Python Forum

I'm trying to make a basic shift, my code is as follows:

alphabet = ['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',]

def shift():
    message = input("Please enter a message:")
    number = int(input('Please enter the number that you would like to shift by (1-26):'))
    new_message = ""
    for letter in message:
        letter = letter.lower()
        if letter.isalpha():
            new_position = alphabet.index(letter) + number
            if new_position > 26:
                new_position = new_position - 27
            new_letter = alphabet[new_position]
        elif letter.isnumeric():
            letter = letter
            print(letter)
        elif letter == " " or letter == "," or letter == "." or letter == ";" or letter == ":":
            letter = letter
            print(letter)
        else:
            print("Error in message please try a different message")
        print(new_message)
        print()


def decode():
    message = input("Please enter a message to decode:")
    number = int(input('Please enter the number shift key (between 1 and 26):'))
    new_message = ""
    for letter in message:
        letter = letter.lower()
        #y = alphabet.index(x)
        #y = y + number
        if letter.isalpha():
            new_position = alphabet.index(letter) - number
            if new_position < 0:
                new_position = new_position + 27
            new_letter = alphabet[new_position]
            new_message = new_message + new_letter
        elif letter.isnumeric():
            letter = letter
            print(letter)
        elif letter == " " or letter == "," or letter == "." or letter == ";" or letter == ":":
            letter = letter
            print(letter)
        else:
            print("Error in message please try a different message")
    print(new_message)
    print()


def main():
    repeat = True
    while repeat == True:
        print("1) Make a code")
        print("2) Decode a message")
        print("3) Quit")
        print()
        choice = int(input("Enter your selection:"))
        if choice == 1:
            shift()
        elif choice == 2:
            decode()
        elif choice == 3:
            repeat = False
        else:
            print("Incorrect choice, please try again")
            choice = int(input("Enter your selection:"))

main()

However I get the following error:

Error:
  File "c:/Users/djwil/Documents/python/learning python/Chapter 19 - Chunky Challenges/Shift Code.py", line 70, in <module>
    main()
  File "c:/Users/djwil/Documents/python/learning python/Chapter 19 - Chunky Challenges/Shift Code.py", line 63, in main
    decode()
  File "c:/Users/djwil/Documents/python/learning python/Chapter 19 - Chunky Challenges/Shift Code.py", line 38, in decode
    new_letter = alphabet[new_position]
IndexError: list index out of range

However I thought that the lines of code:

if new_position < 0:
                new_position = new_position + 27

and:

            if new_position > 26:
                new_position = new_position - 27

should be preventing this from happening. Can anyone help?

There are only 26 letters in alphabet, not 27. The maximum value for new_position is 25, not 26.

Rather than keeping track with another number (so you have something to change in multiple places), you can use the mod operator and the actual array.

Instead of

            if new_position > 26:
                new_position = new_position - 27

Consider

            new_position %= len(alphabet)

if z is in the message, then the program will attempt to shift z to the next available value in the list, this does not exist. Maybe add an if statement (psuedocode):

if z is in the message:
    turn z into a

(Aug-16-2020, 06:22 PM)bowlofred Wrote: [ -> ]Consider

            new_position %= len(alphabet)

Works for positive and negative values. No need for an if.

x = -2 % 26
y = 28 % 26
print(x, y)

Output:
24 2