C/C++ 取整函数ceil(),floor()，向上取整，向下取整

#include <math.h> double floor(double x); 

float floorf(float x); 
long double floorl(long double x);

double floor(double x);

double ceil(double x);

使用floor函数。floor(x)返回的是小于或等于x的最大整数。
如：     floor(10.5) == 10    floor(-10.5) == -11

使用ceil函数。ceil(x)返回的是大于x的最小整数。
如：     ceil(10.5) == 11    ceil(-10.5) ==-10

    
floor()是向负无穷大舍入，floor(-10.5) == -11；
ceil()是向正无穷大舍入，ceil(-10.5) == -10

 

不得不说点：/ % 四舍五入 向上取整（ceil()） 向下取整（floor）

 

 1. /

    //Test "/"
    cout << "Test \"/\"!" << endl;
    cout << "7   / 2   = " << 7/2 << endl;      //3
    cout << "7   / 2.0 = " << 7/2.0 << endl;    //3.5
    cout << "7.0 / 2   = " << 7.0/2 << endl;    //3.5
    cout << "7.0 / 2.0 = " << 7.0/2.0 << endl; //3.5
    cout << "7   / 3   = " << 7/3 << endl;      //2
    cout << endl;

2. %
    //Test "%"
    cout << "Test \"%\"!" << endl;
    cout << "9   % 3   = " << 9%3 << endl;      //0
    cout << "9   % 4   = " << 9%4 << endl;      //1
    //cout << "9.0 % 3   = " << 9.0%3 << endl;
    //cout << "9   % 3.0 = " << 9%3.0 << endl;
    cout << endl;

3. 四舍五入
    //Test round()
    cout << "Test \"四舍五入\"!" << endl;
    double dRoundA = 1.4;
    double dRoundB = 1.6;
    double dRoundLowA = -1.4;
    double dRoundLowB = -1.6;
    double dRoundLowC = 0.0;
    cout << dRoundA << " = " << RoundEx(dRoundA) << endl;         //1
    cout << dRoundB << " = " << RoundEx(dRoundB) << endl;         //2
    cout << dRoundLowA << " = " << RoundEx(dRoundLowA) << endl;   //-1
    cout << dRoundLowB << " = " << RoundEx(dRoundLowB) << endl;   //-2
    cout << dRoundLowC << " = " << RoundEx(dRoundLowC) << endl;   //0
    cout << endl;

double RoundEx(const double& dInput)
{
    double dIn = dInput;
    if (dInput >= 0.0)    //???
    {
        return int(dIn + 0.5);
    } 
    else
    {
        return int(dIn - 0.5);
    }
}

4. ceil() 向上取整
    //Test ceil() 向上取整
    cout << "Test ceil() 向上取整!" << endl; 
    cout << "ceil 1.2 = " << ceil(1.2) << endl;      //2
    cout << "ceil 1.8 = " << ceil(1.8) << endl;      //2
    cout << "ceil -1.2 = " << ceil(-1.2) << endl;    //-1
    cout << "ceil -1.8 = " << ceil(-1.8) << endl;    //-1
    cout << "ceil 0.0 = " << ceil(0.0) << endl;      //0
    cout << endl;

5. floor() 向下取整
    //Test floor() 向下取整
    cout << "Test floor() 向下取整!" << endl;
    cout << "floor 1.2 = " << floor(1.2) << endl;    //1
    cout << "floor 1.8 = " << floor(1.8) << endl;    //1
    cout << "floor -1.2 = " << floor(-1.2) << endl; //-2
    cout << "floor -1.8 = " << floor(-1.8) << endl; //-2
    cout << "floor 0.0 = " << floor(0.0) << endl;    //0
    cout << endl;

16.1.21:标红的部分需要记住，便于理解记忆