Attempt to call global (a nil value) - lua

Question

I'm trying to spawn asteroids in this game every few seconds. But whenever I run the game, I get an error that says
stack traceback:
main.lua:277: in function '_listener'
?: in function '?'
?: in function <?:172>
I've tried moving some of the code around but it's not helping. My professor looked at my code and doesn't know exactly what the issue is, but that the loop that I'm using is missing a something. Here is all the code that includes "createAsteroid"
EDIT: I added my entire main.lua file to this question. The separate files that I do have in this project are the character and the background image, everything else besides the config file is in here. I did try deleting an extra end In the gameLoop function but when I did, I got an error that basically said it expected an end there.
display.setStatusBar( display.HiddenStatusBar )
------------------------------
-- RENDER THE SAMPLE CODE UI
------------------------------
local sampleUI = require( "sampleUI.sampleUI" )
local physics = require( "physics" )
physics.start()
math.randomseed( os.time() )
------------------------------
-- CONFIGURE STAGE
------------------------------
local composer = require( "composer" )
local mainScene = display.newGroup()
display.getCurrentStage():insert( mainScene )
----------------------
-- BEGIN SAMPLE CODE
----------------------
-- Frequently used variables
local centerX = display.contentCenterX
local centerY = display.contentCenterY
local originX = display.screenOriginX
local originY = display.screenOriginY
local width = display.actualContentWidth
local height = display.actualContentHeight
local score = 0
local died = false
local asteroidsTable = {}
local player
local gameLoopTimer
local scoreText
local uiGroup = display.newGroup() -- Display group for UI objects like the score
-- Load background and character from "background.lua" and "character.lua" respectively
local background = require( "background" )
mainScene:insert( background )
local character = require( "character" )
mainScene:insert( character )
local asteroid = display.newImage( "asteroid.png", 180, -50 )
asteroid.rotation = 5
physics.addBody( asteroid, { density=3.0, friction=0.5, bounce=0.3, radius=25 })
-------------------------
-- BEGIN MOVEMENT LOGIC
-------------------------
-- Movement logic variables
local movementSpeed = 1.5
local moving = false
local moveDirection = "down"
-- Sets if the character should move and updates sprite animation
local function setMoving( state )
moving = state
if ( moving ) then
character:play()
else
character:pause()
character:setFrame(2)
end
end
-- Sets the direction that the player should move and updates the character's sprite facing
local function setMovementDirection( direction )
-- Don't change anything if we haven't altered direction
if ( moveDirection == direction ) then
return
end
-- Update the sprite playback
moveDirection = direction
character:setSequence( "walk-" .. direction )
-- Refresh animation playback, which can pause after changing the sprite sequence
setMoving( moving )
end
-- Set movement magnitudes
local movementX = 0
local movementY = 0
local function setMovement( x, y )
local updatedMovement = false
-- Horizontal movement checks
if ( movementX ~= x and nil ~= x ) then
movementX = x
updatedMovement = true
end
-- Abort if nothing is updating
-- We do this since axis/key events can fire multiple times with the same values
if ( not updatedMovement ) then
return
end
-- Determine movement direction
if ( 0 ~= movementX or 0 ~= movementY ) then
-- Favor horizontal animations over vertical ones
if ( math.abs( movementX ) >= math.abs( movementY ) ) then
if ( 0 < movementX ) then
setMovementDirection( "right" )
else
setMovementDirection( "left" )
end
else
if ( 0 < movementY ) then
setMovementDirection( "down" )
else
setMovementDirection( "up" )
end
end
end
-- Update moving animation/variable
