My program won't seem to recognize spaces in Lua - lua

Question

Have to make a program for school. Basically you have a string and you need to make it secret by using a code. The secret string is each letter + the secret code in the alphabet. So BALL would be EDOO with the code 3. A space has location zero in the alphabet and if the location + the code is bigger than 26 the new letter is string + code - 26 now whenever I execute my program and add spaces it just "translates" the first part before the space. FYI, the comments next to it are in Dutch and not that important.
local code = 3
local string = string.upper("spaghetti carbonara") --string
local alfabet = " ABCDEFGHIJKLMNOPQRSTUVWXYZ" --alfabet
local lengte = 6 --lengte van string
for getal1 = 1, lengte do -- for want moet herhaald worden/getal
local letter = string.sub(string, getal1, getal1) -- letter uit de string halen
local locatie = string.find(alfabet, letter) -- letter vinden in alfabet
if locatie + code < 26 then
locatie2 = locatie + code -- nieuwe locatie vinden met code
elseif locatie + code > 26 then
locatie2 = locatie + code - 26
end
local letter2 = string.sub(alfabet, locatie2, locatie2) -- letter die overeenkomt met locatie
io.write(letter2)
end

Answer 1

First of all, you shouldn't be using the variable string like you're doing on line 2. That doesn't break your program only by coincidence because all strings have a metamethod with __index pointing to string. You should rename that variable to something else instead (text, input, str, etc.)
With that out of the way, your program seems to work just fine for me. Adding a bunch of spaces to the input adds a bunch of "C"s to the output. Maybe you've just forgotten to change that local lengte = 6 to local lengte = #string? As it currently is, it will only "translate" the first 6 characters in the string.
As a hint on how to improve this: you can replace the whole encryption / decryption part with a single call to string.gsub if you pass it a table as its third argument:
local code = {}
code["a"] = "b"
code["b"] = "a"
print(string.gsub("abc", ".", code))
-- Prints "bac"
-- "a" gets replaced with "b", as per the table
-- same with "b"
-- "c" does not appear as a key in the table, so it's simply ignored
Then all you need to do is build a table that maps every character you want to replace to what you want to replace it with, which is a bit easier to achieve:
local function decrypt_encrypt_dictionaries(alphabet, offset)
local encrypt, decrypt = {}, {}
-- Code that fills the two tables goes here :D
return encrypt, decrypt
end
local ROT13_encrypt, ROT13_decrypt = dectypt_encrypt_dictionaries("abcde"--[[...and so on]], 13)
local some_text = io.read()
-- Encrypt some text:
local encrypted = string.gsub(some_string, ".", ROT13_encrypt)
-- Decrypt it again:
local original = string.gsub(encrypted, ".", ROT13_decrypt)
-- See if it's the same:
print(original)
As I mentioned above, this only shifts the difficulty from the encryption/decryption to building the two Lua tables, which I'm leaving for you to figure out.