Returning A Sorted List's Index in Lua - sorting

Question

I access object properties with an index number
object = {}
object.y = {60,20,40}
object.g = {box1,box2,box3} -- graphic
object.c = {false,false,false} -- collision
-- object.y[2] is 20 and its graphic is box2
-- sorted by y location, index should be, object.sort = {2,3,1}
I know table.sort sorts a list, but how can I sort the y list that returns index for the purpose of drawing each object in-front depending on the y location.
Maybe the quicksort function can be edited, I don't understand it.
http://rosettacode.org/wiki/Sorting_algorithms/Quicksort#Lua
https://github.com/mirven/lua_snippets/blob/master/lua/quicksort.lua
Is this possible?

Answer 1

Do not store the data as you're currently doing. Use something like:
object = {
{
y = 60,
g = box1,
c = false,
},
{
y = 20,
g = box2,
c = false,
},
{
y = 40,
g = box3,
c = false,
},
}
and then use the following callback function in table.sort:
function CustomSort(L, R)
return L.y > R.y
end
as shown below:
table.sort(object, CustomSort)

Answer 2

This should work:
local temp = {}
local values = object.y
-- filling temp with all indexes
for i=1,#values do
temp[i] = i
end
-- sorting the indexes, using object.y as comparison
table.sort(temp,function(a,b)
return values[a] < values[b]
end)
-- sorting is done here, have fun with it
object.sort = temp
temp will be {2,3,1} when using this code combined with yours.

Answer 3

#EinsteinK #hjpotter92 : Thank you
RESULT: This is the final version of the answers I received. My question is solved.
Use sortIndex(object) to get sorted list in object.sort . Update sort after objects move.
box1 = love.graphics.newImage("tile1.png")
box2 = love.graphics.newImage("tile2.png")
box3 = love.graphics.newImage("tile3.png")
hero = love.graphics.newImage("hero.png")
object = {
{ x = 200, y = 50, g = box1 },
{ x = 50, y = 100, g = box2 },
{ x = 150, y = 200, g = box3 },
{ x = 0, y = 0, g = hero }
}
function sortIndex(item)
-- Sort id, using item values
local function sortY(a,b)
return item[a].y < item[b].y
end
--------------------------------
local i
local id = {} -- id list
for i = 1, #item do -- Fill id list
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
end
sortIndex(object) -- print( unpack(object.sort) ) -- Check sorted id's
function drawObject()
local i,v, g,x,y
for i = 1, #object do
v = object.sort[i] -- Draw in order
x = object[v].x
y = object[v].y
g = object[v].g
love.graphics.draw(g,x,y)
end
end