nextafter
nextafter, nextafterf, nextafterl, nexttoward, nexttowardf, nexttowardl
| 在头文件 | | |
|---|---|---|
| float nextafterf( float from, float to | (1) | (since C99) |
| double nextafter( double from, double to | (2) | (since C99) |
| long double nextafterl( long double from, long double to | (3) | (since C99) |
| float nexttowardf( float from, long double to | (4) | (since C99) |
| double nexttoward( double from, long double to | (5) | (since C99) |
| long double nexttowardl( long double from, long double to | (6) | (since C99) |
| Defined in header <tgmath.h> | | |
| #define nextafter(from, to) | (7) | (since C99) |
| #define nexttoward(from, to) | (8) | (since C99) |
1-3）首先，将两个参数都转换为函数的类型，然后返回下一个可表示值from的方向to。如果from等于to，to则返回。
4-6）首先，将第一个参数转换为函数的类型，然后返回下一个可表示值from的方向to。如果from等于to，to则返回，转换long double为函数的返回类型，但不会丢失范围或精度。
7）类型 - 通用宏：如果任何参数具有类型long double，nextafterl则被调用。否则，如果任何参数具有整数类型或具有类型double，nextafter则被调用。否则，nextafterf被调用。
8）类型 - 通用宏：如果参数from具有类型long double，nexttowardl则被调用。否则，如果from有整数类型或类型double，nexttoward则调用。否则，nexttowardf被调用。
参数
| from, to | - | 浮点值 |
|---|
返回值
如果没有错误发生，下一个可表示值from的方向to。返回。如果from等于to，则to返回，转换为函数的类型。
如果范围误差由于发生溢出，±HUGE_VAL，±HUGE_VALF，或±HUGE_VALL被返回（与相同的符号from）。
如果由于下溢而发生范围错误，则返回正确的结果。
错误处理
按照math_errhandling中的指定报告错误。
如果实现支持IEEE浮点运算（IEC 60559），
- 如果
from是有限的，但预期的结果是无穷大，提高FE_INEXACT和FE_OVERFLOW
笔记
POSIX指定溢出和下溢条件是范围错误（可能会设置错误）。
IEC 60559建议from每当返回from==to。这些函数会返回to，这会使零之间的行为保持一致：nextafter(-0.0, +0.0)返回+0.0并nextafter(+0.0, -0.0)返回–0.0。
例
#include <math.h>
#include <stdio.h>
#include <float.h>
#include <fenv.h>
int main(void)
{
float from1 = 0, to1 = nextafterf(from1, 1
printf("The next representable float after %.2f is %.20g (%a)\n", from1, to1, to1
float from2 = 1, to2 = nextafterf(from2, 2
printf("The next representable float after %.2f is %.20f (%a)\n", from2, to2, to2
double from3 = nextafter(0.1, 0), to3 = 0.1;
printf("The number 0.1 lies between two valid doubles:\n"
" %.56f (%a)\nand %.55f (%a)\n", from3, from3, to3, to3
// difference between nextafter and nexttoward:
long double dir = nextafterl(from1, 1 // first subnormal long double
float x = nextafterf(from1, dir // first converts dir to float, giving 0
printf("Using nextafter, next float after %.2f (%a) is %.20g (%a)\n",
from1, from1, x, x
x = nexttowardf(from1, dir
printf("Using nexttoward, next float after %.2f (%a) is %.20g (%a)\n",
from1, from1, x, x
// special values
{
#pragma STDC FENV_ACCESS ON
feclearexcept(FE_ALL_EXCEPT
double from4 = DBL_MAX, to4 = nextafter(from4, INFINITY
printf("The next representable double after %.2g (%a) is %.23f (%a)\n",
from4, from4, to4, to4
if(fetestexcept(FE_OVERFLOW)) puts(" raised FE_OVERFLOW"
if(fetestexcept(FE_INEXACT)) puts(" raised FE_INEXACT"
} // end FENV_ACCESS block
float from5 = 0.0, to5 = nextafter(from5, -0.0
printf("nextafter(+0.0, -0.0) gives %.2g (%a)\n", to5, to5
}
输出：
The next representable float after 0.00 is 1.4012984643248170709e-45 (0x1p-149)
The next representable float after 1.00 is 1.00000011920928955078 (0x1.000002p+0)
The number 0.1 lies between two valid doubles:
0.09999999999999999167332731531132594682276248931884765625 (0x1.9999999999999p-4)
and 0.1000000000000000055511151231257827021181583404541015625 (0x1.999999999999ap-4)
Using nextafter, next float after 0.00 (0x0p+0) is 0 (0x0p+0)
Using nexttoward, next float after 0.00 (0x0p+0) is 1.4012984643248170709e-45 (0x1p-149)
The next representable double after 1.8e+308 (0x1.fffffffffffffp+1023) is inf (inf)
raised FE_OVERFLOW
raised FE_INEXACT
nextafter(+0.0, -0.0) gives -0 (-0x0p+0)
参考
- C11标准（ISO / IEC 9899：2011）：
