Assignment confusion in Lua - lua

Question

I am analysing Fishies projest from sample codes of Corona and i couldn't understand that assignment.
background = ( backgroundLandscape == background and backgroundPortrait ) or backgroundLandscape
Here is the full code:
-- Seed randomizer
local seed = os.time();
math.randomseed( seed )
display.setStatusBar( display.HiddenStatusBar )
-- Preload the sound file (theoretically, we should also dispose of it when we are completely done with it)
local soundID = audio.loadSound( "bubble_strong_wav.wav" )
-- Background
local halfW = display.viewableContentWidth / 2
local halfH = display.viewableContentHeight / 2
-- Create a table to store all the fish and register this table as the
-- "enterFrame" listener to animate all the fish.
local bounceAnimation = {
container = display.newRect( 0, 0, display.viewableContentWidth, display.viewableContentHeight ),
reflectX = true,
}
local backgroundPortrait = display.newImage( "aquariumbackgroundIPhone.jpg", 0, 0 )
local backgroundLandscape = display.newImage( "aquariumbackgroundIPhoneLandscape.jpg", -80, 80 )
backgroundLandscape.isVisible = false
local background = backgroundPortrait
-- Handle changes in orientation for the background images
local backgroundOrientation = function( event )
-- TODO: This requires some setup, i.e. the landscape needs to be centered
-- Need to add a centering operation. For now, the position is hard coded
local delta = event.delta
if ( delta ~= 0 ) then
local rotateParams = { rotation=-delta, time=500, delta=true }
if ( delta == 90 or delta == -90 ) then
local src = background
-- toggle background to refer to correct dst
background = ( backgroundLandscape == background and backgroundPortrait ) or backgroundLandscape
background.rotation = src.rotation
transition.dissolve( src, background )
transition.to( src, rotateParams )
else
assert( 180 == delta or -180 == delta )
end
transition.to( background, rotateParams )
audio.play( soundID ) -- play preloaded sound file
end
end
-- Add a global listener
Runtime:addEventListener( "orientation", backgroundOrientation )
--
-- Fishies
local numFish = 10
local file1 = "fish.small.red.png"
local file2 = "fish.small.blue.png"
--
-- Define touch listener for fish so that fish can behave like buttons.
-- The listener will receive an 'event' argument containing a "target" property
-- corresponding to the object that was the target of the interaction.
-- This eliminates closure overhead (i.e. the need to reference non-local variables )
local buttonListener = function( event )
if "ended" == event.phase then
local group = event.target
-- tap only triggers change from original to different color
local topObject = group[1]
if ( topObject.isVisible ) then
local bottomObject = group[2]
-- Dissolve to bottomObject (different color)
transition.dissolve( topObject, bottomObject, 500 )
-- Restore after some random delay
transition.dissolve( bottomObject, topObject, 500, math.random( 3000, 10000 ) )
end
-- we handled it so return true to stop propagation
return true
end
end
--
--
--
-- Add fish to the screen
for i=1,numFish do
-- create group which will represent our fish, storing both images (file1 and file2)
local group = display.newGroup()
local fishOriginal = display.newImage( file1 )
group:insert( fishOriginal, true ) -- accessed in buttonListener as group[1]
local fishDifferent = display.newImage( file2 )
group:insert( fishDifferent, true ) -- accessed in buttonListener as group[2]
fishDifferent.isVisible = false -- make file2 invisible
-- move to random position in a 200x200 region in the middle of the screen
group:translate( halfW + math.random( -100, 100 ), halfH + math.random( -100, 100 ) )
-- connect buttonListener. touching the fish will cause it to change to file2's image
group:addEventListener( "touch", buttonListener )
-- assign each fish a random velocity
group.vx = math.random( 1, 5 )
group.vy = math.random( -2, 2 )
-- add fish to animation group so that it will bounce
bounceAnimation[ #bounceAnimation + 1 ] = group
end
--
-- Function to animate all the fish
function bounceAnimation:enterFrame( event )
local container = self.container
container:setFillColor( 0, 0, 0, 0) -- make invisible
local containerBounds = container.contentBounds
local xMin = containerBounds.xMin
local xMax = containerBounds.xMax
local yMin = containerBounds.yMin
local yMax = containerBounds.yMax
local orientation = self.currentOrientation
local isLandscape = "landscapeLeft" == orientation or "landscapeRight" == orientation
local reflectX = nil ~= self.reflectX
local reflectY = nil ~= self.reflectY
-- the fish groups are stored in integer arrays, so iterate through all the
-- integer arrays
for i,v in ipairs( self ) do
local object = v -- the display object to animate, e.g. the fish group
local vx = object.vx
local vy = object.vy
if ( isLandscape ) then
if ( "landscapeLeft" == orientation ) then
local vxOld = vx
vx = -vy
vy = -vxOld
elseif ( "landscapeRight" == orientation ) then
local vxOld = vx
vx = vy
vy = vxOld
end
elseif ( "portraitUpsideDown" == orientation ) then
vx = -vx
vy = -vy
end
-- TODO: for now, time is measured in frames instead of seconds...
local dx = vx
local dy = vy
local bounds = object.contentBounds
local flipX = false
local flipY = false
if (bounds.xMax + dx) > xMax then
flipX = true
dx = xMax - bounds.xMax
elseif (bounds.xMin + dx) < xMin then
flipX = true
dx = xMin - bounds.xMin
end
if (bounds.yMax + dy) > yMax then
flipY = true
dy = yMax - bounds.yMax
elseif (bounds.yMin + dy) < yMin then
flipY = true
dy = yMin - bounds.yMin
end
if ( isLandscape ) then flipX,flipY = flipY,flipX end
if ( flipX ) then
object.vx = -object.vx
if ( reflectX ) then object:scale( -1, 1 ) end
end
if ( flipY ) then
object.vy = -object.vy
if ( reflectY ) then object:scale( 1, -1 ) end
end
object:translate( dx, dy )
end
end
-- Handle orientation of the fish
function bounceAnimation:orientation( event )
print( "bounceAnimation" )
for k,v in pairs( event ) do
print( " " .. tostring( k ) .. "(" .. tostring( v ) .. ")" )
end
if ( event.delta ~= 0 ) then
local rotateParameters = { rotation = -event.delta, time=500, delta=true }
Runtime:removeEventListener( "enterFrame", self )
self.currentOrientation = event.type
for i,object in ipairs( self ) do
transition.to( object, rotateParameters )
end
local function resume(event)
Runtime:addEventListener( "enterFrame", self )
end
timer.performWithDelay( 500, resume )
end
end
Runtime:addEventListener( "enterFrame", bounceAnimation );
Runtime:addEventListener( "orientation", bounceAnimation )
-- This function is never called,
-- but shows how we would unload the sound if we wanted to
function unloadSound()
audio.dispose(soundID)
soundID = nil
end

