std::logb

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C++
 
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Defined in header <cmath>
float       logb( float arg );
 （C++11 起）
double      logb( double arg );
 （C++11 起）
long double logb( long double arg );
 （C++11 起）
double      logb( Integral arg );
 （C++11 起）
提取的指数值从浮点参数arg，并返回一个浮点值。从形式上看，结果是log
r|arg|组成部分，作为一个符号浮点值，非零阿根廷，在那里r是std::numeic_limits<T>::radixT是浮点类型的arg。 arg是低于正常，它被视为仿佛被归.
Original:
Extracts the value of the exponent from the floating-point argument arg, and returns it as a floating-point value. Formally, the result is the integral part of log
r|arg| as a signed floating-point value, for non-zero arg, where r is std::numeic_limits<T>::radix and T is the floating-point type of arg. If arg is subnormal, it is treated as though it was normalized.
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目录

[编辑] 参数

arg -
浮点值
Original:
floating point value
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[编辑] 返回值

浮点指数.
Original:
The floating-point exponent.
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域或范围可能会出现错误，如果arg是零.
Original:
Domain or range error may occur if arg is zero.
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[编辑] 注释

指数的值返回std::logb始终是1小于由std::frexp因为不同的正常化要求重新调谐指数：经由e指数std::logb|arg*r-e
|是介于1和r（通常之间1和2），但对于指数e返回std::frexp，|arg*2-e
|之间0.5和1.
Original:
The value of the exponent returned by std::logb is always 1 less than the exponent retuned by std::frexp because of the different normalization requirements: for the exponent e returned by std::logb, |arg*r-e
| is between 1 and r (typically between 1 and 2), but for the exponent e returned by std::frexp, |arg*2-e
| is between 0.5 and 1.
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[编辑] 为例

比较不同的浮点分解的功能
Original:
Compares different floating-point decomposition functions
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#include <iostream>
#include <cmath>
#include <limits>
int main()
{
    double f = 123.45;
    std::cout << "Given the number " << f << " or " << std::hexfloat
              << f << std::defaultfloat << " in hex,\n";
 
    double f3;
    double f2 = std::modf(f, &f3);
    std::cout << "modf() makes " << f3 << " + " << f2 << '\n';
 
    int i;
    f2 = std::frexp(f, &i);
    std::cout << "frexp() makes " << f2 << " * 2^" << i << '\n';
 
    i = std::ilogb(f);
    std::cout << "logb()/ilogb() make " << f/std::scalbn(1.0, i) << " * "
              << std::numeric_limits<double>::radix
              << "^" << std::ilogb(f) << '\n';
}

Output: 

Given the number 123.45 or 0x1.edccccccccccdp+6 in hex,
modf() makes 123 + 0.45
frexp() makes 0.964453 * 2^7
logb()/ilogb() make 1.92891 * 2^6

[编辑] 另请参阅
分解一个数字，位数和电源2
Original:
decomposes a number into significand and a power of 2
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（函数） [edit]
（C++11）
提取指数的数量
Original:
extracts exponent of the number
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（函数） [edit]
（C++11）
（C++11）
FLT_RADIX次方乘以一个数
Original:
multiplies a number by FLT_RADIX raised to a power
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（函数） [edit]
来自“http://zh.cppreference.com/mwiki/index.php?title=cpp/numeric/math/logb&oldid=28937”