attempt to perform arithmetic on field 'y' (a nil value)

[goldenpotato]

I'm not sure if this is the right forum to post this but I recently got this issue when making a "collides" function in a game I'm working on. Before this function every call to "self.y" works just fine but the moment I enable this function, love gives me the following error:
************************
Error

RCar.lua:29: attempt to perform arithmetic on field 'y' (a nil value)


Traceback

RCar.lua:29: in function 'collides'
RCar.lua:99: in function 'update'
states/PlayState.lua:13: in function 'update'
StateMachine.lua:59: in function 'update'
main.lua:90: in function 'update'
[C]: in function 'xpcall'
*************************

This is really slowing down my progression on this project so I would love any help I could get.
Here's my code, the issue is in the collides function:

Code: Select all

RCar = Class{}

local count = 0
local groundHeight = 0


function RCar:init()
  self.image = love.graphics.newImage('red_car.png')

  groundHeight = VIRTUAL_HEIGHT - 37

  self.x = 120
  self.y = groundHeight

  self.width = self.image:getWidth()
  self.height = self.image:getHeight()

  self.dx = 0
  self.dy = 0

  self.timer = 0

  MOVEMENT_SPEED = 100

  GRAVITY = 10
end

function RCar:collides()
  if BallY >= self.y + 3 and self.x <= BallX and BallX <= self.x + 3 then --Back trunk hit
    BallDy = -5
  elseif BallX >= self.x + 3 and self.y + 3 <= BallY <= self.y then --Back window hit
    BallDx = -BallDx - 2
  elseif BallY <= self.y and self.x + 3 <= BallX <= self.x + 11 then --top of the car
    BallDy = -5
  elseif BallX <= self.x + 11 and self.y <= BallY <= self.y + 3 then --front window
    BallDx = -BallDx + 2
  elseif BallY >= self.y + 3 and self.x + 11 <= BallX <= self.x + 16 then --hood
    BallDy = -5
  elseif BallX <= self.x + 16 and self.y + 3 <= BallY <= self.y + 7 then --bumper
    BallDx = -BallDx + 2
  elseif BallY <= self.y + 7 and self.x <= BallX <= self.x + 16 then --bottom
    BallDy = -BallDy
    if BallDx < 0 then
      BallDx = -BallDx + 2
    elseif BallDx > 0 then
      BallDx = BallDx + 2
    end
  elseif BallX > self.x and self.y + 3 <= BallY <= self.y + 7 then --back bumper
    BallDx = -BallDx - 2
  end
end

function RCar:update(dt)




  if self.y < groundHeight then
    self.dy = self.dy + GRAVITY * dt
  else
    self.dy = 0
  end


  if love.keyboard.isDown('d') then
    self.dx = MOVEMENT_SPEED
    self.x = self.x + self.dx * dt
  end

  if love.keyboard.isDown('a') then
    self.dx = -MOVEMENT_SPEED
    self.x = self.x + self.dx * dt
  end


  if love.keyboard.wasPressed('w') and count == 0 then
    count = 1
    self.dy = -2
  end

  if count == 1 then
    if love.keyboard.isDown('w') then
      self.dy = -2
      self.timer = self.timer + dt
      if self.timer > .5 then
        count = 2
        self.timer = 0
      end
    end
  end


  if self.y >= groundHeight then
    self.y = groundHeight
    count = 0
  end

  self.y = self.y + self.dy
  RCar:collides()

end



function RCar:render()
  love.graphics.draw(self.image, self.x, self.y, 0, 3, 3)
end

[sphyrth]

I can't really see what's wrong in your code as well.

All I can think of is that you didn't call RCar:init() before using RCar:update() or something.

[goldenpotato]

I can't really see what's wrong in your code as well.

All I can think of is that you didn't call RCar:init() before using RCar:update() or something.

This is my code for where I initialized all the Classes.

Code: Select all

PlayState = Class{__includes = BaseState}


function PlayState:init()
  self.redCar = RCar()
  self.blueCar = BCar()
  self.ball = Ball()
  self.goalr = GoalR()

end

function PlayState:update(dt)
  self.redCar:update(dt)
  self.blueCar:update(dt)
  self.ball:update(dt)
  self.goalr:update(dt)
end

function PlayState:render()
  self.redCar:render()
  self.blueCar:render()
  self.ball:render()
  self.goalr:render()
end

[Xugro]

Please upload a .love of your game. Then we can take a look at what is going on here. Without seeing how the different classes interact we can only guess what the problem may be.

[goldenpotato]

Please upload a .love of your game. Then we can take a look at what is going on here. Without seeing how the different classes interact we can only guess what the problem may be.

I'm unsure of how to do that. I'm pretty new to love2D and I'm still exploring many of its features.

[MrFariator]

The problem is at the end of your RCar class' update method:

Code: Select all

function RCar:update(dt)
-- ...rest of code
  RCar:collides()
end

Instead calling the function with an instance of the class (eq. writing "self:collides()"), you are passing the class itself to the function. With lua's syntax, what you have written is syntactically equivalent to:

Code: Select all

RCar.collides(RCar) -- the RCar class itself becomes 'self' in the collides() function

But because the RCar class' table has no member "y" (or "x", for that matter), you're effectively trying to perform arithmetic on non-existent values, causing your crash. What you probably meant to write instead is:

Code: Select all

function RCar:update(dt)
-- ...rest of code
  self:collides()
end