lua - how to represent nil in a table -

let's suppose want store list of element. including nil values. position of values significant, , need represent absence of value in list @ given position.

here problem:

a = {1,2,3,nil,4}  k,v in ipairs(a)   print(k,v) end  print(a[4])  print(a[5]) 

the loop print elements 1,2 , 3. stops @ nil. first print statement prints nil, i'm not sure if stored in table or not. (who knows?) second print statement prints 4 - expected.

so here question: how represent list of elements in table, , iterate through them efficiently? given conditions above, e.g. position significant, , of positions "empty". in other words: have no value, absence of value @ position has meaning.

this module "null.lua"

local function null(...)    local t, n = {...}, select('#', ...)    k = 1, n       local v = t[k]       if     v == null t[k] = nil       elseif v == nil  t[k] = null       end    end    return (table.unpack or unpack)(t, 1, n) end _g.null = null 

use null() encoder , decoder

require("null")  = {null(1,2,3,nil,4)} -- same done element-by-element -- = {null(1),null(2),null(3),null(nil),null(4)}  k,v in ipairs(a)    v = null(v)    print(k,v) end  print(null(a[4])) print(null(a[5]))