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I seem to be having some problems with Vigenere.py for problem set 6. I am attempting to use ord to change the ith character in the plaintext into ASCii, but it is not working. Everything else seems to be logical I think?

Thank you for your help.

import sys

if len(sys.argv) == 2:
    key = str(sys.argv[1])

    if str.isalpha(key) == True:
        plaintext = str(input("Enter the plaintext you wish you encrypt: "))
        print ("ciphertext = ", end='')

        for i in plaintext:
            plaintext = ord(str(plaintext[i]))

            #preventing long plaintext words from not being encrypted
            if i > len(ciphertext) or k > len(ciphertext):
                k = 0
                cipher = ord(str(ciphertext[k]))
                k += 1
            else:
                ciphertext = ord(str(ciphertext[i]))
            #

            final = (plaintext + ciphertext)%26

            print (chr(final), end='')
    else:
        print ("Enter only letters")

else:
    print ("Usage: Enter a key to use to ciphers")
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Go back to your C solution and try to translate it into Python. Not word by word, but by meaning. This code is not a transcript of a working C solution, it's far from a working solution at all.

    key = str(sys.argv[1])

No need to convert this to a string, as it already is.

    if str.isalpha(key) == True:

Just if str.isalpha(key): would be enough.

        plaintext = str(input("Enter the plaintext you wish you encrypt: "))
        print ("ciphertext = ", end='')

Are those the strings required per specification?

        for i in plaintext:

i will assume the characters of plaintext, it's not a number. You can use i instead of plaintext[i], or use for i in range(len(plaintext)):

            plaintext = ord(str(plaintext[i]))

No need for str. You replace string plaintext with a number. Better find another variable.

            if i > len(ciphertext) or k > len(ciphertext):

You never created variables k and ciphertext, so what should k > len(ciphertext) mean?

                k = 0
                cipher = ord(str(ciphertext[k]))
                k += 1

So after this code block, which is the only place where k is assigned to, it always has value 1. Also, str not needed.

                ciphertext = ord(str(ciphertext[i]))

Multiple things from above apply here, too.

            final = (plaintext + ciphertext)%26
            print (chr(final), end='')

In the C solution, you shifted the characters back and forth a bit, ignoring case for the key and preserving case for the character to encrypt.

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