Most efficient way to determine if a Lua table is empty (contains no entries)?

Question

What's the most efficient way to determine if a table is empty (that is, currently contains neither array-style values nor dict-style values)?

Currently, I'm using next():

if not next(myTable) then
    -- Table is empty
end

Is there a more efficient way?

Note: The # operator does not suffice here, as it only operates on the array-style values in the table - thus #{test=2} is indistinguishable from #{} because both return 0. Also note that checking if the table variable is nil does not suffice as I am not looking for nil values, but rather tables with 0 entries (i.e. {}).

Answer 1

One possibility would be to count the number of elements, by using the metatable "newindex" key. When assigning something not nil, increment the counter (the counter could live in the metatable as well) and when assigning nil, decrement the counter.

Testing for empty table would be to test the counter with 0.

Here's a pointer to metatable documentation

I do like your solution though, and I honestly can't assume that my solution is faster overall.

Answer 2

Your code is efficient but wrong. (Consider {[false]=0}.) The correct code is

if next(myTable) == nil then
   -- myTable is empty
end

For maximum efficiency you'll want to bind next to a local variable, e.g.,

...
local next = next 
...
... if next(...) ...