ex20.2.lua

local Set = {}
local mt = {}
-- create a new set with the values of a given list
function Set.new (l)
  local set = {}
  setmetatable(set, mt)
  for _, v in ipairs(l) do set[v] = true end
  return set
end
function Set.union (a, b)
  local res = Set.new{}
  for k in pairs(a) do res[k] = true end
  for k in pairs(b) do res[k] = true end
  return res
end
function Set.intersection (a, b)
  local res = Set.new{}
  for k in pairs(a) do
    res[k] = b[k]
  end
  return res
end
function Set.diff (a, b)
  local res = Set.new{}
  for k in pairs(a) do
    if not b[k] then
      res[k] = true
    end
  end
  return res
end
-- presents a set as a string
function Set.tostring (set)
  local l = {}
  -- list to put all elements from the set
  for e in pairs(set) do
    l[#l + 1] = tostring(e)
  end
  return "{" .. table.concat(l, ", ") .. "}"
end

function Set.len (set)
  local count = 0
  for _ in pairs(set) do
    count = count + 1
  end
  return count
end

mt.__add = Set.union
mt.__mul = Set.intersection
mt.__sub = Set.diff
mt.__tostring = Set.tostring
mt.__len = Set.len

local s1 = Set.new({10, 20, 30})

print(#s1)
-- return Set