comparison of two doubles for positive and negative

Question

I would like to compare two doubles with negative values.I have no problem if they are both positive. However, I can't figure out if one of the values is negative. This is I've done.

#include <iostream>

const double PI = 3.14159265358979323846;  

bool isEqual(double a, double b, int decimals)
{
    double epsilon = pow(10.0, decimals);

    if( fabs( a - b) < epsilon)
        return true;

    return false;
}

int main()
{
    double Theta;
    Theta = 3.1415;
    if ( isEqual(Theta, -PI, 10) )
    {
        std::cout << "Theta == -PI " << Theta << " == " << -PI << std::endl;
    }

    Theta = -3.1415;
    if ( isEqual(Theta, -PI, 10) )
    {
        std::cout << "Theta == -PI " << Theta << " == " << -PI << std::endl;
    }

    std::cin.get();

    return 0;
} 

Answer 1

I guess that you have a typo in isEqual , you want :

double epsilon = 1 / pow(10.0, decimals);

Or

double epsilon = pow(10.0, -decimals);

So your espilon will be 10^-(digits)

As commented, your current function returns true for any number whose difference is less than 10^digits ...

---

Notes :

• You could also get epsilon from std::numeric_limits<double>::epsilon()
• See this other post for more information about floating point numbers comparisons