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Showing content from https://cplusplus.com/reference/cmath/fpclassify/ below:

macro/function

fpclassify function

int fpclassify (float x);int fpclassify (double x);int fpclassify (long double x);

Classify floating-point value

Returns a value of type int that matches one of the classification macro constants, depending on the value of x:

value description FP_INFINITE Positive or negative infinity (overflow) FP_NAN Not-A-Number FP_ZERO Value of zero FP_SUBNORMAL Sub-normal value (underflow) FP_NORMAL Normal value (none of the above) Note that each value pertains to a single category: zero is not a normal value.

These macro constants of type int are defined in header <cmath> (<math.h>).

In C, this is implemented as a macro, but the type of x shall be float, double or long double.

Parameters

x

Return value One of the followoing int values: FP_INFINITE, FP_NAN, FP_ZERO, FP_SUBNORMAL or FP_NORMAL.

Example

/* fpclassify example */
#include <stdio.h>      /* printf */
#include <math.h>       /* fpclassify, signbit, FP_* */

int main()
{
  double d = 1.0 / 0.0;
  switch (fpclassify(d)) {
    case FP_INFINITE:  printf ("infinite");  break;
    case FP_NAN:       printf ("NaN");       break;
    case FP_ZERO:      printf ("zero");      break;
    case FP_SUBNORMAL: printf ("subnormal"); break;
    case FP_NORMAL:    printf ("normal");    break;
  }
  if (signbit(d)) printf (" negative\n");
  else printf (" positive or unsigned\n");
  return 0;
}

Output:

infinite positive or unsigned