if ( 0 == movementX and 0 == movementY ) then
setMoving( false )
else
setMoving( true )
end
end
-- Handle character translation on the screen per frame
local function onFrameEnter()
if ( 0 ~= movementX ) then
character.x = character.x + ( movementSpeed * movementX )
setMoving( true )
end
if ( 0 ~= movementY ) then
character.y = character.y + ( movementSpeed * movementY )
setMoving( true )
end
end
Runtime:addEventListener( "enterFrame", onFrameEnter )
---------------------------
-- BEGIN INPUT CODE: TOUCH
---------------------------
local padGraphic, padButtonUp, padButtonDown, padButtonLeft, padButtonRight
-- Determine if we have a joystick connected or not
local inputDevices = system.getInputDevices()
local function getHasJoystick()
for i = 1, #inputDevices do
if ( "joystick" == inputDevices[i].type ) then
return true
end
end
return false
end
local hasJoystick = getHasJoystick()
-- If we don't have any controllers found, create a virtual D-pad controller
if ( not hasJoystick ) then
-- Called when one of the virtual D-pad buttons are used
local function onTouchEvent( event )
local phase = event.phase
local targetID = event.target.id
if ( "began" == phase or "moved" == phase ) then
if ( "up" == targetID ) then
setMovement( 0, -1 )
elseif ( "down" == targetID ) then
setMovement( 0, 1 )
elseif ( "left" == targetID ) then
setMovement( -1, 0 )
elseif ( "right" == targetID ) then
setMovement( 1, 0 )
elseif ( "padGraphic" == targetID ) then
setMovement( 0, 0 )
end
elseif ( "ended" == phase or "cancelled" == phase ) then
-- An alternative to checking for "cancelled" is to set focus on the control
-- However, we don't want an incoming phone call to bug out input
if ( "up" == targetID or "down" == targetID ) then
setMovement( nil, 0 )
elseif ( "left" == targetID or "right" == targetID ) then
setMovement( 0, nil )
end
end
return true
end
-- Display score
scoreText = display.newText( uiGroup, "Score: " .. score, 400, 40, native.systemFont, 36 )
-- Hide the status bar
display.setStatusBar( display.HiddenStatusBar )
local function updateText()
scoreText.text = "Score: " .. score
end
local function createAsteroid()
local newAsteroid = display.newImageRect( mainGroup, objectSheet, 1, 102, 85 )
table.insert( asteroidsTable, newAsteroid )
physics.addBody( newAsteroid, "dynamic", { radius=25, bounce=0.8 } )
newAsteroid.myName = "asteroid"
local whereFrom = math.random( 3 )
if ( whereFrom == 1 ) then
-- From the left
newAsteroid.x = -60
newAsteroid.y = math.random( 500 )
newAsteroid:setLinearVelocity( math.random( 40,120 ), math.random( 20,60 ) )
elseif ( whereFrom == 2 ) then
-- From the top
newAsteroid.x = math.random( display.contentWidth )
newAsteroid.y = -60
newAsteroid:setLinearVelocity( math.random( -40,40 ), math.random( 40,120 ) )
elseif ( whereFrom == 3 ) then
-- From the right
newAsteroid.x = display.contentWidth + 60
newAsteroid.y = math.random( 500 )
newAsteroid:setLinearVelocity( math.random( -120,-40 ), math.random( 20,60 ) )
end
newAsteroid:applyTorque( math.random( -6,6 ) )
end
-- Create the visuals for the on-screen D-pad
local padSize = 200
padGraphic = display.newImageRect( mainScene, "pad.png", padSize, padSize )
padGraphic.x = originX + padSize/2 - 40
padGraphic.y = height + originY - padSize/2 + 40
padGraphic.alpha = 0.35
padGraphic.id = "padGraphic"
padGraphic:addEventListener( "touch", onTouchEvent )
-- Creates one of the invisible virtual D-pad buttons
local function createPadButton( buttonID, offsetX, offsetY )
local btn = display.newRect( mainScene, padGraphic.x+offsetX, padGraphic.y+offsetY, padSize/5, padSize/5 )
btn:addEventListener( "touch", onTouchEvent )
btn.id = buttonID
btn.isVisible = false
btn.isHitTestable = true
return btn
end
-- Create buttons for handling the D-pad input
padButtonUp = createPadButton( "up", 0, padSize/-5 )
padButtonDown = createPadButton( "down", 0, padSize/5 )
padButtonLeft = createPadButton( "left", padSize/-5, 0 )
