Metatables examples

-- factorial.lua:

function fact (n)
    if n == 0 then
        return 1
    else
        return n * fact(n-1)
    end
end

-- print("enter a number:")
-- a = io.read("*number")        -- read a number
-- print(fact(a))

-- test1.lua:

dofile('factorial.lua')

local x = {1, 2, 3, 4, 5, value = 3}

local mt = {
  __add =
  function (lhs, rhs) -- "add" event handler
    return { value = lhs.value + rhs.value }
  end,
  __mul =
  function (lhs, rhs) -- "add" event handler
    return { value = lhs.value * rhs.value }
  end
}

setmetatable(x, mt) -- use "mt" as the metatable for "x"

local y = x + x

print(y.value)

local z = x * x

print(fact(z.value))

-- test2.lua:
-- Creates 'vector' table with a "vector space" metatable such as it assigns the vector_metatable metatable to every value set on 'vector'. The vector_metatable allows sum and dot product of vectors of same size. Only for testing purposes.

local vector = {}

local vector_metatable = {
    __add =
    function (lhs, rhs)
        local res = {}
        for i=1,table.maxn(lhs) do
            res[i] = lhs[i] + rhs[i]
        end
        return res
    end,
    __mul =
    function (x, y)
        if type(x) == 'table' then
            lhs = y
            rhs = x
            else if type(y) == 'table' then
                lhs = x
                rhs = y
            end
        end
        if type(lhs) == 'table' then
            local sum = 0
            for i=1,table.maxn(rhs) do
                sum = sum + lhs[i] * rhs[i]
            end
            return sum
        else
            local res = {}
            for i=1,table.maxn(rhs) do
                res[i] = lhs * rhs[i]
            end
            return res
        end
    end
}

local vspace_metatable = {
    __newindex =
    function (t, key, value)
        setmetatable(rawset(t, key, value)[key], vector_metatable)
    end
}

setmetatable(vector, vspace_metatable)

vector.x = {1, 2, 3, 4, 5}

vector.z = {3, 7, 5, 1, 0}

local y = vector.x * vector.z

if type(y) == 'table' then
    for i=1,table.maxn(y) do
        print(y[i])
    end
else
    print(y)
end