Floating-point manipulation functions 浮点处理功能

copysign 

返回由x的绝对值和y的符号组成的值。

/* copysign example */
#include <stdio.h>      /* printf */
#include <math.h>       /* copysign */

int main ()
{
  printf ("copysign ( 10.0,-1.0) = %f\n", copysign( 10.0,-1.0));
  printf ("copysign (-10.0,-1.0) = %f\n", copysign(-10.0,-1.0));
  printf ("copysign (-10.0, 1.0) = %f\n", copysign(-10.0, 1.0));

  return 0;
}

输出：

copysign ( 10.0,-1.0) = -10.0
copysign (-10.0,-1.0) = -10.0
copysign (-10.0, 1.0) = 10.0

nan 

： NaN  (Not-A-Number) 不是一个数字，double类型。

nextafter 

返回x到y方向上的x的下一个元素。

printf ("first representable value greater than zero: %e\n", nextafter(0.0,1.0));
printf ("first representable value less than zero: %e\n", nextafter(0.0,-1.0));

first representable value greater than zero: 4.940656e-324 // 比0大一点的数
first representable value less than zero: -4.940656e-324 // 比0小一点的数

nexttoward 

 和nextafter相似，只是比它更精确

printf ("first representable value greater than zero: %e\n", nexttoward(0.0,1.0L));
printf ("first representable value less than zero: %e\n", nexttoward(0.0,-1.0L));

first representable value greater than zero: 4.940656e-324
first representable value less than zero: -4.940656e-324