CS50 2017 Vigenere Python "IndexError: list index out of range"

Question

Um, another error, another thread? O.o Well, anyway, here's the code:

import cs50
import sys

if len(sys.argv) == 1 or len(sys.argv) > 2:
    print("Usage: python vigenere.py <key>")
    exit(1)

key = sys.argv[1]
for i in range(len(key)):
    if key[i].isalpha() is not True:
        print("Keyword must only contain letters A-Z or a-z")
        exit(2)

print("plaintext: ", end="")
plaintext = cs50.get_string()
plaintext_size = len(plaintext)
key_size = len(key)

ciphertext = []
j = 0
for i in range(plaintext_size):
    if j >= key_size:
        j = 0
    if plaintext[i].isalpha():
        k = ord(key[j].upper()) % 65
        ciphertext = []

        c = (ord(ciphertext[i]) + ord(key[i])) % 26
        c += ord('A')
        ciphertext.append(chr(c))
        j += 1

    else:
        c = ord(plaintext[i])
        ciphertext.append(chr(c))

print("ciphertext: ", end="")
print("".join(ciphertext))
exit(0)

The error is on line 28: c = (ord(ciphertext[i]) + ord(key[i])) % 26, and it says, "IndexError: list index out of range".

Answer 1

IndexError probably means that you are trying to access an element in a list that doesn't exist. This particular problems refers to, on line 28, that you are trying to read an index in ciphertext which doesn't even exist yet, and there's also every chance that there aren't i elements in key.