C++ nearbyint()用法及代碼示例

C++ 中的nearbyint() 函數使用當前舍入模式將參數舍入為整數值。當前舍入模式由函數 fesetround() 確定。 nearbyint() 函數類似於 rint() ，隻是它不會像 rint() 那樣引發 FE_INEXACT 異常。

FE_INEXACT exception 是一個浮點異常，當運算結果由於舍入或逐漸下溢而未準確表示時發生。

nearbyint() 原型 [從 C++ 11 標準開始]

double nearbyint(double x);
float nearbyint(float x);
long double nearbyint(long double x);
double nearbyint(T x); // For integral type

nearbyint() 函數采用單個參數並返回 double、float 或 long double 類型的值。該函數在<cmath> 頭文件中定義。

參數：

nearbyint() 函數采用單個參數值進行舍入。

返回：

nearbyint() 函數使用 fegetround() 指定的舍入方向將參數 x 舍入為整數值並返回該值。默認情況下，舍入方向設置為'to-nearest'。可以使用fesetround() 函數將舍入方向設置為其他值。

示例 1：nearbyint() 如何在 C++ 中工作？

#include <iostream>
#include <cmath>
#include <cfenv>
using namespace std;

int main()
{
    // by default, rounding direction is to-nearest i.e. fesetround(FE_TONEAREST)
    double x = 11.87, result;
    result = nearbyint(x);
    cout << "Rounding to-nearest (" << x << ") = " << result << endl;
    
    // upper value is taken for mid-way values
    x = 11.5;
    result = nearbyint(x);
    cout << "Rounding to-nearest (" << x << ") = " << result << endl;

    // setting rounding direction to DOWNWARD
    fesetround(FE_DOWNWARD);
    x = 17.87;
    result = nearbyint(x);
    cout << "Rounding downward (" << x << ") = " << nearbyint(x) << endl;
    
    // setting rounding direction to UPWARD
    x = 33.34;
    fesetround(FE_UPWARD);
    result = nearbyint(x);
    cout << "Rounding upward (" << x << ") = " << result << endl;
    
    return 0;
}

運行程序時，輸出將是：

Rounding to-nearest (11.87) = 12 Rounding to-nearest (11.5) = 12 Rounding downward (17.87) = 17 Rounding upward (33.3401) = 34

示例 2：用於整數類型的 nearbyint() 函數

#include <iostream>
#include <cmath>
#include <cfenv>
using namespace std;

int main()
{
    int x = 15;
    double result;
    
    // setting rounding direction to DOWNWARD
    fesetround(FE_DOWNWARD);
    result = nearbyint(x);
    cout << "Rounding downward (" << x << ") = " << result << endl;

    return 0;
}

運行程序時，輸出將是：

Rounding downward (15) = 15

對於整數值，應用nearbyint 函數返回與輸入相同的值。所以在實踐中它並不常用於整數值。