std::rint, std::lrint, std::llrint

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C++
 
Numerics的图书馆
常见的数学函数
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编译时合理的算法 （C++11）
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Generic numeric operations
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Nearest integer floating point operations
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llrint		（C++11）
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Defined in header <cmath>
int rint( float arg );
 （C++11 起）
int rint( double arg );
 （C++11 起）
int rint( long double arg );
 （C++11 起）
int rint( Integral arg );
 （C++11 起）
long lrint( float arg );
 （C++11 起）
long lrint( double arg );
 （C++11 起）
long lrint( long double arg );
 （C++11 起）
long lrint( Integral arg );
 （C++11 起）
long long llrint( float arg );
 （C++11 起）
long long llrint( double arg );
 （C++11 起）
long long llrint( long double arg );
 （C++11 起）
long long llrint( Integral arg );
 （C++11 起）
舍入浮点参数arg一个整数值，浮点格式，使用当前的舍入模式。如果结果是不同的arg（即arg已经不是一个整数值），的浮点异常FE_INEXACT，提高.
Original:
Rounds the floating-point argument arg to an integer value in floating-point format, using the current rounding mode. If the result differs from arg (i.e., arg was not an integer value already), the floating-point exception FE_INEXACT is raised.
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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

的整数结果四舍五入arg
Original:
The integer result of rounding arg
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[编辑] 注释

唯一的区别std::nearbyint和std::rint的是，std::rint可以提高FE_INEXACT浮点异常，而std::nearbyint不会提高.
Original:
The only difference between std::nearbyint and std::rint is that std::rint may raise the FE_INEXACT floating-point exception, while std::nearbyint never raises it.
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[编辑] 为例

#include <cmath>
#include <cfenv>
#include <iostream>
int main()
{
    #pragma STDC FENV_ACCESS ON
    std::fesetround(FE_DOWNWARD);
    std::cout << "rounding using FE_DOWNWARD:\n" << std::fixed
              << " 12.0 ->  " << std::rint(12.0) << '\n'
              << " 12.1 ->  " << std::rint(12.1) << '\n'
              << "-12.1 -> " << std::rint(-12.1) << '\n'
              << " 12.5 ->  " << std::rint(12.5) << '\n'
              << " 12.9 ->  " << std::rint(12.9) << '\n'
              << "-12.9 -> " << std::rint(-12.9) << '\n'
              << " 13.0 ->  " << std::rint(13.0) << '\n';
    std::fesetround(FE_TONEAREST);
    std::cout << "rounding using FE_TONEAREST: \n"
              << " 12.0 ->  " << std::rint(12.0) << '\n'
              << " 12.1 ->  " << std::rint(12.1) << '\n'
              << "-12.1 -> " << std::rint(-12.1) << '\n'
              << " 12.5 ->  " << std::rint(12.5) << '\n'
              << " 12.9 ->  " << std::rint(12.9) << '\n'
              << "-12.9 -> " << std::rint(-12.9) << '\n'
              << " 13.0 ->  " << std::rint(13.0) << '\n';
 
    std::cout << "When rounding 12.0 ";
    std::feclearexcept(FE_ALL_EXCEPT);
    std::rint(12.0);
    if(std::fetestexcept(FE_ALL_EXCEPT) & FE_INEXACT) {
        std::cout << "inexact result reported\n";
    } else {
        std::cout << "inexact result not reported\n";
    }
 
    std::cout << "When rounding 12.1 ";
    std::rint(12.1);
    if(std::fetestexcept(FE_ALL_EXCEPT) & FE_INEXACT) {
        std::cout << "inexact result reported\n";
    } else {
        std::cout << "inexact result not reported\n";
    }
}

Output: 

rounding using FE_DOWNWARD:
 12.0 ->  12.000000
 12.1 ->  12.000000
-12.1 -> -13.000000
 12.5 ->  12.000000
 12.9 ->  12.000000
-12.9 -> -13.000000
 13.0 ->  13.000000
rounding using FE_TONEAREST: 
 12.0 ->  12.000000
 12.1 ->  12.000000
-12.1 -> -12.000000
 12.5 ->  12.000000
 12.9 ->  13.000000
-12.9 -> -13.000000
 13.0 ->  13.000000
When rounding 12.0 inexact result not reported
When rounding 12.1 inexact result reported

[编辑] 另请参阅

（C++11）
最接近的整数，使用当前的舍入模式
Original:
nearest integer using current rounding mode
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（函数） [edit]
（C++11）
最接近的整数的幅值比不大于给定值
Original:
nearest integer not greater in magnitude than the given value
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（函数） [edit]
（C++11）
（C++11）
获取或设置舍入的方向
Original:
gets or sets rounding direction
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（函数） [edit]
来自“http://zh.cppreference.com/mwiki/index.php?title=cpp/numeric/math/rint&oldid=28898”