due 31 December 2014, per schedule

if unable to view embedded YouTube videos, visit CS50.tv for downloadable MP4s and SRTs


  • Introduce some fundamental programming constructs.

  • Empower you to design your own animation, game, or interactive art.

  • Impress your friends.

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 problem sets is not permitted 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 to others, but you may not view theirs, so long as you and they respect this policy’s other constraints. Collaboration on the course’s 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.


  • Communicating with classmates about problem sets' 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, 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.

  • 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 code on Reddit or elsewhere so that others might help you identify and fix a bug.

  • 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 problem set’s problems or your own final project.

  • Whiteboarding solutions to problem sets 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 in CS50 Vault to some problem prior to (re-)submitting your own.

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

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

  • Failing to cite (as with comments) the origins of code 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 your solution to a problem set’s problem when it is he or she, and not you, who is struggling to solve it.

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

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

  • Searching for or soliciting outright solutions to problem sets online or elsewhere.

  • Splitting a problem set’s workload with another individual and combining your work.

  • 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.

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

Important Pre-Course Survey

  • If you haven’t filled it out already, first take a few minutes to fill out HarvardX’s Important Pre-Course Survey, then return here!



Like to discuss CS50 with classmates? Have a question? Visit CS50’s "subreddit" at http://www.reddit.com/r/cs50. You’re welcome to browse and search for answers without a reddit account. But if you’d like to ask questions or upvote or downvote posts, you’ll want to create a reddit account and log in at https://ssl.reddit.com/login, then return to http://www.reddit.com/r/cs50. That account will be distinct from your edX account, but you’re welcome to (try to!) choose the same username.

Be sure to read up on "reddiquette" at http://www.reddit.com/wiki/reddiquette so that you don’t get downvoted!

Finish CS50x already? Like to field questions on CS50’s subreddit as an alum (i.e., graduate) of CS50? Let us know your reddit username at cs50.net/alum so that we can give you some "flair" so that "alum" appears next to your name when you answer questions, so that students know you’re there to help!


Prefer to interact with classmates via Facebook? Visit https://www.facebook.com/groups/cs50 and join CS50’s Facebook Group!


Prefer to interact with classmates via Twitter? Follow @cs50 and tweet with hashtag #cs50!


  • Curl up with Nate’s short on binary, if not too familiar:

    And then with Nate’s short on ASCII:

  • Consider these questions rhetorical for now, but odds are they’ll come up again!

    • How do you represent the (decimal) integer 50 in binary?

    • How many bits must be "flipped" (i.e., changed from 0 to 1 or from 1 to 0) in order to capitalize a lowercase a that’s represented in ASCII?

    • How do you represent the (decimal) integer 50 in, oh, "hexadecimal," otherwise known as "base-16"? Know that decimal is considered "base-10" (since it employs 10 digits, 0 through 9), and binary is considered "base-2" (since it employs 2 digits, 0 and 1). Infer from those base systems how to represent base-16! (We’ll see base-16 again in the context of graphics and web design.)

  • Next dive into Allison’s short on Scratch:

    No questions on that one, though, for now!

Itching to Program?

  • Now join Zamyla for a walkthrough of this problem set, if you’d like a bit of a tour:

  • Next head to http://scratch.mit.edu/ and sign up for an account on MIT’s website by clicking Join Scratch atop the page. Any username (that’s available) is fine, but take care to remember it and your choice of password.

  • Then head to http://scratch.mit.edu/help/ and take note of the resources available to you before you dive into Scratch itself. In particular, you might want to skim the Getting Started Guide.

  • Next head to http://scratch.mit.edu/projects/266919/ to see Scratch Scratch Revolution by CS50 alumna Ann Chi, which Vanessa played on stage in Week 0. Click the blue square above the game’s top-left corner if you’d like to full-screen the user interface (UI). Then click either of the green flags. Per Ann’s instructions, as soon as you hit your keyboard’s spacebar, the game will begin! Feel free to procrastinate a bit. And if you’d like to try out Frogger, by CS50 alumnus Blake Walsh, head to http://scratch.mit.edu/projects/12221773/.

  • If you’ve no experience (or comfort) whatsoever with programming, rest assured that Ann’s and Blake’s projects are more complex than what we expect for this first problem set. (Click See inside in Scratch’s top-right corner to look at each project’s underlying "implementation details.") But they do reveal what you can do with Scratch. And if your computer has a webcam, you might also want to try Move the Butterfly at http://scratch.mit.edu/projects/10016382/. Recall from Week 0 how that program utilizes a webcam to detect movement to which sprites can respond.

