C++最接近整数的浮点运算

Function	return
ceil	不小于给定值的最接近整数值
floor	不大于给定值的最接近整数
trunc (C++11)	绝对值不大于给定值的最接近整数
round(C++11)	最接近整数，中间情况下舍入到远离零
lround(C++11)	最接近整数，中间情况下舍入到远离零
llround (C++11)	最接近整数，中间情况下舍入到远离零

1.ceil–向上取整

/*
函数原型
float       ceil(float arg);(1)
double      ceil(double arg);(2)
long double ceil(long double arg);(3)
double      ceil(Integral arg);(4)  (C++11 起)
*/
#include <cmath>
#include <iostream>
using namespace std;
int main()
{
    cout << fixed
        << "ceil(+2.4) = " << ceil(+2.4) << '\n'
        << "ceil(-2.4) = " << ceil(-2.4) << '\n'
        << "ceil(-0.0) = " << ceil(-0.0) << '\n'
        << "ceil(-Inf) = " << ceil(-INFINITY) << '\n';
}

输出：

ceil(+2.4) = 3.000000
ceil(-2.4) = -2.000000
ceil(-0.0) = -0.000000
ceil(-Inf) = -INF

2.floor–向下取整

/*函数原型
float       floor( float arg );(1)  
double      floor( double arg );(2) 
long double floor( long double arg );(3)    
double      floor( Integral arg );(4)   (C++11 起)
*/

#include <cmath>
#include <iostream>
using namespace std;
int main()
{
    cout << fixed
              << "floor(+2.7) = " <<floor(+2.7) << '\n'
              << "floor(-2.7) = " << floor(-2.7) << '\n'
              << "floor(-0.0) = " << floor(-0.0) << '\n'
              << "floor(-Inf) = " << floor(-INFINITY) << '\n';
}
输出：

floor(+2.7) = 2.000000
floor(-2.7) = -3.000000
floor(-0.0) = -0.000000
floor(-Inf) = -inf

3.trunc—保留整数部分

//函数原型
float       trunc( float arg );
double      trunc( double arg );
long double trunc( long double arg );
double      trunc( Integral arg );

#include <cmath>
#include <iostream>
using namespace std;
int main()
{
    cout << fixed
              << "trunc(+2.7) = " << trunc(+2.7) << '\n'
              << "trunc(-2.9) = " << trunc(-2.9) << '\n'
              << "trunc(-0.0) = " << strunc(-0.0) << '\n'
              << "trunc(-Inf) = " << strunc(-INFINITY) << '\n';
}

可能的输出：

trunc(+2.7) = 2.000000
trunc(-2.9) = -2.000000
trunc(-0.0) = -0.000000
trunc(-Inf) = -inf

4.round,lround,llround–四舍五入
函数原型：

float round( float arg );
double round( double arg );
long double round( long double arg );
double round( Integral arg );
long lround( float arg );
long lround( double arg );
long lround( long double arg );
long lround( Integral arg );
long long llround( float arg );
long long llround( double arg );
long long llround( long double arg );
long long llround( Integral arg );

#include <iostream>
#include <cmath>
using namespace std;
int main()
{
    // round
    cout << "round(+2.3) = " << round(2.3)
        << "  round(+2.5) = " << round(2.5)
        << "  round(+2.7) = " << round(2.7) << endl
        << "round(-2.3) = " << round(-2.3)
        << "  round(-2.5) = " << round(-2.5)
        << "  round(-2.7) = " << round(-2.7) << endl;

    cout << "round(-0.0) = " << round(-0.0) << endl
        << "round(-Inf) = " << round(-INFINITY) << endl;

    // lround
    cout << "lround(+2.3) = " << lround(2.3)
        << "  lround(+2.5) = " << lround(2.5)
        << "  lround(+2.7) = " << lround(2.7) << endl
        << "lround(-2.3) = " << lround(-2.3)
        << "  lround(-2.5) = " << lround(-2.5)
        << "  lround(-2.7) = " << lround(-2.7) << endl;

    cout << "lround(-0.0) = " << lround(-0.0) << endl
        << "lround(-Inf) = " << lround(-INFINITY) << endl;
    return 0;
}

输出

round(+2.3) = 2  round(+2.5) = 3  round(+2.7) = 3
round(-2.3) = -2  round(-2.5) = -3  round(-2.7) = -3
round(-0.0) = -0
round(-Inf) = -inf
lround(+2.3) = 2  lround(+2.5) = 3  lround(+2.7) = 3
lround(-2.3) = -2  lround(-2.5) = -3  lround(-2.7) = -3
lround(-0.0) = 0
lround(-Inf) = 0