Squareroot returning not a number in C++

Question

In the program below, I am trying to calculate the distance between two points. For this, I have made two Point objects. In the method that returns the distance, I have used the distance formula to calculate distance between two points in space. However, every time I run the program, I get a not a number value, which shouldn't be there. Please help.

#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <cmath>

using namespace std;

class Point
{
    public:
        Point(int a, int b);
        ~Point();
        double getDistance(Point& P2);
        void setPoints(int a, int b);
        int getX();
        int getY();
    private:
        int x;
        int y;
};

Point::Point(int a, int b)
{
    setPoints(a,b); 
}

Point::~Point()
{
    //Nothing much to do
}

void Point::setPoints(int a, int b)
{
    x = a;
    y = b;
}

double Point::getDistance(Point& P2)
{
    int xdiff = P2.getX()-this->getX();
    int ydiff = P2.getY()-this->getY();
    xdiff = xdiff*xdiff;
    ydiff = ydiff*ydiff;
    double retval =  sqrt((xdiff) - (ydiff));
    return retval;
}

int Point::getX()
{
    return x;
}

int Point::getY()
{
    return y;
}
int main(int argc, char* argv[])
{
    Point P1(0,0);
    Point P2(0,1);
    Point& pr = P2;
    cout<<P1.getDistance(pr)<<endl;
    return 0;
}

Answer 1

Your formula is wrong. It's not

sqrt(xdiff - ydiff)

but

sqrt(xdiff + ydiff)

You're trying to get the sqrt(-1) which is indeed not a number (or not a real number).

Answer 2

Here's how to figure this sort of thing out for yourself, or at least get a lot closer to a good StackOverflow question:

You know the problem is in the sqrt() call. So, what is it being called with? In this case, you could trace through the computation manually:

int xdiff = P2.getX()-this->getX();    // this is 0 - 0, which is 0.
int ydiff = P2.getY()-this->getY();    // this is 1 - 0, which is 1.
xdiff = xdiff*xdiff;                   // this is still 0.
ydiff = ydiff*ydiff;                   // this is still 1.
double retval =  sqrt((xdiff) - (ydiff));  // this is sqrt(0 - 1), or sqrt(-1).

Alternately, in more complicated cases -- and to check your work, you could either use a debugger to print out the values of the arguments, or you could insert print statements:

xdiff = xdiff*xdiff;
ydiff = ydiff*ydiff;
cout << 'xdiff: ' << xdiff << ' ydiff: ' << ydiff << endl
cout << 'computing sqrt(' << xdiff - ydiff << ')' << endl
double retval =  sqrt((xdiff) - (ydiff));

Either way, you now know that you're computing sqrt(-1), and you can try running that directly to confirm that it does indeed produce the same result. So either you have a question of "Why is sqrt(-1) returning NaN?" or a question of "Why is my distance calculation trying to compute the square root of a negative number?"

Hopefully you already know the answer to the first question, and the second question should indicate that you need to double-check your distance formula, which should have showed you the answer pretty quickly -- but even if you can't figure out why it's doing that, it at least makes a more useful question to ask here.

Answer 3

You should be adding here, not subtracting:

double retval =  sqrt((xdiff) - (ydiff));  // correct is +

Subtracting causes you to take the square root of -1 due to the input data, which is not a (real) number.

Answer 4

As craigmj said, the formula for distance is sqrt ((x1-x2) + (y1-y2)). It's addition not subtraction. What your doing is generating an imaginary number (sqrt (-1)) which will cause an error.

Just a tip of advice, but do not create the destructor if it doesn't do anything; a destructor will be provided for you. Adding a destructor that doesn't do anything just adds unneeded code and makes it look messier.

Also in the getDistance function, you do not need to use this ->getX() and this-> getY(). Since this is a member function it has access to private data, therefore you can directly access the variables through x and y.