Lua table variables the same even for new objects - lua

Question

I'm trying to create a Lua table that represents a matrix, however I keep running into a problem where if I create two Matrices, and initialize some values they both have the same values.
--Test.Lua
require"Matrix"
M1 = Matrix.Matrix:New()
M2 = Matrix.Matrix:New()
M1._11 = 2
print(M1._11) --Prints 2
print(M2._11) --Prints 2
--Matrix.lua
module("Matrix", package.seeall)
Matrix = {}
Matrix = { _11 = 0, _12 = 0, _13 = 0,
_21 = 0, _22 = 0, _23 = 0,
_31 = 0, _32 = 0, _33 = 0
}
function Matrix:New()
object = object or {}
setmetatable(object, self)
self.__index = self
return object
end

Answer 1

object = object or {}
This is why that happens. You only ever create one Matrix object. There is only every one object table which you return, and there is only ever one self table that you use as a metatable.
So how can you expect different instances when Matrix:New will always return the exact same value on every call?
You need to return a new table for each New call; that's why we use that name ;) Because of the way you're using a metatable, you also have to return a new metatable; you can't return the same metatable attached to new tables and expect it to work.

Answer 2

As nicol is explaining, on one hand you are trying to "reuse the same object over and over" (probably to "make it faster") and on the other you want to have different objects.
The solution is - don't reuse object on New call.
local Matrix = {} -- don't use the module function. Make Matrix local ...
Matrix.__index = Matrix
function Matrix:New()
local object = { -- create one local variable on every call to New
_11 = 0, _12 = 0, _13 = 0,
_21 = 0, _22 = 0, _23 = 0,
_31 = 0, _32 = 0, _33 = 0
}
setmetatable(object, self)
return object
end
return Matrix -- ... and return the Matrix local var at the end
A couple notes:
You really must learn how to use local
Usage of the module function is not recommended. Return a local table instead, as in my example.
Usage: assuming that that file is called "Matrix.lua":
local Matrix = require 'Matrix'
local M1 = Matrix:New()
local M2 = Matrix:New()
-- etc
As a sidenote, the Matrix:New() function can be made shorter (and faster). The following implementation works exactly as the one above, but it's slightly more efficient:
function Matrix:New()
return setmetatable({
_11 = 0, _12 = 0, _13 = 0,
_21 = 0, _22 = 0, _23 = 0,
_31 = 0, _32 = 0, _33 = 0
},
self)
end
This works because setmetatable(t,m) returns t with m already set as its metatable.