Billiard Ball bounce from bottom and right borders problem

Hello everyone.

I've been working on problem 133(Billiard Ball) and I stuck with bouncing from bottom and right borders, as I may suggest by wrong output.

Here is the code of bounces(bottom, right)

Bounce from bottom border: if(Y >= H-R) { Y += 2 * (H - R - Y); Vy *= -1; }

Bounce from right border: if(X >= W-R) { X += 2 * (W - R - X); Vx *= -1; }

I'm thinking I made it right, did I?

I don't write code for top and left borders, because it can be found in the problem.

Here it is:

#include <iostream>
using std::cout;
using std::endl;
#include <fstream>
using std::ifstream;
#include <cmath>
using std::sqrt;
using std::pow;
#include <utility>
using std::pair;

constexpr double dt = 0.0001;

typedef pair<double, double> coord_type;
typedef const size_t c_size_t;

coord_type find_final_position(c_size_t &W, c_size_t &H, double &X, 
                               double &Y, c_size_t &R, double &Vx,
                               double &Vy, c_size_t &A)
{
    double V = sqrt(pow(Vx, 2) + pow(Vy, 2));

    while(V > 0.0)
    {
        X += Vx * dt;
        if(X < R) 
        {
            X += 2 * (R - X);
            Vx *= -1;
        }   
        if(X >= W-R)    
        {
            X += 2 * (W - R - X);
            Vx *= -1;
        }

        Y += Vy * dt;
        if(Y < R) 
        {
            Y += 2 * (R - Y);
            Vy *= -1;
        }   
        if(Y >= H-R)    
        {
            Y += 2 * (H - R - Y);
            Vy *= -1;
        }

        V -= A * dt;
    }

    return {X, Y};
}

int main()
{
    size_t W{}, H{}, R{}, A{};
    double X{}, Y{}, Vx{}, Vy{};
    ifstream fin("data.txt");
    fin >> W >> H >> X >> Y >> R >> Vx >> Vy >> A;
    coord_type final_position = 
        find_final_position(W, H, X, Y, R, Vx, Vy, A);

    cout << final_position.first << " " << final_position.second;

    return 0;
}

Didn't want to post all code first, because of those who didn't solve this problem yet, to let them solve it by themselves.

Hi, Quandray.

Sorry for delay.

1) My code decreases only V (not Vx and Vy), because it says in problem statement in "Slowing down" section V -= A * dt

2) {} those brackets just initialize variables with default-initialization, so I can be sure that they are not uninitialize.

3) I was sure, that all variables were initialize correctly, but I did what you said and read input from stdin. Nothing changed.

BTW, I copy my code into the "your solution" area and run it, just in case.

Any more suggestions?

1) Damn, it's logical.

But how can I calculate a - angle between OX asix and vector of velocity?

And why I should calculate this angle to use it in Vx = V * cos(a) and Vy = V * sin(a) if I can simpy do this Vx = sqrt(pow(V, 2) - pow(Vy, 2)) and Vy = sqrt(pow(V, 2) - pow(Vx, 2)) isn't this one correct?

Thank you very much, Quandray!

Solve it with reduce percentage too.

Admin, please delete source code above if posible.