Attempt to index field (a nil value), for an Object - lua

Question

I have a problem with trying to add things to a LOVE2D version of Match-3. (From the CS50 course)
I added a function swapTiles() into my Board class and made a class object called self.board in a Class called PlayState. Then when I try to access the new function, it says this error:
Error
src/states/PlayState.lua:155: attempt to index field 'board' (a nil value)
I'll provide my Board and PlayState class below:
Board: (keep in mind the new function is literally in the code)
Board = Class{}
function Board:init(x, y, level) -- Added "level" as an integer for the block variations.
self.x = x
self.y = y
self.matches = {}
self.level = level
self:initializeTiles()
end
function Board:swapTiles(tile1, tile2)
-- swap grid positions of tiles
local tempX = tile1.gridX
local tempY = tile1.gridY
tile1.gridX = tile2.gridX
tile1.gridY = tile2.gridY
tile2.gridX = tempX
tile2.gridY = tempY
-- swap tiles in the tiles table
self.tiles[tile1.gridY][tile1.gridX] = tile1
self.tiles[tile2.gridY][tile2.gridX] = tile2
end
function Board:initializeTiles()
self.tiles = {}
-- There should only be two shiny tiles.
for tileY = 1, 8 do
-- empty table that will serve as a new row
table.insert(self.tiles, {})
for tileX = 1, 8 do
self.isPowerup = false
if math.random(1, 25) == 4 then
self.isPowerup = true
end
-- create a new tile at X,Y with a random color and variety
table.insert(self.tiles[tileY], Tile(tileX, tileY, math.random(18), math.min(8, math.random(1, self.level)), self.isPowerup))
end
end
while self:calculateMatches() do
-- recursively initialize if matches were returned so we always have
-- a matchless board on start
self:initializeTiles()
end
end
--[[
Goes left to right, top to bottom in the board, calculating matches by counting consecutive
tiles of the same color. Doesn't need to check the last tile in every row or column if the
last two haven't been a match.
]]
function Board:calculateMatches()
local matches = {}
-- how many of the same color blocks in a row we've found
local matchNum = 1
-- horizontal matches first
for y = 1, 8 do
local colorToMatch = self.tiles[y][1].color
matchNum = 1
-- every horizontal tile
for x = 2, 8 do
-- if this is the same color as the one we're trying to match...
if self.tiles[y][x].color == colorToMatch then
matchNum = matchNum + 1
else
-- set this as the new color we want to watch for
colorToMatch = self.tiles[y][x].color
-- if we have a match of 3 or more up to now, add it to our matches table
if matchNum >= 3 then
local match = {}
-- go backwards from here by matchNum
for x2 = x - 1, x - matchNum, -1 do
-- add each tile to the match that's in that match
table.insert(match, self.tiles[y][x2])
-- Shiny Check
if self.tiles[y][x2].isShiny == true then
for i = 1, 8 do
table.insert(match, self.tiles[y][i])
end
end
end
-- add this match to our total matches table
table.insert(matches, match)
end
-- don't need to check last two if they won't be in a match
if x >= 7 then
break
end
matchNum = 1
end
end
-- account for the last row ending with a match
if matchNum >= 3 then
local match = {}
-- go backwards from end of last row by matchNum
for x = 8, 8 - matchNum + 1, -1 do
table.insert(match, self.tiles[y][x])
end
table.insert(matches, match)
end
end
-- vertical matches
for x = 1, 8 do
local colorToMatch = self.tiles[1][x].color
matchNum = 1
-- every vertical tile
for y = 2, 8 do
if self.tiles[y][x].color == colorToMatch then
matchNum = matchNum + 1
else
colorToMatch = self.tiles[y][x].color
if matchNum >= 3 then
local match = {}
for y2 = y - 1, y - matchNum, -1 do
table.insert(match, self.tiles[y2][x])
if self.tiles[y2][x].isShiny == true then
for i = 1, 8 do
table.insert(match, self.tiles[i][x])
end
end
end
table.insert(matches, match)
end
matchNum = 1
-- don't need to check last two if they won't be in a match
if y >= 7 then
break
end
end
end
-- account for the last column ending with a match
if matchNum >= 3 then
local match = {}
-- go backwards from end of last row by matchNum
for y = 8, 8 - matchNum, -1 do
table.insert(match, self.tiles[y][x])
end
table.insert(matches, match)
end
end
-- store matches for later reference
self.matches = matches
-- return matches table if > 0, else just return false
return #self.matches > 0 and self.matches or false
end
--[[
Remove the matches from the Board by just setting the Tile slots within
them to nil, then setting self.matches to nil.