Answer 1

Lua has a slightly strange behaviour when it comes to ands and ors.
The expression a and b evaluates to a if a is considered false (only nil and false is considered false, all other values including 0 are considered true) and if a is considered true the expression evaluates to b.
The expression a or b evaluates to a if a is considered true and b if a is considered false.
Note: in both cases if the expression evaluates to a that value of b isn't even evaluated. This is called short circuit logic. In and if a is false, the and can't possibly be true, so there is no point in wasting computation time to evaluate b. Similarly, if a is true in a or b there is no point to evaluate b as the or can't possibly be false.
The construct cond and valiftrue or valiffalse (or the equivalent, due to operator precedence, (cond and valiftrue) or valiffalse) is equivalent to other language's ternary if statement, with one caveat: valiftrue must not evaluate to false. If that is the case, the whole expression will always evaluate to valiffalse. (Try and reason it out, that's the best way I find to get a grip on this construct.)

Answer 2

was about to post in detail but #JPvdMerwe got it spot on.
To be precise,
background = ( backgroundLandscape == background and backgroundPortrait ) or backgroundLandscape
translates to
if backgroundLandscape == background then
background = backgroundPortrait
else
background = backgroundLandscape
end
EDIT
As #JPvdMerwe pointed out, in this case, if backgroundPortrait is false then
background = backgroundLandscape
will be executed all the time.