If you have an array in Lua, which is a table used as a contiguous list of items, this is how you get the number of items:

    num_items = #my_array

Your qualm, apparently, is that this # is not built for all use cases of tables.

In C++, what operator tells you how many instance variables a class has? There is none, because that wouldn't make sense in many use cases. Lua tables, offering prototype-based inheritance, experience a similar conceptual ambiguity in the case where you want to take the length of an object.

Lua has made a design decision that the built-in length operator only works on arrays = tables-used-as-a-list and on strings -- cases without ambiguity. If you want the length operator to make sense for a hash map usage, it's a few extra lines of work for you. It's consistent with the langauge being small and flexible, a theme which Lua embraces with elegance.