Click dictionary? How to achieve this in Lua?

Question

Say I have this dictionary in Lua

places = {dest1 = 10, dest2 = 20, dest3 = 30}

In my program, I check if the dictionary matches my size limit in this case 3, how can I get the oldest key / value pair from the dictionary and add a new one?

places["newdest"] = 50

--places should now look like this, dest3 pushed off and newdest added and dictionary has kept its size

places = {newdest = 50, dest1 = 10, dest2 = 20}

Answer 1

This is not too difficult to do if you really need it, and it can be easily reused.

local function ld_next(t, i) -- This is an ordered iterator, oldest first.
  if i <= #t then
    return i + 1, t[i], t[t[i]]
  end
end

local limited_dict = {__newindex = function(t,k,v)
  if #t == t[0] then -- Pop the last entry.
    t[table.remove(t, 1)] = nil
  end
  table.insert(t, k)
  rawset(t, k, v)
end, __pairs = function(t)
  return ld_next, t, 1
end}

local t = setmetatable({[0] = 3}, limited_dict)

t['dest1'] = 10
t['dest2'] = 20
t['dest3'] = 30
t['dest4'] = 50

for i, k, v in pairs(t) do print(k, v) end

dest2 20
dest3 30
dest4 50

The order is stored in numerical indexes with an index 0th indicating the limit of the unique keys that the table can have.

Answer 2

, , -, , , .

function push_old(t, k, v)
  local z = fifo[1]
  t[z] = nil
  t[k] = v
  table.insert(fifo, k)
  table.remove(fifo, 1)
end

fifo, , (, fifo = { "dest3", "dest2", "dest1" }, , ), :

push_old(places, "newdest", 50)

. !