how to i change the positions of elements in a list to the actual elements (keyword cipher)?

Question

this cipher uses a keyword and repeats it to the length of a message inputted then both are converted to number (positions of each letter of the keyword and message in the alphabet list) it then adds them together and is SUPPOSED to then be converted back to letters in the alphabet list.

alpha =         ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]
keyword = input("Please enter a keyword: ")

sentence = input("Enter message: ")

new_keyword = []
while len(keyword) < len(sentence):
    keyword = keyword + keyword
keyword = (keyword.lower())


for letters in keyword:
    pos1 = alpha.index(letters) + 1
    new_keyword.append(pos1)
print (new_keyword)

new_sentence = []   

for letters in sentence:
    pos2 = alpha.index(letters) + 1
    new_sentence.append(pos2)
print (new_sentence)

joined = [x + y for x, y in zip(new_keyword, new_sentence)]
print (joined)

that is my code

i need to find a way to turn the joined list into letters again aka the encrypted message

PLEASE HELP

Answer 1

So far you have shifted the letters by the appropriate amount but still have the ordinal (or numeric) value stored.

Problem 1 is to deal with letters that wrap past 26 (e.g. z (26) + a (1) = 27).

Problem 2 is to use the alpha array to turn the values back into numbers.

I've left the print statements in so you can see what is happening.

# Original code - unedited
alpha = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]
keyword = input("Please enter a keyword: ")

sentence = input("Enter message: ")

new_keyword = []
while len(keyword) < len(sentence):
    keyword = keyword + keyword
keyword = (keyword.lower())


for letters in keyword:
    pos1 = alpha.index(letters) + 1
    new_keyword.append(pos1)
print (new_keyword)

new_sentence = []   

for letters in sentence:
    pos2 = alpha.index(letters) + 1
    new_sentence.append(pos2)
print (new_sentence)

joined = [x + y for x, y in zip(new_keyword, new_sentence)]
print (joined)

# Take any value over 26 and wrap it back round (so z + 1 = a)

for i in range(len(joined)):
    if joined[i] > 26:
        joined[i] -= 26

print (joined)

# Convert each numeric value back into its character, using the alpha list

ciphertext = []

for letters in joined:
    char = alpha[letters - 1]
    ciphertext.append(char)

print(ciphertext)

A slightly easier way is to use the ord() and chr() functions. These convert the characters into their ASCII values and back, respectively.