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C/C++ 取整函数ceil(),floor()

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

<pre>float floorf(float x);
long double floorl(long double x); </pre>

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