Caesar

tl;dr

Implement a program that encrypts messages using Caesar’s cipher, per the below.

$ python caesar.py 13
plaintext:  HELLO
ciphertext: URYYB

Design and implement a program, caesar, that encrypts messages using Caesar’s cipher, exactly as you did in Problem Set 2, except that your program this time should be written (a) in Python and (b) in CS50 IDE.

Implement your program in a file called caesar.py in your ~/workspace/pset6/caesar directory (if it doesn’t already exist, create it now!).
Your program must accept a single command-line argument, a non-negative integer. Let’s call it k for the sake of discussion.
If your program is executed without any command-line arguments or with more than one command-line argument, your program should print an error message of your choice (with print) and exit immediately with a status code of 1.
You can assume that, if a user does provide a command-line argument, it will be a non-negative integer (e.g., 1). No need to check that it’s indeed numeric.
Do not assume that k will be less than or equal to 26. Your program should work for all non-negative integral values of k less than 2³¹ - 26. In other words, you don’t need to worry if your program eventually breaks if the user chooses a value for k that’s too big or almost too big to fit in an int. (Recall that an int can overflow.) But, even if k is greater than 26, alphabetical characters in your program’s input should remain alphabetical characters in your program’s output. For instance, if k is 27, A should not become [ even though [ is 27 positions away from A in ASCII, per asciichart.com; A should become B, since B is 27 positions away from A, provided you wrap around from Z to A.
Your program must output plaintext: (without a newline) and then prompt the user for a string of plaintext (using get_string).
Your program must output ciphertext: (without a newline) followed by the plaintext’s corresponding ciphertext, with each alphabetical character in the plaintext "rotated" by k positions; non-alphabetical characters should be outputted unchanged.
Your program must preserve case: capitalized letters, though rotated, must remain capitalized letters; lowercase letters, though rotated, must remain lowercase letters.
After outputting ciphertext, you should print a newline.

Walkthrough

Usage

Your program should behave per the examples below. Assume that the underlined text is what some user has typed.

$ python caesar.py 1
plaintext:  HELLO
ciphertext: IFMMP

$ python caesar.py 13
plaintext:  hello, world
ciphertext: uryyb, jbeyq

$ python caesar.py 13
plaintext:  be sure to drink your Ovaltine
ciphertext: or fher gb qevax lbhe Binygvar

$ python caesar.py
Usage: python caesar.py k

$ python caesar.py 1 2 3 4 5
Usage: python caesar.py k

Testing

Correctness

check50 cs50/problems/2019/x/sentimental/caesar

Style

style50 caesar.py

Staff’s Solution

~cs50/2019/x/pset6/caesar

How to Submit

Execute the below, logging in with your GitHub username and password when prompted. For security, you’ll see asterisks (*) instead of the actual characters in your password.

submit50 cs50/problems/2019/x/sentimental/caesar

You can then go to https://cs50.me/cs50x to view your current scores!

Hints

Recall that argv is a list of strings representing the command line arguments. Recall that we can use len(argv) in order to figure out how many strings exist in that list; this is the equivalent idea to argc, from C.

And so you can access k with code like

k = argv[1]

assuming it’s actually there! And assuming you’ve imported argv, as by:

from sys import argv

Once you have both k and some plaintext, p, it’s time to encrypt the latter with the former. Recall that you can iterate over the characters in a string, printing each one at a time, with code like the below:

for c in p:
    print(c, end="")

That end="" line just overrides Python’s default behavior when printing which, unlike C, tacks on a newline by default!

You may also wish to have a look at Python’s ord() and chr() functions!