, 1993                         PAGE  :  1/3

    TITLE  :  Testing for Not a Number (NaN)




  /***************************************************************
     TEST4NAN.C
     How to test for NaN (Not a Number)

     NaN is an IEEE standard constant which stands for Not a
     Number. It is returned if an invalid operation exception
     occurs within an operation.  For example, sqrt(-1) will
     return a number with a bit configuration that is recognized
     as NaN because this operation would result in an imaginary
     number.

     NaN will be returned if the exponent of the resultant number
     contains all 1's and the mantissa does not consist of all 0's.

     The following is a code sample which demonstrates how to
     test for NaN.

  ***************************************************************/

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

  struct LongDouble
  {
    unsigned char mant[8];   // the mantissa is the lower 8 bytes.
    unsigned int exp;        // the exponent is the upper 2 bytes
  };

  union Real
  // This union allows us to write in a long double and read out a
  // struct LongDouble.
  {
    struct LongDouble sld;
    long double ld;
  };


  int IsNaN (long double x)
  // This function returns 1 if the parameter is any NaN type.  A
  // float or double may be passed too, and the compiler will
  // promote them to long double so the test will still work.
  {













  PRODUCT  :  Borland C++                           NUMBER  :  1735
  VERSION  :  All
       OS  :  All
     DATE  :  October 25, 1993                         PAGE  :  2/3

    TITLE  :  Testing for Not a Number (NaN)




    union Real real;
    real.ld = x;
    if ((real.sld.exp &0x7FFF) == 0x7FFF)
       return 1;                     // ignore sign of exponent in
                                     // high bit of exponent.

    return 0;
  }

  int matherr (struct exception *a)
  // This function prevents the log function from complaining
  // about taking the log of a negative number.  It simply
  // returns 1, preventing the default error message from being
  // printed.
  {
    return 1;
  }

  #pragma argsused
  int main (int argc, char *argv[])
  {
    float f = 0.0;
    int j;

    for (j=7;j>-5;j-=2)
    {
      f = log (j);
      printf ("log of %d = ",j);

      if (IsNaN (f))
        printf ("(Not a Number)\n");
      else
        printf ("%f\n", f);
    }
    printf ("\n");

    for (j=7;j>-5;j-=2)
    {
      f = sqrt (j);
      printf ("sqrt of %d = ",j);

      if (IsNaN (f))
        printf ("(Not a Number)\n");













  PRODUCT  :  Borland C++                           NUMBER  :  1735
  VERSION  :  All
       OS  :  All
     DATE  :  October 25, 1993                         PAGE  :  3/3

    TITLE  :  Testing for Not a Number (NaN)




      else
        printf ("%f\n", f);
    }

    return 0;
  }


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