parabola ifndef _PARABOLA_H define _PARABOLA_H class parabola private

// parabola.h

#ifndef _PARABOLA_H
#define _PARABOLA_H

class parabola
{
    private:
        double p, d;
    public:
        parabola();
        parabola(double p);
        void getYByX(double x);
        void setP(double p);
        void setD(double d);
        void getXByY(double y);
        void operator+=(double d);
        void operator-=(double d);
};

#endif  /* _PARABOLA_H */

// parabola.cpp

#include "parabola.h"
#include <iostream>
#include <stdlib.h>
#include <cmath>

using namespace std;

parabola::parabola()
{
}

parabola::parabola(double p)
{
    this->setP(p);
    this->setD(0);
}

void parabola::setP(double p)
{
    if (p != 0)
        this->p = p;
    else
    {
        cout << "Error. Wrong value is failed.";
        exit(0);
    }
}
void parabola::setD(double d)
{
    this->d = fabs(d);
}
void parabola::getYByX(double x)
{
    double y1, y2;
    if (p * (x - d) >= 0)
    {
        y1 = -(sqrt(2 * p * (x - d)) + d);
        y2 = sqrt(2 * p * (x - d)) + d;
        cout << "x = " << x << endl;
        cout << "y1 = " << y1 << ";y2 = " << y2 << endl;
    }
    else
    {
        cout << "Error. Invalid p or x values.";
        exit(0);
    }
}

void parabola::getXByY(double y)
{
    double x;
    x = pow((y-d), 2) / (2 * p)+d;
    cout << "y = " << y << endl;
    cout << "x = " << x << endl;
}
void parabola::operator+=(double d)
{
    this->d = d;
}
void parabola::operator-=(double d)
{
    this->d = -d;
}

// main

#include <stdlib.h>
#include <iostream>
#include <cmath>
#include "parabola.h"

using namespace std;

int main(int argc, char** argv)
{
    parabola p(1);
    p+=6;
    p.getXByY(8);
    return 0;
}