初学VC，各位大虾帮忙看下程序的问题。

#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="<<endl;
return 0;
}