]]
function Board:removeMatches()
for k, match in pairs(self.matches) do
for k, tile in pairs(match) do
self.tiles[tile.gridY][tile.gridX] = nil
end
end
self.matches = nil
end
--[[
Shifts down all of the tiles that now have spaces below them, then returns a table that
contains tweening information for these new tiles.
]]
function Board:getFallingTiles()
-- tween table, with tiles as keys and their x and y as the to values
local tweens = {}
-- for each column, go up tile by tile till we hit a space
for x = 1, 8 do
local space = false
local spaceY = 0
local y = 8
while y >= 1 do
-- if our last tile was a space...
local tile = self.tiles[y][x]
if space then
-- if the current tile is *not* a space, bring this down to the lowest space
if tile then
-- put the tile in the correct spot in the board and fix its grid positions
self.tiles[spaceY][x] = tile
tile.gridY = spaceY
-- set its prior position to nil
self.tiles[y][x] = nil
-- tween the Y position to 32 x its grid position
tweens[tile] = {
y = (tile.gridY - 1) * 32
}
-- set space back to 0, set Y to spaceY so we start back from here again
space = false
y = spaceY
spaceY = 0
end
elseif tile == nil then
space = true
if spaceY == 0 then
spaceY = y
end
end
y = y - 1
end
end
-- create replacement tiles at the top of the screen
for x = 1, 8 do
for y = 8, 1, -1 do
local tile = self.tiles[y][x]
-- if the tile is nil, we need to add a new one
if not tile then
local tile = Tile(x, y, math.random(18), math.random(1, self.level))
tile.y = -32
self.tiles[y][x] = tile
tweens[tile] = {
y = (tile.gridY - 1) * 32
}
end
end
end
return tweens
end
function Board:getNewTiles()
return {}
end
function Board:testForMatches()
for y = 1, 8 do
for x = 1, 8 do
-- Test for left swap
if x > 1 then
end
end
end
end
function Board:render()
for y = 1, #self.tiles do
for x = 1, #self.tiles[1] do
self.tiles[y][x]:render(self.x, self.y)
end
end
end
Here's my PlayState: (look for PlayState:swapTiles(), keep in mind that the self.board is being used several times for and works fine, except when i try calling self.board:swapTiles().)
PlayState = Class{__includes = BaseState}
function PlayState:init()
-- start our transition alpha at full, so we fade in
self.transitionAlpha = 255
-- position in the grid which we're highlighting
self.boardHighlightX = 0
self.boardHighlightY = 0
-- timer used to switch the highlight rect's color
self.rectHighlighted = false
-- flag to show whether we're able to process input (not swapping or clearing)
self.canInput = true
-- tile we're currently highlighting (preparing to swap)
self.highlightedTile = nil
self.score = 0
self.timer = 60
-- set our Timer class to turn cursor highlight on and off
Timer.every(0.5, function()
self.rectHighlighted = not self.rectHighlighted
end)
-- subtract 1 from timer every second
Timer.every(1, function()
self.timer = self.timer - 1
-- play warning sound on timer if we get low
if self.timer <= 5 then
gSounds['clock']:play()
end
end)
end
function PlayState:enter(params)
-- grab level # from the params we're passed
self.level = params.level
-- spawn a board and place it toward the right
self.board = params.board or Board(VIRTUAL_WIDTH - 272, 16)
-- grab score from params if it was passed
self.score = params.score or 0
-- score we have to reach to get to the next level
self.scoreGoal = self.level * 1.25 * 1000
end
function PlayState:update(dt)
if love.keyboard.wasPressed('escape') then
love.event.quit()
end
-- go back to start if time runs out
if self.timer <= 0 then
-- clear timers from prior PlayStates
Timer.clear()
gSounds['game-over']:play()
gStateMachine:change('game-over', {
score = self.score
})
end
-- go to next level if we surpass score goal
if self.score >= self.scoreGoal then
-- clear timers from prior PlayStates
-- always clear before you change state, else next state's timers
-- will also clear!
