C++ |不使用库函数实现 arctan x

原理是利用 arctan(x) 的泰勒展开式；同时当 x 的绝对值大于 1 时，利用arctan(x) + arctan(1/x) = pi/2 计算。

#include 
#include 

using namespace std;
const double pi = 3.1415926;

double tarctan(double x){
    double sqr = x * x;
    double e = x;
    double r = 0;
    int i = 1;
    while (fabs(e / i) > 1e-15){
        double f = e / i;
        r = (i % 4 == 1)? r + f : r - f;
        e *= sqr;
        i += 2;
    }
    return r;
}

double arctan(double x){
    if (x >= -1 && x <= 1)
        return tarctan(x)
    else{
        if (x > 0)
            return (pi/2 - tarctan(1 / x));
        else
            return (-pi/2 - tarctan(1 / x));
   }//else
}//arctan

int main(){
    double x;
    cin >> x;
    cout << "arctan(x): " << arctan(x) << endl;
    cout << "atan(x): " << atan(x) << endl;
    return 0;
}