计算圆周率π的值

#include <iostream>
using namespace std;

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

int main()
{
    double a = 16.0 * arctan(1/5.0);
    double b = 4.0 * arctan(1/239.0);

    cout << "PI = " << a - b << endl;
    return 0;
}