Variable is nii? - lua

Question

I'm using LOVE2D and Lua for making games recently. I'm updating Breakout and I have an error in Paddle.lua.
Code:
Paddle = Class{}
--[[
Our Paddle will initialize at the same spot every time, in the middle
of the world horizontally, toward the bottom.
]]
size = math.random(4)
function Paddle:init(skin, size)
-- x is placed in the middle
self.x = VIRTUAL_WIDTH / 2 - 32
-- y is placed a little above the bottom edge of the screen
self.y = VIRTUAL_HEIGHT - 32
-- start us off with no velocity
self.dx = 0
self.size = size
self.height = 16
if self.size == 1 then
self.width = 32
elseif self.size == 3 then
self.width = 96
elseif self.size == 4 then
self.width = 128
else
self.width = 64
end
-- the skin only has the effect of changing our color, used to offset us
-- into the gPaddleSkins table later
self.skin = skin
end
function Paddle:update(dt)
-- keyboard input
if love.keyboard.isDown('left') then
self.dx = -PADDLE_SPEED
elseif love.keyboard.isDown('right') then
self.dx = PADDLE_SPEED
else
self.dx = 0
end
-- math.max here ensures that we're the greater of 0 or the player's
-- current calculated Y position when pressing up so that we don't
-- go into the negatives; the movement calculation is simply our
-- previously-defined paddle speed scaled by dt
if self.dx < 0 then
self.x = math.max(0, self.x + self.dx * dt)
-- similar to before, this time we use math.min to ensure we don't
-- go any farther than the bottom of the screen minus the paddle's
-- height (or else it will go partially below, since position is
-- based on its top left corner)
else
self.x = math.min(VIRTUAL_WIDTH - self.width, self.x + self.dx * dt)
end
end
--[[
Render the paddle by drawing the main texture, passing in the quad
that corresponds to the proper skin and size.
]]
function Paddle:render()
love.graphics.draw(gTextures['main'], gFrames['paddles'][self.size + 4 * (self.skin - 1)],
self.x, self.y)
end
Error:
Error
src/Paddle.lua:83: attempt to perform arithmetic on field 'size' (a nil value)
Traceback
src/Paddle.lua:83: in function 'render'
src/states/ServeState.lua:68: in function 'render'
src/StateMachine.lua:26: in function 'render'
main.lua:210: in function 'draw'
[C]: in function 'xpcall'
Saying that the value is nii even though I assigned it. How do I fix this?

Answer 1

Function parameters are local to the function's body. So inside Paddle.Init size is a local variable that shadows the global variable size.
As you call Paddle.Init without providing the size parameter, size is nil within the function leading to the observed error.
See Wikipedia: Variable Shadowing
Many people use a prefix to denote global variables. g_size for example. Then you could do something like:
g_size = 4
function Paddle:Init(skin, size)
self.size = size or g_size
end
Which would make self.size default to g_size if the size parameter is not provided.
Another option is to call the paddle constructor with your global size as input.