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在头文件<math.h>中定义



float       ldexpf( float arg, int exp );

(1)

(since C99)

double      ldexp( double arg, int exp );

(2)


long double ldexpl( long double arg, int exp );

(3)

(since C99)

Defined in header <tgmath.h>



#define ldexp( arg, exp )

(4)

(since C99)

1-3）arg将数字2 的浮点值乘以exp功率。

4）类型 - 通用宏：如果arg有类型long double，ldexpl被调用。否则，如果arg有整数类型或类型double，ldexp则调用。否则ldexpf，分别称为。

参数

arg

-

浮点值

exp

-

整数值

返回值

如果没有错误发生，arg乘以2的幂exp（arg×2exp

）返回。

如果范围误差由于发生溢出，±HUGE_VAL，±HUGE_VALF，或±HUGE_VALL返回。

如果发生下溢引起的范围错误，则返回正确的结果（舍入后）。

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），

  • 除非发生范围错误，否则FE_INEXACT不会引发（结果是确切的）

  • 除非发生范围错误，否则当前舍入模式将被忽略

  • 如果arg为±0，则返回，未修改

  • 如果arg是±∞，则返回，未修改

  • 如果exp为0，则arg返回，未修改

  • 如果arg是NaN，则返回NaN

函数ldexp（“加载指数”）及其双数，frexp可以用来操纵浮点数的表示，而无需直接位操作。

在许多实现中，ldexp使用算术运算符的乘法或除法乘以二的幂的效率较低。

例

#include <stdio.h>#include <math.h>#include <float.h>#include <errno.h>#include <fenv.h>#pragma STDC FENV_ACCESS ON
int main(void){    printf("ldexp(7, -4) = %f\n", ldexp(7, -4));    printf("ldexp(1, -1074) = %g (minimum positive subnormal double)\n",            ldexp(1, -1074));    printf("ldexp(nextafter(1,0), 1024) = %g (largest finite double)\n",            ldexp(nextafter(1,0), 1024));    // special values    printf("ldexp(-0, 10) = %f\n", ldexp(-0.0, 10));    printf("ldexp(-Inf, -1) = %f\n", ldexp(-INFINITY, -1));    //error handling
    errno = 0; feclearexcept(FE_ALL_EXCEPT);    printf("ldexp(1, 1024) = %f\n", ldexp(1, 1024));    if(errno == ERANGE) perror("    errno == ERANGE");    if(fetestexcept(FE_OVERFLOW)) puts("    FE_OVERFLOW raised");}

可能的输出：

ldexp(7, -4) = 0.437500ldexp(1, -1074) = 4.94066e-324 (minimum positive subnormal double)ldexp(nextafter(1,0), 1024) = 1.79769e+308 (largest finite double)ldexp(-0, 10) = -0.000000ldexp(-Inf, -1) = -infldexp(1, 1024) = inf
    errno == ERANGE: Numerical result out of range
    FE_OVERFLOW raised

参考

  • C11标准（ISO / IEC 9899：2011）：

    • 7.12.6.6 ldexp函数（p：244）

    • 7.25类型通用数学<tgmath.h>（p：373-375）

    • F.10.3.6 ldexp函数（p：522）

  • C99标准（ISO / IEC 9899：1999）：

    • 7.12.6.6 ldexp函数（p：225）

    • 7.22类型通用数学<tgmath.h>（p：335-337）

    • F.9.3.6 ldexp函数（p：459）

  • C89 / C90标准（ISO / IEC 9899：1990）：

    • 4.5.4.3 ldexp函数

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