Sort multiple lists lua - sorting

Question

How can I sort 2 lists y location ( map tiles and people ) and draw them in order dependent of y. 2 lists I want to use:
map = {}
map.y = {60,10,40,80}
map.t = {0,0,1,1} -- type
people = {}
people.y = {0,100}
people.t = {0,1} -- type
I can currently sort and draw a single list of hero and boxes.
Sort / draw code:
box1 = love.graphics.newImage("box1.png")
box2 = love.graphics.newImage("box2.png")
box3 = love.graphics.newImage("box3.png")
hero = love.graphics.newImage("hero.png")
object = {
x = {0, 50,100,200},
y = {0,200, 50,100},
g = {0,1,2,3}
}
function sortIndex(item)
local i
local id = {} -- id list
for i = 1, #item.x do -- Fill id list (1 to length)
id[i] = i
end
-- print( unpack(id) ) -- Check before
table.sort(id,sortY)-- Sort list
-- print( unpack(id) ) -- Check after
item.sort = id -- List added to object.sort
-- Sort id, using item values
function sortY(a,b)
return item.y[a] < item.y[b]
end
end
function drawObject()
local i,v, g,x,y
for i = 1, #object.x do
v = object.sort[i] -- Draw in order
x = object.x[v]
y = object.y[v]
g = object.g[v]
if g == 0 then g = hero -- set to an image value
elseif g == 1 then g = box1
elseif g == 2 then g = box2
elseif g == 3 then g = box3
end
love.graphics.draw(g,x,y,0,7,7)
end
end
Update sort:
sortIndex(object)
My function sorts an id list comparing a y location list. The id is used to draw objects in order dependent of their y. How can I sort 2 id lists together comparing 2 y location lists, then draw them in order?
Maybe when drawing, switch from map tiles to people dependent on y, but I don't know how.

Answer 1

Might be related to your previous question a lot: Returning A Sorted List's Index in Lua
I assume if your height can be 1,2 and 3 (with 1 being on the top), you first want to render all tiles at Y1, then all people at Y1, then Y2 and Y3. To do that, you'll have to make a combined list and sort that:
map = {}
map.y = {60,10,40,80}
map.t = {0,0,1,1} -- type
people = {}
people.y = {0,100}
people.t = {0,1} -- type
local all = {}
local map_y = map.y
local offset = #map_y
local people_y = people.y
-- Fill the list with map tiles
for i=1,offset do
all[i] = {1,i,map_y[i]} --{type,index,y}
end
-- Fill the list with people
for i=1,#people_y do
all[i+offset] = {2,i,people_y[i]}
end
-- Do the sorting
-- It works a bit like your previous question:
-- 'all' contains "references":
-- They tell us is it's from map/people + the index
-- We sort the references using the third element in it:
-- The 'y' variable we put there during the first 2 loops
table.sort(all,function(a,b)
return a[3] < b[3]
end)
-- Printing example
-- The references are sorted using the 'y' field of your objects
-- With v[1] we know if it's from map/people
-- The v[2] tells us the index in that ^ table
-- The v[3] is the 'y'-field. No real need to remove it
for k,v in pairs(all) do
print(v[1] == 1 and "Map" or "Person",v[2],"with y being",v[3])
end
Output:
Person 1 with y being 0
Map 2 with y being 10
Map 3 with y being 40
Map 1 with y being 60
Map 4 with y being 80
Person 2 with y being 100
There are 2 things I want to add, that doesn't have anything to do with the question of my answer:
Maybe it would be easier if you have a table for each element.
Your people would be {0,0} and {100,1} which might be easier to manipulate.
If you prefer your stuff always sorted, you might want to use this: Sorted List. If you keep a sorted list of all your objects, you don't have to sort the list everytime you add/remove an element, or worse, each time you render. (depending if people move) This might help with performance if you're planning to have a lot of map/people objects. (Sorted List could be useful for your current data system, but also the {y=1,t=1} one)

Answer 2

function sortIndex(...)
sorted = {} -- global
local arrays_order = {}
for arr_index, array in ipairs{...} do
arrays_order[array] = arr_index
for index = 1, #array.y do
table.insert(sorted, {array = array, index = index})
end
end
table.sort(sorted,
function (a,b)
local arr1, arr2 = a.array, b.array
local ind1, ind2 = a.index, b.index
return arr1.y[ind1] < arr2.y[ind2] or
arr1.y[ind1] == arr2.y[ind2] and arrays_order[arr1] < arrays_order[arr2]
end)
end
function drawAll()
for _, elem_info in ipairs(sorted) do
local array = elem_info.array
local index = elem_info.index
local x = array.x[index]
local y = array.y[index]
if array == map then
-- draw a map tile with love.graphics.draw()
elseif array == people then
-- draw a human with love.graphics.draw()
end
end
end
sortIndex(map, people) -- to draw map tiles before people for the same y