How to change the velocity in each frame in Love2D? - lua

Question

I'm trying to code a Pong game on Lua using the framework Love2D with some extra features. Among others, I want curves to occur. In order to do so, I'm trying to implement the trajectory of horizontally launched projectiles. I have a ball table with a position attribute for x (ball.x) and another for y (ball.y). I, also, have a an attribute for the x velocity (ball.dx) and another for the y velocity (ball.dy). Finally, I have an acceleration variable (gravity)
In my game, if the paddle moves and the ball hits it, the ball should follow and horizontal curve. In order to create my curves, I want to change the y-axis velocity on each frame in order to make my ball move in an arc across the screen. The main issue that I have is that I don't know how to change this velocity in each frame in order to create the expected arc. The most recent attempt that I made was to create a while loop like the following code. However, it creates an infinite loop. Can someone enlighten me please?
Clarification:
-Player.x and player.y are the player coordinate
-Player2.x and Player2.y are the opponent coordinate
-This piece of code is inside another if statement detecting a collision. It is inside the love.update(dt) function.
Thank you so much!
if love.keyboard.isDown("up") and distanceBetween(ball.x,ball.y,player.x,player.y)>30 then
ball.dy=0
while CollisionDetector(ball.x,player2.x,ball.y,player2.y, player2.width,player2.height,ball.size)==false or ball.x>0 or ball.y-20>0 or ball.y+20<love.graphics.getHeight() do
ball.dy=ball.dy+gravity*dt
ball.y=ball.y+ball.dy
end
end

Answer 1

I believe the snippet you've provided is a part of love.update function. (At least it should be.) That function is supposed to perform a single step of the game at a time and return. It is not supposed to handle the whole flight in a single call.
In your particular case, while loop expects the ball to move, but that is not something that will happen until the loop and encompassing function end.
In general, to simulate ongoing processes you'll have to store the information about such process. For example, if you want gravity be dependent on the paddle movement direction, then you'll have to modify the corresponding variable and store it between the call.
In your code, there are multiple other design flaws that make it do not what you think it should do. With some functions and code parts left to be implemented by yourself, the code outline would look as follows:
function collides(ball,player)
return (ball.x,player.x,ball.y,player.y, player.width,player.height,ball.size)
end
function love.update(dt)
handle_paddle_movements()
--handle the ball-paddle interaction
if collides(ball,player1) or collides(ball,player2) then
ball.dx=-ball.dx
if love.keyboard.isDown("up") then
--store the info you need to know about the interaction
ball.d2y = -gravity
else if love.keyboard.isDown("down")
ball.d2y = gravity
else
ball.d2y = 0
end
end
handle_the_wall_collision()
--ball movement code should be separate from collision handling
--concrete equations can be whatever you want
ball.x = ball.x+ball.dx*dt
ball.dy = ball.dy + ball.d2y * dt
ball.y = ball.y + ball.dy * dt
end