Lua - Removing an element from a List
Removing an element from a linked list requires updates in the references of prev node and next node of the element to be removed.
Case of Last element
We can set the next of prev node as nil, set the last as prev node as shown in psuedo code below−
-- get the last node local removedElement = self.last -- if prev is present -- then set prev node as last node if removedElement._prev then removedElement._prev._next = nil self.last = removedElement._prev removedElement._prev = nil else -- last node was only node in the list self.first = nil self.last = nil end
Case of Other element, t
If t has both next and prev nodes, then update prev and next references of t.next and t.prev. If t is the first node, set first as t.next.
See the pseudo code below−
if t._next then
if t._prev then
t._next._prev = t._prev
t._prev._next = t._next
else
-- this was the first node
t._next._prev = nil
self._first = t._next
end
elseif t._prev then -- if node has prev node only
-- this was the last node
t._prev._next = nil
self._last = t._prev
else
-- this was the only node
self._first = nil
self._last = nil
end
Now We'll build the list, add method to remove an element from the end of the list and to remove a specific element from the list step by step as shown below:
Step 1: Create List
Create a List with a push method to add an element to the end of the list.
-- List Implementation
list = {}
list.__index = list
-- push an element to the end of the list
function list:push(t)
-- move till last node
if self.last then
self.last._next = t
t._prev = self.last
self.last = t
else
-- set the node as first node
self.first = t
self.last = t
end
-- increment the length of the list
self.length = self.length + 1
end
Step 2: Using setmetatable
modify list behavior when list is called to push elements.
setmetatable(list, { __call = function(_, ...)
local t = setmetatable({ length = 0 }, list)
for _, v in ipairs{...}
do t:push(v)
end
return t
end })
Step 3: Create iterator over list
Create an iterator to navigate through elements of the list.
-- iterate through the list
local function iterate(self, current)
if not current then
current = self.first
elseif current then
current = current._next
end
return current
end
function list:iterate()
return iterate, self, nil
end
Step 4: Create removeLast() and remove() methods
Create removeLast() to remove element from the end of the list and remove(t) to remove a given element.
-- remove last element from the list
function list:removeLast()
-- if last is nil then return
if not self.last then
return
end
local removedElement = self.last
-- if prev is present
-- then set prev node as last node
if removedElement._prev then
removedElement._prev._next = nil
self.last = removedElement._prev
removedElement._prev = nil
else
-- last node was only node in the list
self.first = nil
self.last = nil
end
-- decrement the length
self.length = self.length - 1
return removedElement
end
-- remove a specific element, t
function list:remove(t)
-- if node to delete has next node
-- update the pointers of prev and next nodes
if t._next then
if t._prev then
t._next._prev = t._prev
t._prev._next = t._next
else
-- this was the first node
t._next._prev = nil
self._first = t._next
end
elseif t._prev then -- if node has prev node only
-- this was the last node
t._prev._next = nil
self._last = t._prev
else
-- this was the only node
self._first = nil
self._last = nil
end
t._next = nil
t._prev = nil
self.length = self.length - 1
end
Step 5: Test remove operations on List
First initialize a list with few values and then remove few elements.
-- create a new list with values
local l = list({ "Mon" }, { "Tue" }, wed, { "Thu" }, { "Fri" })
print("Original List")
-- iterate throgh entries
for v in l:iterate() do
print(v[1])
end
l:removeLast()
print("Updated List")
-- iterate throgh entries
for v in l:iterate() do
print(v[1])
end
l:remove (wed)
print("Updated List")
-- iterate throgh entries
for v in l:iterate() do
print(v[1])
end
Complete Example - Removal of Elements from a List
Following is the complete example of inserting and traversing elements of a list.
main.lua
-- List Implementation
list = {}
list.__index = list
setmetatable(list, { __call = function(_, ...)
local t = setmetatable({ length = 0 }, list)
for _, v in ipairs{...}
do t:push(v)
end
return t
end })
-- push an element to the end of the list
function list:push(t)
-- move till last node
if self.last then
self.last._next = t
t._prev = self.last
self.last = t
else
-- set the node as first node
self.first = t
self.last = t
end
-- increment the length of the list
self.length = self.length + 1
end
-- iterate through the list
local function iterate(self, current)
if not current then
current = self.first
elseif current then
current = current._next
end
return current
end
function list:iterate()
return iterate, self, nil
end
-- remove last element from the list
function list:removeLast()
-- if last is nil then return
if not self.last then
return
end
local removedElement = self.last
-- if prev is present
-- then set prev node as last node
if removedElement._prev then
removedElement._prev._next = nil
self.last = removedElement._prev
removedElement._prev = nil
else
-- last node was only node in the list
self.first = nil
self.last = nil
end
-- decrement the length
self.length = self.length - 1
return removedElement
end
-- remove a specific element, t
function list:remove(t)
-- if node to delete has next node
-- update the pointers of prev and next nodes
if t._next then
if t._prev then
t._next._prev = t._prev
t._prev._next = t._next
else
-- this was the first node
t._next._prev = nil
self._first = t._next
end
elseif t._prev then -- if node has prev node only
-- this was the last node
t._prev._next = nil
self._last = t._prev
else
-- this was the only node
self._first = nil
self._last = nil
end
t._next = nil
t._prev = nil
self.length = self.length - 1
end
local wed = { "Wed" }
-- create a new list with values
local l = list({ "Mon" }, { "Tue" }, wed, { "Thu" }, { "Fri" })
print("Original List")
-- iterate throgh entries
for v in l:iterate() do
print(v[1])
end
l:removeLast()
print("Updated List after removing last element")
-- iterate throgh entries
for v in l:iterate() do
print(v[1])
end
l:remove (wed)
print("Updated List after removing Wed as well")
-- iterate throgh entries
for v in l:iterate() do
print(v[1])
end
Output
When we run the above code, we will get the following output−
Original List Mon Tue Wed Thu Fri Updated List after removing last element Mon Tue Wed Thu Updated List after removing Wed as well Mon Tue Thu
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