Lua: Merge 2 strings collections in a case-insensitive manner - lua

Question

I want to merge two strings collections in a case-insensitive manner:
string_collection1 = {"hello","buddy","world","ciao"}
string_collection2 = {"Hello","Buddy","holly","Bye", "bYe"}
merged_string_collection = merge_case_insensitive(string_collection1,string_collection2) --> {"hello","buddy","world","holly","bye","ciao"}
Here's an attempt, but it does not work...
function merge_case_insensitive(t1,t2)
t3 = {}
for _,s1 in pairs(t1) do
for _,s2 in pairs(t2) do
if string.lower(s1) == string.lower(s2) then
t3[s1] = s1
end
end
end
t4 = {}
i = 1
for s,_ in pairs(t3) do
t4[i] = string.lower(s)
i = i + 1
end
return t4
end
string_collection1 = {"hello","buddy","world","ciao"}
string_collection2 = {"Hello","Buddy","holly","Bye", "bYe"}
merged_string_collection = merge_case_insensitive(string_collection1,string_collection2)
for k,v in pairs(merged_string_collection) do print(k,v) end

Answer 1

It does not work because you use == to compare both strings which is case-sensitive.
You could do something like string.lower(s1) == string.lower(s2) to fix that.
Edit:
As you can't figure out the rest yourself, here's some code:
local t1 = {"hello","buddy","world","ciao"}
local t2 = {"Hello","Buddy","holly","Bye", "bYe"}
local aux_table = {}
local merged_table = {}
for k,v in pairs(t1) do
aux_table[v:lower()] = true
end
for k,v in pairs(t2) do
aux_table[v:lower()] = true
end
for k,v in pairs(aux_table) do
table.insert(merged_table, k)
end
merged_table now contains the lower case version of every word in both input tables.
Now pour that into a function that takes any number of input tables and you are done.
What we did here: we use the lower case version of every word in those tables and store them in a list. aux_table[string.lower("Hello")] will index the same value as aux_table[string.lower("hello")]. So we end up with one entry for each word, even if a word comes in multiple variations.
Using the keys saves us the hassle of comparing strings and distiguishing between unique words and others.

Answer 2

To get a table with all strings from two other tables appearing once (without regard to case), you need something like this:
function merge_case_insensitive(t1,t2)
local ans = {}
for _,v in pairs(t1) do ans[v:lower()] = true end
for _,v in pairs(t2) do ans[v:lower()] = true end
return ans
end
string_collection1 = {"hello","buddy","world","ciao"}
string_collection2 = {"Hello","Buddy","holly","Bye", "bYe"}
merged_string_collection = merge_case_insensitive(string_collection1,string_collection2)
for k in pairs(merged_string_collection) do print(k) end
Edit: And in case you want an array result (without adding another iteration)
function merge_case_insensitive(t1,t2)
local ans = {}
local
function add(t)
for _,v in pairs(t) do
v = v:lower()
if ans[v] == nil then ans[#ans+1] = v end
ans[v] = true
end
end
add(t1)
add(t2)
return ans
end
string_collection1 = {"hello","buddy","world","ciao"}
string_collection2 = {"Hello","Buddy","holly","Bye", "bYe"}
merged_string_collection = merge_case_insensitive(string_collection1,string_collection2)
for _,v in ipairs(merged_string_collection) do print(v) end

Answer 3

We can do this by simply iterations over both tables, and storing a temporary dictionary for checking what words we have already found, and if not there yet, putting them in our new array:
function Merge(t1, t2)
local found = {} --Temporary dictionary
local new = {} --New array
local low --Value to store low versions of words in later
for i,v in ipairs(t1) do --Begin iterating over table one
low = v:lower()
if not found[low] then --If not found yet
new[#new+1] = low --Put it in the new table
found[low] = true --Add it to found
end
end
for i,v in ipairs(t2) do --Repeat with table 2
low = v:lower()
if not found[low] then
new[#new+1] = low
found[low] = true
end
end
return new --Return the new array
end
This method eliminates the need for a third iteration, like in Piglet's answer, and doesn't keep redefining a function and closure and calling them like in tonypdmtr's answer.