Problem: Caesar

tl;dr

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

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

Academic Honesty

This course’s philosophy on academic honesty is best stated as "be reasonable." The course recognizes that interactions with classmates and others can facilitate mastery of the course’s material. However, there remains a line between enlisting the help of another and submitting the work of another. This policy characterizes both sides of that line.

The essence of all work that you submit to this course must be your own. Collaboration on problems is not permitted (unless explicitly stated otherwise) except to the extent that you may ask classmates and others for help so long as that help does not reduce to another doing your work for you. Generally speaking, when asking for help, you may show your code or writing to others, but you may not view theirs, so long as you and they respect this policy’s other constraints. Collaboration on quizzes and tests is not permitted at all. Collaboration on the final project is permitted to the extent prescribed by its specification.

Below are rules of thumb that (inexhaustively) characterize acts that the course considers reasonable and not reasonable. If in doubt as to whether some act is reasonable, do not commit it until you solicit and receive approval in writing from your instructor. If a violation of this policy is suspected and confirmed, your instructor reserves the right to impose local sanctions on top of any disciplinary outcome that may include an unsatisfactory or failing grade for work submitted or for the course itself.

Reasonable

  • Communicating with classmates about problems in English (or some other spoken language).

  • Discussing the course’s material with others in order to understand it better.

  • Helping a classmate identify a bug in his or her code, such as by viewing, compiling, or running his or her code, even on your own computer.

  • Incorporating snippets of code that you find online or elsewhere into your own code, provided that those snippets are not themselves solutions to assigned problems and that you cite the snippets' origins.

  • Reviewing past years' quizzes, tests, and solutions thereto.

  • Sending or showing code that you’ve written to someone, possibly a classmate, so that he or she might help you identify and fix a bug.

  • Sharing snippets of your own solutions to problems online so that others might help you identify and fix a bug or other issue.

  • Turning to the web or elsewhere for instruction beyond the course’s own, for references, and for solutions to technical difficulties, but not for outright solutions to problems or your own final project.

  • Whiteboarding solutions to problems with others using diagrams or pseudocode but not actual code.

  • Working with (and even paying) a tutor to help you with the course, provided the tutor does not do your work for you.

Not Reasonable

  • Accessing a solution to some problem prior to (re-)submitting your own.

  • Asking a classmate to see his or her solution to a problem before (re-)submitting your own.

  • Decompiling, deobfuscating, or disassembling the staff’s solutions to problems.

  • Failing to cite (as with comments) the origins of code, writing, or techniques that you discover outside of the course’s own lessons and integrate into your own work, even while respecting this policy’s other constraints.

  • Giving or showing to a classmate a solution to a problem when it is he or she, and not you, who is struggling to solve it.

  • Looking at another individual’s work during a quiz or test.

  • Paying or offering to pay an individual for work that you may submit as (part of) your own.

  • Providing or making available solutions to problems to individuals who might take this course in the future.

  • Searching for, soliciting, or viewing a quiz’s questions or answers prior to taking the quiz.

  • Searching for or soliciting outright solutions to problems online or elsewhere.

  • Splitting a problem’s workload with another individual and combining your work (unless explicitly authorized by the problem itself).

  • Submitting (after possibly modifying) the work of another individual beyond allowed snippets.

  • Submitting the same or similar work to this course that you have submitted or will submit to another.

  • Using resources during a quiz beyond those explicitly allowed in the quiz’s instructions.

  • Viewing another’s solution to a problem and basing your own solution on it.

Specification

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 ~/chapter6/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/ap/sentimental/caesar

Style

style50 caesar.py

Staff’s Solution

If you’d like to play with the staff’s own implementation of caesar, you may execute the below.

~cs50/2019/ap/chapter6/caesar

How to Submit

Step 1 of 2

Head back to the CS50 IDE and ensure that caesar.py is in ~/chapter6/caesar, as with:

cd ~/chapter6/caesar
ls

If caesar.py is not in ~/chapter6/caesar, move it into that directory, as via mv (or via CS50 IDE’s lefthand file browser).

Step 2 of 2

  • To submit caesar, execute

    cd ~/chapter6/caesar
submit50 cs50/problems/2019/ap/sentimental/caesar

    inputting your GitHub username and GitHub password as prompted.

If you run into any trouble, email sysadmins@cs50.harvard.edu!

You may resubmit any problem as many times as you’d like before the deadline.

Your submission should be graded for correctness within 2 minutes, at which point your score will appear at submit.cs50.io!

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!

This was Caesar.