A table that can be iterated over the array part and the non integer keys separatedly

array_table.lua

local arrayTable do
  local function isinteger(num)
    if type(num) == "number" then
      local _, f = math.modf(num)
      return f == 0
    else
      return false
    end
  end

  arrayTable = function(initial)
    local table = { ___array = {}, ___table = {} }

    local metatable = {
      __index = function(_, key)
        return rawget(table.___table, key) or rawget(table.___array, key)
      end,

      __newindex = function(_, key, val)
        if isinteger(key) then
          return rawset(table.___array, key, val)
        else
          return rawset(table.___table, key, val)
        end
      end
    }

    table = setmetatable(table, metatable)

    for k, v in pairs(initial) do
      table[k] = v
    end

    return table
  end
end

function at_ipairs(at)
  return ipairs(at.___array)
end

function at_pairs(at)
  return pairs(at.___table)
end

a = arrayTable{1, 2, a = 1}

a[1.5]=1.5
a[3] = "a"
a["b"] = 2
a[5] = "c"
a["c"] = 3
a[4] = "b"

for i, v in at_ipairs(a) do
  print(i, v)
end

print('---------')

for i, v in at_pairs(a) do
  print(i, v)
end

output = [[
▸ lua arrayandtable.lua 
1	1
2	2
3	a
4	b
5	c
---------
a	1
c	3
1.5	1.5
b	2
]]