  • Let’s take a look at one other project. Head to http://scratch.mit.edu/projects/37413/ to see a project you probably haven’t yet seen by Carlos Herrera. Take a moment to play the game, then click See inside in Scratch’s top-right corner to see how it’s implemented. Spend some time looking over Carlos’s scripts. Don’t forget that each sprite has its own set of scripts. Try to get a sense of how the overall program works. Try making some changes, even while the program is running, to see how the program responds. Note that this project is probably a bit simpler than we expect of you for this problem set, but it’s a good one to learn from because it’s pretty easy to follow. And do appreciate that this game, like all Scratch projects, reduces quite literally to some basic building blocks.

    Feel free to download the source code for a few more projects, either from http://scratch.mit.edu/explore/ or from Week 0 at http://scratch.mit.edu/studios/247678/, even if you already saw some of the latter in lecture. For each program, run it to see how it works overall and then look over its scripts to understand how it works underneath the hood. Feel free to make changes to scripts and observe the effects. Once you can say to yourself, "Okay, I think I get this," you’re ready to proceed.

  • Now it’s time to choose your own adventure! Your mission is, quite simply, to have fun with Scratch and implement a project of your choice (be it an animation, a game, interactive art, or anything else), subject only to the following requirements.

    • Your project must have at least two sprites, at least one of which must resemble something other than a cat.

    • Your project must have at least three scripts total (i.e., not necessarily three per sprite).

    • Your project must use at least one condition, one loop, and one variable.

    • Your project must use at least one sound.

    • Your project should be more complex than most of those demonstrated in lecture (many of which, though instructive, were quite short) but it can be less complex than, say, Scratch Scratch Revolution. As such, your project should probably use a few dozen puzzle pieces overall.

    Feel free to peruse additional projects online for inspiration, but your own project should not be terribly similar to any of them. Try to think of an idea on your own, and then set out to implement it. But don’t try to implement the entirety of your project all at once: pluck off one piece at a time. Ann, for instance, probably implemented just one arrow first, before she moved on to her game’s other three. And Carlos probably implemented a stationary goal before he tried to make it move up and down on its own.

    If, along the way, you find it too difficult to implement some feature, try not to fret; alter your design or work around the problem. If you set out to implement an idea that you find fun, you should not find it hard to satisfy this problem set’s requirements.

    Alright, off you go. Make us proud!

  • Once finished with your project, click See project page in Scratch’s top-right corner. Ensure your project has a title (in Scratch’s top-left corner), some instructions (in Scratch’s top-right corner), and some notes and/or credits (in Scratch’s bottom-right corner). Then click Share in Scratch’s top-right corner so that others your project. Finally, take note of the URL in your browser’s address bar. That’s your project’s URL on MIT’s website, and you’ll need to know it later.

  • Oh, and if you’d like to exhibit your project in CS50x’s gallery, head to http://scratch.mit.edu/studios/317457/, then click Add projects, and paste in your own project’s URL.

About You

  • Suffice it to say it’s a bit harder to meet classmates when taking a course online. But, thanks to technology, everyone can at least say hello!

    If you have a digital camera or phone, allow us to invite you to record a 1- to 2-minute video of yourself saying hello to classmates, perhaps stating where in the world you are, why you’re taking CS50x, and something interesting about you! In the interests of a fun montage, allow us to suggest that you begin your video by saying "hello, world" and end it with "my name is …., and this is CS50," much like the teaching staff do in CS50’s shorts! But, ultimately, it’s totally up to you. Indeed, consider this part of the problem set quite optional, but encouraged! We’ll do our best to post as many of the videos as possible publicly over time.

    If you do record a video, upload it to YouTube (unless blocked in your country, in which case you’re welcome to upload it elsewhere) so that you can provide us with its URL when you submit!

CS50 Show

  • Have a question about CS50? About computer science? About Harvard? About anything else?

    When you submit this problem set, you’ll have an opportunity to submit one or more questions that David and CS50’s staff will try to answer for you (and your classmates!) on live, streaming video. (For those unable to tune in live, the videos will also be viewable on demand afterward.) Best would be questions whose answers might appeal to your classmates as well. (For questions about problem sets, the appliance, and the like, best to turn to /r/cs50.)

    If one or more questions do come to mind, record yourself asking them on video, then upload them to YouTube (unless blocked in your country, in which case you’re welcome to upload it elsewhere)! Allow us to suggest that you begin your videos by saying something like "Hello, my name is …, and I’m from …" so that we can embed you into the live stream for classmates to see! Alternatively, you’re welcome to ask your question simply by typing it out when submitting this problem set, but that’ll be less fun! We’ll do our best to answer as many questions as possible on camera.

How to Submit

  • To submit this problem set, head to the URL below.

    You’ll find that a few questions await. Be extra-sure that your answers are correct, particularly your email address(es) and your project’s URL, else we may overlook your submission! And be sure to click Submit on the last page of that form in order to submit this, your first CS50 problem set!

  • This was Problem Set 0.