求助：c++求pi,编译通过，但运行后无任何结果？

#include "stdafx.h"
#include
using namespace std;

int main()
{
double arctan(double);
cout << "pi= " << 16 * arctan(1 / 5.0) - 4 * arctan(1 / 239.0) << endl;
return 0;
}

double arctan(double x)
{
double result = 0;
int i = 1, k = 1;
do
{
for (k = 1; k <= (2 * i - 1); k++)
k *= x;
switch (i % 2)
{
case 1:
result += (k / (2 * i - 1)); break;
case 0:
result -= (k / (2 * i - 1)); break;
}
i++;
} while (k > 1e-15);
return result;
}

 #include<iostream>
using namespace std;

double arctan(double);

int main()
{
    cout << "pi= " << 16 * arctan(1 / 5.0) - 4 * arctan(1 / 239.0) << endl;
    return 0;
}

double arctan(double x)
{
    double result = 0;
    int i = 1, s = 1;
        double k = 1.0;
    do
    {
        double t = 1;
        for (int j = 0; j < (i * 2 - 1); j++)
            t *= x;
                k = t / (double)(i * 2 - 1) * (double)s;
        result = result + k;
        s = -s;
        i++;
    } while (k > 1e-15);
    return result;
}

pi= 3.1406

什么无结果？你是怎样运行的？

 #include "stdafx.h"
#include<iostream>
using namespace std;

double arctan(double);

int main()
{
    double a = 16 * arctan(1 / 5.0);
    double b = 4 * arctan(1 / 239.0);
    cout << "pi= " << a-b << endl;
    return 0;
}

double arctan(double x)
{
    double result = 0;
    int i = 1, j = 1;
    double k = 1;
    do
    {
        for (j = 1; j <= (2 * i - 1); j++)
            k *= x;
        switch (i % 2)
        {
        case 1:
            result += (k / (2 * i - 1)); break;
        case 0:
            result -= (k / (2 * i - 1)); break;
        }
        i++;
    } while (k > 1e-15);
    return result;
}

vs2015运行的结果怎么是3.17473？

答案是这样的：

 #include "stdafx.h"
#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;
}