Re: how to work with tables with holes

[Sean Conner <sean@...>]

It was thus said that the Great Rafis Ganeyev once stated:
> Tables can't be used so easily as array() in PHP. All Lua developers should
> know their peculiarities to use them properly: distinctions between vector
> and hash parts, no count() function, maybe vector and hash parts grew
> strategy, etc.
> 
> My question is simple. If I have some collection of objects in vector part
> and I create holes by setting nil to middle elements, length operator and
> ipairs begin work improperly, how to I traverse this collection? Only with
> pairs() or should I use table.getn(), which seems have been removed in Lua
> 5.2? Or should I create element called "n" in hash part and create
> add/remove functions for collection? What are best practices for that case?

  1) Don't set an element to nil.  Use table.remove() instead.  That way, #
     will alway work:

	t = { 1 , 2 , 3 , 4 }
	print(#t)
	table.remove(t,2)
	print(#t)

  2) Use a sentinel value to mark removal.  

	empty = {}
	t = { 1 , 2 , 3 , 4 }
	print(#t)
	t[2] = empty
	print(#t)

	for i = 1 , #t do
	  if t[i] ~= empty then
	    print(t[i])
	  end
	end

  3) Keep track of both the size and the maximum index.

	m = { __newindex = function(t,i,v)
			     if v == nil then
			       print(">>>")
			       t.N = t.N - 1
                               t.P[i] = v
			       return
			     end
			     
			     if i > t.MAX then
			       t.MAX = i
			     end

			     t.N = t.N + 1
			     t.P[i] = v
			   end
	}
	t = { P = { 1 , 2 , 3 , 4 } , N = 4 , MAX = 4 }
	setmetatable(t,m)
	print(t.N)
	t[2] = nil
	print(t.N)

  In my opintion, the best practice is #1.

  -spc