GObject object = detectCollision(window, ball);

if (object != NULL)
{
    // bounce on paddle
    if (object == paddle)
    {
        if (y > 0)
            y = -y;
    }

    // bounce on bricks
    else if (strcmp(getType(object), "GRect") == 0)
    {
        if (y < 0)
        {
            y = -y;
            removeGWindow(window, object);

            // convert i from int to string
            sprintf(s, "%i", counter);
            setLabel(label, s);
            counter++;
        }
    }
}

if(object !== NULL)

{
     // The object can be a brick or paddle

    if(object == paddle)
    {
        /*change the velocity in the y-direction
          When the ball hits the paddle, you know that before hitting,
          the velocity of the ball was negative bcoz it's coming down.
          Hence, you don't need "if y>0 in your code"
        */
    }
    else if (strcmp(getType(object),"GRect") == 0)
    {
        //change the velocity in the y-direction again
        velocity_y = -velocity_y;

        //remove the brick from the window
        removeGWindow (window, object);

        //increase points and decrease bricks
        points++;
        bricks--;

        //update scoreboard
        updateScoreboard(window, label, points);
    }
}