Question

I’m using Lua tables as sets by placing the value of the set in the table key and 1 as the table value, e.g.

function addToSet(s,...)      for _,e in ipairs{...} do s[e]=1   end end
function removeFromSet(s,...) for _,e in ipairs{...} do s[e]=nil end end

local logics = {}
addToSet(logics,true,false,"maybe")

To test if two sets are equal I need to ensure that they have exactly the same keys. What’s an efficient way to do this?

Answer 1

Loop through both tables and make sure that the key has a value in the other. Fail as soon as you find a mismatch, return true if you got through both. For sets of size M and N this is O(M+N) complexity.

function sameKeys(t1,t2)
  for k,_ in pairs(t1) do if t2[k]==nil then return false end end
  for k,_ in pairs(t2) do if t1[k]==nil then return false end end
  return true
end

Seen in action:

local a,b,c,d = {},{},{},{}
addToSet(a,1,2,3)
addToSet(b,3,1,2,3,3,1)
addToSet(c,1,2)
addToSet(d,2,1)
print(sameKeys(a,b)) --> true
print(sameKeys(a,c)) --> false
print(sameKeys(d,c)) --> true

Note testing for t[k]==nil is better than just not t[k] to handle the (unlikely) case that you have set a value of false for the table entry and you want the key to be present in the set.