C++ function not updating variables

Question

I have a program designed to simulate a ball bouncing in C++ and save the position, and velocity of it. I have a ball class to simulate many of them at a time and for some reason when I run the update function to update the ball, none of the variables update.

Ball class:

    class Ball
    {
    public:
        float x;
        float y;
        float xVel;
        float yVel;
        float gravity;
        float elasticity;
        float radius;
        float friction;
        float width = 200;
        float height = 200;
        void update() { //Only simulate gravity for testing purposes
            this->yVel += this->gravity;
            std::cout << this->yVel << this->gravity << std::endl;
        }
        void init()
        {
            this->x = randomf(0, width);
            this->y = randomf(0, height);
            this->xVel = randomf(-10, 10);
            this->yVel = randomf(-0.5, 0.5);
            this->gravity = randomf(-1, 1);
            this->elasticity = randomf(0.25, 1);
            this->radius = randomf(2.5, 50);
            this->friction = randomf(0.25, 1);
        }
    };

For some reason when I call update() none of the variables update, only producing the output

@"0.0241846 0.0655345\r\n"
@"0.0241846 0.0655345\r\n"
@"0.0241846 0.0655345\r\n"
@"0.0241846 0.0655345\r\n"
@"0.0241846 0.0655345\r\n"
@"0.0241846 0.0655345\r\n"
@"0.0241846 0.0655345\r\n"

How I'm calling update is I have a for() loop that goes for as many iterations as I want, currently 500, and update is inside of it. Another thing that tends to happen with this program, is that my random function

The for() loop:

for (int i = 0; i < iter; i++)
    {
        balls.clear(); //Clear array
        for (int j = 0; j < batch; j++) //Create new balls
        {
            Ball ball;
            ball.init();
            balls.push_back(ball);
        }
        for (int j = 0; j < simLen; j++) //For simulation length update all balls
        {
            for (int k = 0; k < batch; k++)
            {
                Ball ball = balls.at(k);
                ball.update();
                std::vector<float> temp; //Store ball info to be saved later
                temp.push_back(ball.x);
                temp.push_back(ball.y);
                temp.push_back(ball.xVel);
                temp.push_back(ball.yVel);
                if (j % 2 == 0) {
                    y.push_back(temp);
                } else {
                    temp.push_back(ball.gravity);
                    temp.push_back(ball.elasticity);
                    temp.push_back(ball.radius);
                    temp.push_back(ball.friction);
                    x.push_back(temp);
                }
                temp.clear();
            }
        }
        std::cout << "Simulation " << i + 1 << " out of " << iter << " finished, batch size " << batch << std::endl;
    }

Random number function:

    float randomf(float LO, float HI)
    {
        return LO + static_cast<float>(rand()) / (static_cast<float>(RAND_MAX / (HI - LO)));
    }

Generates the same values most of the time. I have no clue what on earth would cause this, help.

How I'm calling update is I have a for() loop that goes for as many iterations as I want, currently 500, and update is inside of it. -- Post the code, do not just describe it. — PaulMcKenzie
– PaulMcKenzie, Commented Oct 25, 2020 at 2:08
Ball ball = balls.at(k); -- You do know that ball is a copy, right? Don't you really want a reference, i.e. Ball& ball = balls.at(k);? — PaulMcKenzie
– PaulMcKenzie, Commented Oct 25, 2020 at 2:11

Alan Birtles · Accepted Answer · 2020-10-25 03:51:15Z

2

Ball ball = balls.at(k);

This retrieves one of the Ball objects from the vector, and creates a new object called ball that's a copy of it.

ball.update();

This calls update() on this ball object. Since it's a copy of the original object from the vector this, of course, does nothing whatsoever to the object in the vector.

std::vector::at returns a reference, so you simply need to make ball a reference to the object in the array:

Ball &ball = balls.at(k);

See your C++ textbook for a more complete description of what references are and how they work.

edited Oct 25, 2020 at 3:51

Alan Birtles

38.3k4 gold badges39 silver badges73 bronze badges

answered Oct 25, 2020 at 2:12

Sam Varshavchik

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3 Comments

Grimtin10 Over a year ago

oh my god thank you, im a java programmer usually so i didnt know this

paxdiablo Over a year ago

Java programmers should never be allowed to roam the hallowed halls of C++ land, that's like letting Notepad users edit Linux code, or allowing cats in a dog shelter :-) Note the smiley, I don't need any grief over what I think is, despite what my wife usually says, a humorous comment.

Sam Varshavchik Over a year ago

If you review all the previous copies of this question that you posted it should now be quite clear to you why stackoverflow requires a minimal reproducible example. You thought that problem was with your update() function so you only showed that. That's not where the problem was. P.S. C++ is not Java. Objects in C++ work in fundamentally different ways than they do in Java. You will do yourself a big favor if you completely forget everything you know about Java while you learn C++, else you'll kept getting tripped up like that. This is not the only fundamental difference between Java and C++.

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