Why is my lua table object empty? - lua

Question

I create a lua table object called Map in my Map module, and this function creates a new instance:
function Map:new (o)
o = o or {
centers = {},
corners = {},
edges = {}
}
setmetatable(o, self)
self.__index = self
return o
end
and in my island module I put in this code in the first few lines:
local map = require (*map module location*)
Island = map:new ()
and when I print the number of centers, corners, and tables, they all come out to 0.
I have separate modules for Corner:new (), Center:new (), and Edge:new ()
Why does the length of centers, corners, and edges output as 0?
Edit:
This is what I input into the centers table for example(corners and edges is similar)
function pointToKey(point)
return point.x.."_"..point.y
end
function Map:generateCenters(centers)
local N = math.sqrt(self.SIZE)
for xx = 1, N do
for yy = 1, N do
local cntr = Center:new()
cntr.point = {x = 0.5+xx - 1, y = 0.5+yy - 1}
centers[pointToKey(cntr.point)] = cntr
end
end
return centers
end
The size is always a perfect square

Answer 1

This seems to be a problem with variable scope. Firstly, in instantiating a new Map, the o that is returned should be local:
function Map:new (o)
local o = o or { -- this should be local
centers = {},
corners = {},
edges = {}
}
setmetatable(o, self)
self.__index = self
return o
end
When you pass a pointer to a table to Map:generateCenters(), there is no need to return that pointer. The centers have been added to that table:
function Map:generateCenters(centers)
local N = math.sqrt(self.SIZE)
for xx = 1, N do
for yy = 1, N do
local cntr = Center:new()
cntr.point = {x = 0.5+xx - 1, y = 0.5+yy - 1}
centers[pointToKey(cntr.point)] = cntr -- HERE you're adding to the table passed as an argument
end
end
-- return centers --> NO NEED TO RETURN THIS
end
Lastly, you would do:
local map = require( "map" )
local island = map:new()
map:generateCenters( island.centers )
You are saying, "Put the centers into the table pointed to by the table value corresponding to the key called centers in the table called island".
Lastly, note that
local t = island.centers
print( #t )
will still not output the number of elements in the table centers because there are gaps keys (i.e. they don't go {0,1,2,3,4,..} but rather whatever string the pointToKey() function returns ). To count the elements in centers, you could do:
local count = 0
for k,v in pairs( island.centers ) do
count = count + 1
end
print( count )