padButtonRight = createPadButton( "right", padSize/5, 0 )
end
-----------------
-- GAME LOOP --
-----------------
local function gameLoop()
-- Create new asteroid
createAsteroid()
-- Remove asteroids which have drifted off screen
for i = #asteroidsTable, 1, -1 do
local thisAsteroid = asteroidsTable[i]
if ( thisAsteroid.x < -100 or
thisAsteroid.x > display.contentWidth + 100 or
thisAsteroid.y < -100 or
thisAsteroid.y > display.contentHeight + 100 )
then
display.remove( thisAsteroid )
table.remove( asteroidsTable, i )
end
end
end
timer.performWithDelay(1000, gameLoop, 0)
--------------------------------------------------
-- BEGIN INPUT CODE: KEYBOARD & BASIC CONTROLLER
--------------------------------------------------
-- Detect if a joystick axis is being used
local joystickInUse = false
-- Keyboard input configuration
local keyUp = "up"
local keyDown = "down"
local keyLeft = "left"
local keyRight = "right"
-- Called when a key event has been received
local function onKeyEvent( event )
local keyName = event.keyName
local phase = event.phase
-- Handle movement keys events; update movement logic variables
if ( not joystickInUse ) then
if ( "down" == phase ) then
if ( keyUp == keyName ) then
setMovement( nil, -1 )
elseif ( keyDown == keyName ) then
setMovement( nil, 1 )
elseif ( keyLeft == keyName ) then
setMovement( -1, nil )
elseif ( keyRight == keyName ) then
setMovement( 1, nil )
end
elseif ( "up" == phase ) then
if ( keyUp == keyName ) then
setMovement( nil, 0 )
elseif ( keyDown == keyName ) then
setMovement( nil, 0 )
elseif ( keyLeft == keyName ) then
setMovement( 0, nil )
elseif ( keyRight == keyName ) then
setMovement( 0, nil )
end
end
end
return false
end
Runtime:addEventListener( "key", onKeyEvent )
------------------------------------------
-- BEGIN INPUT CODE: ADVANCED CONTROLLER
------------------------------------------
-- We only support advanced controllers when one is detected
if ( getHasJoystick ) then
-- Detect axis event updates
local function onAxisEvent( event )
local value = event.normalizedValue
local axis = event.axis.type
local descriptor = event.axis.descriptor
-- We only care about "x" and "y" input events
-- However, touch-screen events can fire these as well so we filter them out
if ( ( "x" ~= axis and "y" ~= axis ) or ( string.find( descriptor, "Joystick" ) == nil ) ) then
return
end
-- Detect zero movement at a certain cutoff so we don't get the character moving very, very slowly
if ( math.abs(value) < 0.15 ) then
value = 0
end
-- Based on which axis type we are dealing with, set movement variables
if ( "x" == axis ) then
setMovement( value, nil )
elseif ( "y" == axis ) then
setMovement( nil, value )
end
-- Some devices will send both up/down/left/right keys and the axis value
-- We let our code know that we are using a joystick value so they do not conflict
if ( 0 ~= value ) then
joystickInUse = true
else
joystickInUse = false
end
end
Runtime:addEventListener( "axis", onAxisEvent )
end
local function onCollision( event )
if ( ( obj1.myName == "player" and obj2.myName == "asteroid" ) or
( obj1.myName == "asteroid" and obj2.myName == "player" ) )
then
display.remove( player )
else
ship.alpha = 0
timer.performWithDelay( 1000, restoreShip )
end
end
Runtime:addEventListener( "collision", onCollision )

Answer 1

Go to line 277 in your file main.lua
You will find a function call createAsteroid(). Within this scope createAsteroid is not defined.
So let's see if we can find a definition for createAsteroid in this file.
Line 218: local function createAsteroid() ...
Check if the function call is within the scope of that function...
No! It is inside an if-statement while the function call is outside that statement. As createAsteroid is local to that if-statement it is unknown (nil) inside gameLoop and hence may not be called.
Finding such scope issues could be very easy if you had proper indentation!