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Linked Lists in Lua

I've recently been looking up linked lists in Lua and have a quick question that so far I haven't been able to find an answer for

local head = nil

head = {next = head, value = "d"}
head = {next = head, value = "c"}
head = {next = head, value = "b"}
head = {next = head, value = "a"}

local entry = head

while entry do

  entry = entry.next

This will end up printing out "a,b,c,d". I understand why it would print out backwards, as the first 'node' available would be the final one created (with value = a). My question is why the 'head's created before the final one still exist and have not simply been overwritten in the memory.

like image 900
user2087398 Avatar asked Mar 23 '23 21:03


1 Answers

What do you mean by "overwritten in the memory"? Nothing you're doing would cause that to happen.

Let's look at what you're doing step by step.

local head = nil

There now exists a local variable called head. It has the value nil.

head = {next = head, value = "d"}

Let's break this down into the sequence of operations here. This is equivalent to the following:

  local temp = {}
  temp.next = head  --This is still `nil`
  temp.value = "d"
  head = temp

Every table you construct is a unique value. So let's call this first table table-d. It is constructed, stored in the temporary variable temp. The table is given a next value of nil. And it gets a value value of "d". And the result is stored in the local variable head.

So now head has the value table-d. Next step:

head = {next = head, value = "c"}

Same thing:

  local temp = {}
  temp.next = head  --Not nil anymore.
  temp.value = "c"
  head = temp

OK, we create a new table. For the sake of clarity, we'll call this table table-c.

We store this in temp. We then set it's next field to the value in head. That value is table-d. We set the value field to "c". And then store table-c into head.

The table-c table looks like this:

  next = { value = "d" }
  value = "c"

That is the table stored in head.

This continues like this. So where would something be "overwritten"?

like image 165
Nicol Bolas Avatar answered Apr 07 '23 07:04

Nicol Bolas