Re: [Proposal] ## operator

["Thiago L." <fakedme@...>]

On 21/11/14 01:58 PM, Luiz Henrique de Figueiredo wrote:
> > <SoniEx2> I wish Lua had a way to count keys in a table
> This question often appears here and in
> http://stackoverflow.com/questions/tagged/lua and it always strikes me:
> why would you need to count entries in a table? It seems to me that
> this is rarely required, and that it is probably an instance of
> The XY Problem <http://xyproblem.info/> and trying to use in Lua using
> paradigms from other languages. Perhaps I'm missing something.
>
> This requests also springs from a misconception that Lua already
> stores this value internally anyway, which it doesn't.
>
> If you really need to count entries in a table you can use:
>
> function number_of_entries(t)
> 	local n=0
> 	local k=nil
> 	while true do
> 		k=next(t,k)
> 		if k==nil then return n end
> 		n=n+1
> 	end
> end
>
> But there is really no need for an *operator* for that.
Well say you have 2 functions, one for heavy tables and another for 
light tables, and you wanna pick the right one for the right job. The 
current way is to use:

function nkeys(t)
  local c = 0
  for x in next, t do
    c = c + 1
  end
  return c
end

But if you're the Lua internals, you could instead do this:

*something somewhere calls rawset*
1. if key exists and value is nil, then decrement
2. else if key doesn't exist and value is not nil, then increment
*do normal rawset stuff*

*GC collection of weak keys/values*
*GC detects a weak key/value to destroy*
1. call rawset with key and nil
*GC does normal GC stuff*

Which would be much more efficient.