取整函数：
ceil() 右向取整：数轴上右边最靠近的整数，向大的方向取值；ceil “天花板”
floor() 左向取整：数轴上左边最靠近的整数，向小的方向取值；floor “地板”
round() 接近取整：不管正负，即四舍五入后留下整数部分 round(x) = floor(x+0.5)
trunc() 零向取整：不管正负，直接舍掉小数部分只留下整数部分，向原点0的方向取值
rint() nearbyint()：四种方法任选其一，由 fesetround() 配合设置，见例程二说明
例程一：ceil() floor() round() trunc()用法
#include <iostream> #include <cmath> using namespace std; int main() { double num[4] = {13.6, 3.23, -13.6, -3.23}; for (auto n:num){ cout<< " ceil(x) floor(x) round(x) trunc(x)：x = " << n << endl; cout<< "\t" << ceil(n) << "\t" << floor(n) << "\t" << round(n) << "\t" << trunc(n) << endl << endl; } return 0; } /* ceil(x) floor(x) round(x) trunc(x)：x = 13.6 14 13 14 13 ceil(x) floor(x) round(x) trunc(x)：x = 3.23 4 3 3 3 ceil(x) floor(x) round(x) trunc(x)：x = -13.6 -13 -14 -14 -13 ceil(x) floor(x) round(x) trunc(x)：x = -3.23 -3 -4 -3 -3 -------------------------------- Process exited after 0.6186 seconds with return value 0 请按任意键继续. . . */
例程二：fegetround() fesetround()用法
函数原型：
int fesetround(int mode); 设置取整模式
int fegetround(void); 获取当前取整模式
mode 值：
Round to nearest value FE_TONEAREST == 0
Round downward FE_DOWNWARD == 1024
Round upward FE_UPWARD == 2048
Round towards zero FE_TOWARDZERO == 3072
default value： FE_TONEAREST == 0
#include <iostream> #include <cmath> #include <cfenv> using namespace std; int getround(void) { switch (fegetround()) { case FE_TONEAREST: cout << "\nRound to nearest value \t FE_TONEAREST\t== "; break; //round() case FE_DOWNWARD: cout << "\nRound downward \t\t FE_DOWNWARD\t== "; break; //ceil() case FE_UPWARD: cout << "\nRound upward \t\t FE_UPWARD\t== "; break; //floor() case FE_TOWARDZERO: cout << "\nRound towards zero \t FE_TOWARDZERO\t== "; break; //trunc() default: cout << "unknown"; } return fegetround(); } int main(void) { for(int i=0;i<4;i++){ fesetround(i*1024); cout << getround(); } cout << "\n\nDaufult: TONEAREST == 0" << endl; return 0; } /* Round to nearest value FE_TONEAREST == 0 Round downward FE_DOWNWARD == 1024 Round upward FE_UPWARD == 2048 Round towards zero FE_TOWARDZERO == 3072 Daufult: TONEAREST == 0 -------------------------------- Process exited after 0.8902 seconds with return value 0 请按任意键继续. . . */
例程三：nearbyint() rint()用法
#include <iostream> #include <array> #include <cmath> #include <cfenv> using namespace std; int main(void) { array<double,4> num={13.6, 3.23, -13.6, -3.23}; array<string,6> str={"nearbyint(x), x = \n", "\nmode = ", "FE_TONEAREST(default) = ", "FE_DOWNWARD = ", "FE_UPWARD = ", "FE_TOWARDZERO = " }; cout<<str.at(0); for (auto n:num) cout<<"\t"<<n; cout<<endl; for (int i=0;i<4;i++){ fesetround(i*1024); cout<<str.at(1)<<str.at(i+2)<<fegetround()<<endl; for (auto n:num) cout<<"\t"<<nearbyint(n); cout<<endl; } return 0; } /* nearbyint(x), x = 13.6 3.23 -13.6 -3.23 mode = FE_TONEAREST(default) = 0 14 3 -14 -3 mode = FE_DOWNWARD = 1024 13 3 -14 -4 mode = FE_UPWARD = 2048 14 4 -13 -3 mode = FE_TOWARDZERO = 3072 13 3 -13 -3 -------------------------------- Process exited after 0.6 seconds with return value 0 请按任意键继续. . . */
