Question

I want to make a python program that encodes/decodes inputted messages.

There are only three options in the initial menu: 1. encode, 2. decode, 3. quit, and if 1, 2, or 3 are not the input the program says that it is not valid, try again.

If encode is chosen, the user inputs their message, the message should only be alphabetical and spaces. Then, the user can pick a number in the range [-27,27] with space being the 27th character (if the number is not in this range they should be reminded of the range and asked to try again) . The message is then "encoded" according to the number chosen by moving each character forward (or backward if negative) that many letters. For example, if the message is "apple pie" and the number inputted is 3, the encoded message will be "dssohcslh". If decode is selected, the user can put in the same message ("dssohcslh") and number, 3, and this message will be "apple pie" again. After encoding or decoding the initial menu should pop up again.

I would appreciate if you can try to keep it simple as I am a beginner. Inputted messages don't need to be saved in any way.

Answer 1

Please find below the code, code screenshots and output screenshots. Please refer to the screenshot of the code to understand the indentation of the code.  Please get back to me if you need any change in code. Else please upvote

CODE:

#Declare a list of albhabets and space as the 27th letter

alphabet_list = ["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", " "]

                

#Function that reads the shift factor from user, validate it and return a valid shift factor

def get_shift_factor():

    isValid = False

    while not isValid:

        shift = int(input("Enter a number in range [-27, 27]: ")) #Reading the shift factor from user and convert to int

        if(shift >= -27 and shift <= 27): #Check if shift factor entered is in range [-27, 27]

            isValid = True #If yes, set isValid as True

        else:

            print("Number entered is out of range [-27, 27]... Please re-enter") #Else display error

    return shift #return the shift factor

#This function should accept a string to encrypt and the shift factor to use.

#It should construct and return the corresponding encrypted message

def encrypt(message, shift):

    encrypted_msg = "" #declare an empty string for encrypted_msg

    for i in range(len(message)): #loop through each character in message

        index = alphabet_list.index(message[i]) #get the index value of character in alphabet_list

        #Adding the shift factor to index value of character

        if((index + shift) > 26):

            index = (index + shift) - 27

        else:

            index = index + shift

        # append albhabet at index to encrypted_msg

        encrypted_msg += alphabet_list[index]

    return encrypted_msg #return the encrypted_msg

#This function should accept a string to decrypt and the shift factor to use.

#It should construct and return the corresponding decrypted (original) message.

def decrypt(message, shift):

    decrypted_msg = "" #declare an empty string for decrypted_msg

    for i in range(len(message)): #loop through each character in message

        index = alphabet_list.index(message[i]) #get the index value of character in alphabet_list

        #Subtracting the shift factor from index value of character

        if((index - shift) < 0):

            index = 27 + (index - shift)

        else:

            index = index - shift  

        #append albhabet at index to decrypted_msg

        decrypted_msg += alphabet_list[index]

    return decrypted_msg #return the decrypted_msg

#main function

def main():

    repeat = True #set repeat as True

    while repeat: #loop untill repeat is True

        #displaying the menu

        print("\nEnter 1 to encode a message")

        print("Enter 2 to deode a message")

        print("Enter 3 to quit the program")

        choice = int(input("Enter your choice: ")) #reading the choice

       

        #If choice is 1, encrypt a message

        if choice == 1:

            message = input("\nEnter the string to encode: ") #reading string to encrypt

            shift_factor = get_shift_factor() #Getting the shift factor

            encrypted_msg = encrypt(message, shift_factor) #calling encrypt() and get the encrypted_msg

            print("The encoded message is", encrypted_msg) #displaying the encrypted_msg

            

        #If choice is 2, decrypt a message

        elif choice == 2:

            message = input("\nEnter the string to decode: ") #reading string to decrypt

            shift_factor = get_shift_factor() #Getting the shift factor

            decrypted_msg = decrypt(message, shift_factor) #calling decrypt() and get the decrypted_msg

            print("The decoded message is", decrypted_msg) #displaying the decrypted_msg

           

        elif choice == 3:

            print("\nGoodbye!!!") #displaying final message

            repeat = False #set repeat as False

       

        else:

            print("Invalid entry...Please select your choice correctly!!!")

main() #calling main function

OUTPUT: