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Im trying to create a Ceaser Cipher function in Python that shifts letters based off the input you put in.

plainText = input("Secret message: ")

shift = int(input("Shift: "))



def caesar(plainText, shift): 

  cipherText = ""

  for ch in plainText:

    if ch.isalpha():

      stayInAlphabet = ord(ch) + shift 

      if stayInAlphabet > ord('z'):

        stayInAlphabet -= 26

      finalLetter = chr(stayInAlphabet)

      cipherText += finalLetter

  print(cipherText)

  return cipherText



caesar(plainText, shift)

For example, if I put "THE IDES OF MARCH" as my message and put 1 as my shift, it outputs "UIFJEFTPGNBSDI" when it is meant to output "UIF JEFT PG NBSDI." It doesn't keep the spaces and also shifts things like exclamation marks back also when it should leave them as is. Letters should also wrap meaning if I put shift as 3, an X should go back to A.

To fix the spacing issue, you can add an else to if ch.isalpha() and just append the plain text character to the cipher text. This will also handle punctuation and other special, non-alpha characters.

To handle wrapping (e.g. X to A), you'll want to use the modulo operator %. Because A is the 65th ASCII character and not the 0th, you'll need to zero-base the alpha characters, then apply the mod, then add back the offset of 'A'. To shift with wrap-around, you can do something like: final_letter = chr((ord(ch) + shift - ord('A')) % 26 + ord('A')). Note the 26 comes from number of letters in the Latin alphabet.

With these in mind, here is a full example:

plain_text = input("Secret message: ")

shift = int(input("Shift: "))



def caesar(plain_text, shift): 

  cipher_text = ""

  for ch in plain_text:

    if ch.isalpha():

      final_letter = chr((ord(ch) + shift - ord('A')) % 26 + ord('A'))

      cipher_text += final_letter

    else:

      cipher_text += ch

  print(cipher_text)

  return cipher_text



caesar(plain_text, shift)

Sample input:

plain_text = "THE IDES OF MARCH"

shift = 1



cipher_text = caesar(plain_text, shift)

print(cipher_text)

# UIF JEFT PG NBSDI

The reason the cipher does not produce the expected result is your code does not account for the case where it is not a alpha non numerical letter. So, a potential fix is just adding handling for spaces.

Code

plainText = input("Secret message: ")

shift = int(input("Shift: "))





def caesar(plainText, shift):

    cipherText = ""

    for ch in plainText:

        if ch.isalpha():

            stayInAlphabet = ord(ch) + shift

            if stayInAlphabet > ord('z'):

                stayInAlphabet -= 26

            finalLetter = chr(stayInAlphabet)

            cipherText += finalLetter

        elif ch is " ":

            cipherText += " "

    print(cipherText)

    return cipherText





caesar(plainText, shift)

Example

Secret message: THE IDES OF MARCH

Shift: 1

UIF JEFT PG NBSDI