how to represent nil in a table

Question

Let's suppose I want to store a list of element. Including some nil values. The position of the values is significant, and I need to represent the absence of a value in the list at a given position.

Here is a problem:

a = {1,2,3,nil,4}

for k,v in ipairs(a) do
  print(k,v)
end

print(a[4]) 
print(a[5])

The for loop will only print elements 1,2 and 3. It stops at nil. The first print statement prints nil, but I'm not sure if it is actually stored in the table or not. (Who knows?) The second print statement prints 4 - as expected.

So here is the question: how to represent a list of elements in a table, and iterate through them efficiently? Given the conditions above, e.g. the position is significant, and some of the positions are "empty". In other words: have no value, but the absence of that value at that position has a meaning.

Answer 1

This is module "null.lua"

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

Use null() as encoder and decoder

require("null")

a = {null(1,2,3,nil,4)}
-- the same could be done element-by-element
-- a = {null(1),null(2),null(3),null(nil),null(4)}

for k,v in ipairs(a) do
   v = null(v)
   print(k,v)
end

print(null(a[4]))
print(null(a[5]))