圆周率PI

我知道哪错了，我自己重新编写了一个绝对值函数，结果正确，为什么math.h头文件下的abs不行呢。正确为：
#include<iostream>
//#include<math.h>
using namespace std;

double arctan(double x);
double abs(double a);
int main()
{
    double a;
    double b;
    //double arctan(double x);
    a = 16.0 * arctan(1 / 5.0);
    b = 4.0 * arctan(1 / 239.0);
    cout<<"PI= "<<a-b<<endl;
    return 0;
}
double arctan(double x)
{
    double result = 0.0;
    double sqr = x * x;
    double num = x;//fenzi
    int den = 1;//fenmu
    double f = num / den;
    int flag = -1;
    while(abs(f) > 1e-15)
    {
        result += f;
        num = num * sqr * flag;
        den += 2;
        f = num / den;
    }
    return result;
}
double abs(double a)
{
    return a > 0 ? a : -a;
}