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 
    
     #include 
     
      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) = 3.000000ceil(-2.4) = -2.000000ceil(-0.0) = -0.000000ceil(-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 
    
     #include 
     
      using namespace std;int main(){    cout << fixed              << "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) = 2.000000floor(-2.7) = -3.000000floor(-0.0) = -0.000000floor(-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 
    
     #include 
     
      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) = 2.000000trunc(-2.9) = -2.000000trunc(-0.0) = -0.000000trunc(-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 
    
     #include 
     
      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) = 3round(-2.3) = -2  round(-2.5) = -3  round(-2.7) = -3round(-0.0) = -0round(-Inf) = -inflround(+2.3) = 2  lround(+2.5) = 3  lround(+2.7) = 3lround(-2.3) = -2  lround(-2.5) = -3  lround(-2.7) = -3lround(-0.0) = 0lround(-Inf) = 0