Timer.clear()
gSounds['next-level']:play()
-- change to begin game state with new level (incremented)
gStateMachine:change('begin-game', {
level = self.level + 1,
score = self.score
})
end
if self.canInput then
-- move cursor around based on bounds of grid, playing sounds
if love.keyboard.wasPressed('up') then
self.boardHighlightY = math.max(0, self.boardHighlightY - 1)
gSounds['select']:play()
elseif love.keyboard.wasPressed('down') then
self.boardHighlightY = math.min(7, self.boardHighlightY + 1)
gSounds['select']:play()
elseif love.keyboard.wasPressed('left') then
self.boardHighlightX = math.max(0, self.boardHighlightX - 1)
gSounds['select']:play()
elseif love.keyboard.wasPressed('right') then
self.boardHighlightX = math.min(7, self.boardHighlightX + 1)
gSounds['select']:play()
end
-- if we've pressed enter, to select or deselect a tile...
if love.keyboard.wasPressed('enter') or love.keyboard.wasPressed('return') then
-- if same tile as currently highlighted, deselect
local x = self.boardHighlightX + 1
local y = self.boardHighlightY + 1
-- if nothing is highlighted, highlight current tile
if not self.highlightedTile then
self.highlightedTile = self.board.tiles[y][x]
-- if we select the position already highlighted, remove highlight
elseif self.highlightedTile == self.board.tiles[y][x] then
self.highlightedTile = nil
-- if the difference between X and Y combined of this highlighted tile
-- vs the previous is not equal to 1, also remove highlight
elseif math.abs(self.highlightedTile.gridX - x) + math.abs(self.highlightedTile.gridY - y) > 1 then
gSounds['error']:play()
self.highlightedTile = nil
else
self:swapTiles(self.highlightedTile, self.board.tiles[y][x], true)
end
end
end
Timer.update(dt)
end
function PlayState:swapTiles(tile1, tile2, swapBackAtNoMatch)
local tile1 = tile1
local tile2 = tile2
local swapBackAtNoMatch = swapBackAtNoMatch
self.board:swapTiles(tile1, tile2) -- Causes the nil error.
if swapBackAtNoMatch then
-- tween coordinates between two swapping tiles
Timer.tween(0.1, {
[tile1] = {x = tile2.x, y = tile2.y},
[tile2] = {x = tile1.x, y = tile1.y}
})
-- once they've swapped, tween falling blocks
:finish(function ()
local matches = self.board:calculateMatches()
if matches then
self.calculateMatches(matches)
else
-- swap back if there's no match
self.swapTiles(tile1, tile2, false)
gSounds['error']:play()
end
end)
else
-- tween coordinates between the two so they swap
Timer.tween(0.1, {
[tile1] = {x = tile2.x, y = tile2.y},
[tile2] = {x = tile1.x, y = tile1.y}})
end
end
--[[
Calculates whether any matches were found on the board and tweens the needed
tiles to their new destinations if so. Also removes tiles from the board that
have matched and replaces them with new randomized tiles, deferring most of this
to the Board class.
]]
function PlayState:calculateMatches()
self.highlightedTile = nil
-- if we have any matches, remove them and tween the falling blocks that result
local matches = self.board:calculateMatches()
if matches then
gSounds['match']:stop()
gSounds['match']:play()
-- add score for each match
for k, match in pairs(matches) do
local varietyPoints = 0 -- We'll keep track of the bonus variety points here
-- We'll use vareity to calculate points for each tile within a match
for j, tiles in pairs(match) do
varietyPoints = varietyPoints + tiles.variety * 25
end
self.score = self.score + (#match * 50) + varietyPoints
-- Also add one second times the number of match to the timer
self.timer = self.timer + #match * 1
end
-- remove any tiles that matched from the board, making empty spaces
self.board:removeMatches()
-- gets a table with tween values for tiles that should now fall
local tilesToFall = self.board:getFallingTiles()
-- first, tween the falling tiles over 0.25s
Timer.tween(0.25, tilesToFall):finish(function()
local newTiles = self.board:getNewTiles()
-- then, tween new tiles that spawn from the ceiling over 0.25s to fill in
-- the new upper gaps that exist
Timer.tween(0.25, newTiles):finish(function()
-- recursively call function in case new matches have been created
-- as a result of falling blocks once new blocks have finished falling
self:calculateMatches()
end)
end)
-- if no matches, we can continue playing
else
self.canInput = true
end
end
Honestly doesn't make much sense. Please help!

Answer 1

function PlayState:swapTiles(tile1, tile2, swapBackAtNoMatch) end
is syntactic sugar for
PlayState.swaptiles = function(self, tile1, tile2, swapBackAtNoMatch) end
That's the reason why you can work with self inside that function.
A function defined like that needs to be called using the colon operator as well to make this work. Or you explicitly provide the table as first parameter.
Hence in your call self.swapTiles(tile1, tile2, false)
self is going to be tile1
tile1.board is nil so self.board is nil in this function call which causes the error
You have to call self:swapTiles(tile1, tile2, false) or self.swapTiles(self, tile1, tile2, false)
Please make sure you fully understand the colon syntax.