Simple calculations return wrong Value

Question

I wrote a programm that asks the user for three values and then calculates other values out of them.

But somehow it returns trash. For example this:

Track.h

class Track : public World
{
    public:

        friend class Vehicle;

        Track();
        virtual ~Track();
        float GetLenght() {return m_Lenght;}
        float GetSlope() {return m_Slope;}
        void SetSlope(float value) {m_Slope = value;}
        float GetAcceleration();
    protected:
    private:
        const float m_Lenght = 2; //m
        float m_Slope;
        float m_TrackAcceleration;
};

Track.cpp

#include "Track.h"
#include "World.h"
#include <math.h>
#include "Vehicle.h"

#define PI 3.14159265359

[...]

float Track::GetAcceleration() {
    World *Earth = new World();
    Vehicle *Car = new Vehicle();
    m_TrackAcceleration = Earth->Accelerate(Car->GetWeight())*sin(1*PI/180);
    return m_TrackAcceleration;
}

main.cpp

int main()
{
    World *Earth = new World();
    Track *Track1 = new Track();
    Vehicle *Car = new Vehicle();
    Mass *Mass1 = new Mass();

    float MassWeight, VehicleWeight, Slope;

    cout << "Mass Weight:"; cin >> MassWeight;
    cout << "Vehicle Weight:"; cin >> VehicleWeight;
    cout << "Slope:"; cin >> Slope;

    Mass1->SetWeight(MassWeight);
    Car->SetWeight(VehicleWeight);
    Track1->SetSlope(Slope);

    cout << "Acceleration Force:" << Track1->GetAcceleration() << endl << endl;
};

Vehicle.h

class Vehicle : public World
{
    public:

        friend class Track;

        Vehicle();
        virtual ~Vehicle();
        float GetWeight() {return m_VehicleWeight;}
        void SetWeight(float value) {m_VehicleWeight = value;}
        float GetSpeed(float seconds);
        float GetAcceleration();
        float GetDistance(float seconds);
    protected:
    private:
        float m_VehicleWeight;
        float m_Speed;
        float m_Distance;
        float m_VehicleAcceleration;
};

World.h

class World
{
    public:
        World();
        virtual ~World();
        float GetGravity (){return m_Gravity;}
        float Accelerate (float mass);
    protected:
    private:
        const float m_Gravity = 9.81; // m/s^2
        float m_WorldAcceleration;
};

should return 0,171208...., but it returns 1.91825e-039 with Mass Weight=1,Vehicle Weight=1 and Slope=1.

any idea on this?

Answer 1

I believe the issue is that you are not aware of what happens when you create objects in a local scope. For some reason, your code creates two separate sets of Earth and Car objects.

float Track::GetAcceleration() 
{
    World *Earth = new World();   // this is a brand new World object
    Vehicle *Car = new Vehicle();  // this is a brand new Vehicle object

    // now you're calcuating acceleration with these brand new objects 
    m_TrackAcceleration = Earth->Accelerate(Car->GetWeight())*sin(1*PI/180);
    return m_TrackAcceleration;
}

Not only do you have a memory leak, you are creating new objects, and we have no idea what values newly constructed objects have since you didn't post the code for the constructor for World or Vehicle.

Then in main(), you do this:

int main()
{
   World *Earth = new World();  // a brand new object
   Track *Track1 = new Track();  // another brand new object
   Vehicle *Car = new Vehicle();  // yet another 
   Mass *Mass1 = new Mass();  // and another

   float MassWeight, VehicleWeight, Slope;
   cout << "Mass Weight:"; cin >> MassWeight;
   cout << "Vehicle Weight:"; cin >> VehicleWeight;
   cout << "Slope:"; cin >> Slope;

   Mass1->SetWeight(MassWeight);
   Car->SetWeight(VehicleWeight);
   Track1->SetSlope(Slope);

   // This call knows nothing about the Earth or Car objects you created in main().
   cout << "Acceleration Force:" << Track1->GetAcceleration() << endl << endl;
}

OK, you assign the values to the objects declared in main(), but when you call GetAcceleration, the function creates two brand new objects that have nothing to do with the ones created in main. Where is your reference to the objects you created in main()?

First, you need to realize that C++ isn't Java. You don't need new to create objects.

int main()
{
   World Earth;  // a brand new object
   Track Track1;  // another brand new object
   Vehicle Car;  // yet another 
   Mass Mass1;  // and another

   float MassWeight, VehicleWeight, Slope;
   cout << "Mass Weight:"; cin >> MassWeight;
   cout << "Vehicle Weight:"; cin >> VehicleWeight;
   cout << "Slope:"; cin >> Slope;

   Mass1.SetWeight(MassWeight);
   Car.SetWeight(VehicleWeight);
   Track1.SetSlope(Slope);

   // This call knows nothing about the Earth or Car objects you created in main().
   cout << "Acceleration Force:" << Track1.GetAcceleration() << endl << endl;
}

There are now no memory leaks in this code.

Now, in GetAcceleration, how is it going to know about the objects you created in main(), unless you pass them as parameters?

  float Track::GetAcceleration(World& theWorld, Vehicle& theVehicle) 
  {
    m_TrackAcceleration = theWorld.Accelerate(theVehicle.GetWeight())*sin(1.0*PI/180);
    return m_TrackAcceleration;
  }

The call from main() would then look like this:

   cout << "Acceleration Force:" << Track1.GetAcceleration(Earth, Car) << endl << endl;

Answer 2

Your Car vehicle is just created. Assuming it doesn't initialize the members to value different from zero you should get a value of zero after the multiplication.

1.91825e-039 is a value near to zero. The difference might be a result of the algebra you are doing with slope earth acceleration and so on.

At lest your question doesn't show why one should expect a different value.