《用C++語言編寫一個程序，求PI的值》

//編寫一個C++程序求PI的值
/*
 PI=16arctan(1/5)-4arctan(1/239)
 其中arctan用如下形式的極數計算：
    arctan=x-(x^3/3)+(x^5/7)-(x^7/7)+...
*/
#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-16){
        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;
    system("pause");
    return 0;
}