Lua - Modifying List Elements
We can access the list element while traversing them and as tables are references, the modification done will reflect. In the similar fashion, after accessing the first and last element we can modify them.
We'll build the list and then add method to get first element, last element and to iterate the list and in this process, we'll update and print the values of the list.
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: Get first element and then we can modify it.
Create a function to get first element
function list:getFirst()
-- if first is nil then return
if not self.first then
return
end
return self.first
end
Step 4: Get Last element and then we can modify it.
Create a function to get last element
function list:getLast()
-- if last is nil then return
if not self.last then
return
end
return self.last
end
Step 5: Create iterator over list and update complete list during iteration.
Create an iterator to navigate through elements of the list.
-- iterate through the list
local function iterate(self, current)
-- if current is nil
-- set the current as first node
if not current then
current = self.first
-- if current is present
-- set current as current next
elseif current then
current = current._next
end
-- return current
return current
end
-- return the iterator
function list:iterate()
return iterate, self, nil
end
Step 6: Test Modification of elements on List
In list, we can insert objects,
-- create a new list with values
local l = list({ "Mon" }, { "Tue" }, { "Wed" }, { "Thu" }, { "Fri" })
print("Update List to make each entry in Uppercase")
-- iterate throgh entries
for v in l:iterate() do
print(string.upper(v[1]))
end
-- modify first element
l:getFirst()[1] = "MONDAY"
print("First Element: " , l:getFirst()[1])
-- modify last element
l:getLast()[1] = "FRIDAY"
print("Last Element: " , l:getLast()[1])
Complete Example - Modify an element of a List
Following is the complete example of modifying 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
function list:getLast()
-- if last is nil then return
if not self.last then
return
end
return self.last
end
function list:getFirst()
-- if first is nil then return
if not self.first then
return
end
return self.first
end
-- create a new list with values
local l = list({ "Mon" }, { "Tue" }, { "Wed" }, { "Thu" }, { "Fri" })
print("Update List to make each entry in Uppercase")
-- iterate throgh entries
for v in l:iterate() do
print(string.upper(v[1]))
end
-- modify first element
l:getFirst()[1] = "MONDAY"
print("First Element: " , l:getFirst()[1])
-- modify last element
l:getLast()[1] = "FRIDAY"
print("Last Element: " , l:getLast()[1])
Output
When we run the above code, we will get the following output−
Update List to make each entry in Uppercase MON TUE WED THU FRI First Element: MONDAY Last Element: FRIDAY
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