This is the Hacker Edition of Problem Set 1. It cannot be submitted for credit.


  • Get comfortable with Linux.

  • Start thinking more carefully.

  • Solve some problems in C.

diff pset1 hacker1

  • Hacker Edition plays with credit cards instead of coins.

  • Hacker Edition demands two half pyramids.

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.

Getting Started

  • Recall that the CS50 Appliance is a "virtual machine" (running an operating system called Fedora, which itself is a flavor of Linux) that you can run inside of a window on your own computer, whether you run Windows, Mac OS, or even Linux itself. To do so, all you need is a "hypervisor" (otherwise known as a "virtual machine monitor"), software that tricks the appliance into thinking that it’s running on "bare metal."

    Alternatively, you could buy a new computer, install Fedora on it (i.e., bare metal), and use that! But a hypervisor lets you do all that for free with whatever computer you already have. Plus, the CS50 Appliance is pre-configured for CS50, so, as soon as you install it, you can hit the ground running.

    So let’s get a hypervisor and the CS50 Appliance installed on your computer. Head to, where instructions await. In particular, if running Mac OS, follow the instructions for VMware Fusion. If running Windows or Linux, follow the instructions for VMware Player.

  • Once you have the CS50 Appliance installed, go ahead and start it (per those same instructions). A small window should open, inside of which the appliance should boot. A few seconds or minutes later, you should find yourself logged in as John Harvard (whose username is jharvard and whose password is crimson), with John Harvard’s desktop before you.

    If you find that the appliance runs unbearably slow on your PC, particularly if several years old or a somewhat slow netbook, or if you see a hint about "long mode," try the instructions at

    Feel free to poke around, particularly the 50 Menu in the appliance’s bottom-left corner. You should find the graphical user interface (GUI), called Xfce, reminiscent of both Mac OS and Windows. Linux actually comes with a bunch of GUIs; Xfce is just one. If you’re already familiar with Linux, you’re welcome to install other software via Menu > Administration > Add/Remove Software, but the appliance should have everything you need for now. You’re also welcome to play with the appliance’s various features, per the instructions at, but this problem set will explicitly mention anything that you need know or do.

  • Even if you just downloaded the appliance, ensure that it’s completely up-to-date by opening a terminal window, as via Menu > Programming > Terminal, typing


and then hitting Enter on your keyboard. So long as your computer (and, thus, the appliance) has Internet access, the appliance should proceed to download and install any available updates.

  • Next, follow the instructions at to configure the appliance to use Dropbox so that your work is automatically backed up, just in case something goes wrong with your appliance. (If you really don’t want to use Dropbox, that’s fine, but realize your files won’t be backed up as a result!) If you don’t yet have a Dropbox account, sign up when prompted for the free (2 GB) plan. You’re welcome to install Dropbox on your own computer as well (outside of the appliance), per, but no need if you’d rather not; just inside the appliance is fine.

    If you’re already a Dropbox user but don’t want your personal files to be synched into the appliance, simply enable Selective Sync, per the CS50 Manual’s instructions.

  • Okay, let’s create a folder (otherwise known as a "directory") in which your code for this problem set will soon live. Go ahead and double-click Home on John Harvard’s desktop (in the appliance’s top-left corner). A window entitled Home should appear, indicating that you’re inside of John Harvard’s "home directory" (i.e., personal folder). Then double-click the folder called Dropbox, at which point the window’s title should change to Dropbox. Next select New Folder under the gear icon in the window’s top-right corner, at which point a new folder called Untitled Folder should appear. Rename it hacker1 (in all lowercase, with no spaces). (If the folder’s name doesn’t seem to be editable, control-click (i.e., click while holding your keyboard’s control key) the Untitled Folder once, then select Rename…, at which point its name should become editable.) Then double-click that hacker1 folder to open it. The window’s title should change to hacker1, and you should see an otherwise empty folder (since you just created it). Notice, though, that atop the window are three buttons, Home, Dropbox, and hacker1, that indicate where you were and where you are; you can click buttons like those to navigate back and forth easily.

  • Okay, go ahead and close any open windows, then select Menu > Programming > gedit. (Recall that Menu is in the appliance’s bottom-left corner.) A window entitled Unsaved Document 1 - gedit should appear, inside of which is a tab entitled Unsaved Document 1. Clearly the document is just begging to be saved. Go ahead and type hello (or the ever-popular asdf) in the tab, and then notice how the tab’s name is now prefixed with an asterisk (*), indicating that you’ve made changes since the file was first opened. Select File > Save, and a window entitled Save As should appear. Input hello.txt next to Name, then click jharvard under Places. You should then see the contents of John Harvard’s home directory. Double-click Dropbox, then double-click hacker1, and you should find yourself inside that empty folder you created. Now, at the bottom of this same window, you should see that the file’s default Character Encoding is Unicode (UTF-8) and that the file’s default Line Ending is Unix/Linux. No need to change either; just notice they’re there. That the file’s Line Ending is Unix/Linux just means that gedit will insert (invisibly) \n at the end of any line of text that you type. Windows, by contrast, uses \r\n, and Mac OS uses \r, but more on those details some other time.

  • Okay, click Save in the window’s bottom-right corner. The window should close, and you should see that the original window’s title is now hello.txt (~/Dropbox/hacker1) - gedit. The parenthetical just means that hello.txt is inside of hacker1, which is inside of Dropbox, which is inside of ~, which is shorthand notation for John Harvard’s home directory. A useful reminder is all. The tab, meanwhile, should now be entitled hello.txt (with no asterisk, unless you accidentally hit the keyboard again).

  • Okay, with hello.txt still open in gedit, notice that beneath your document is a "terminal window," a command-line (i.e., text-based) interface via which you can navigate the appliance’s hard drive and run programs (by typing their name). Notice that the window’s "prompt" is

    jharvard@appliance (~):

    which means that you are logged into the appliance as John Harvard and that you are currently inside of ~ (i.e., John Harvard’s home directory). If that’s the case, there should be a Dropbox directory somewhere inside. Let’s confirm as much.

    Click somewhere inside of that terminal window, and the prompt should start to blink. Type


    and then Enter. That’s a lowercase L and a lowercase S, which is shorthand notation for "list." Indeed, you should then see a list of the folders inside of John Harvard’s home directory, among which is Dropbox! Let’s open that folder, followed immediately by the hacker1 folder therein. Type

    cd Dropbox/hacker1

    or even

    cd ~/Dropbox/hacker1

    followed by Enter to change your directory to ~/Dropbox/hacker1 (ergo, cd). You should find that your prompt changes to

    jharvard@appliance (~/Dropbox/hacker1):

    confirming that you are indeed now inside of ~/Dropbox/hacker1 (i.e., a directory called hacker1 inside of a directory called Dropbox inside of John Harvard’s home directory). Now type


    followed by Enter. You should see hello.txt! Now, you can’t click or double-click on that file’s name there; it’s just text. But that listing does confirm that hello.txt is where we hoped it would be.

    Let’s poke around a bit more. Go ahead and type


    and then Enter. If you don’t provide cd with a "command-line argument" (i.e., a directory’s name), it whisks you back to your home directory by default. Indeed, your prompt should now be:

    jharvard@appliance (~):

    Phew, home sweet home. Make sense? If not, no worries; it soon will! It’s in this terminal window that you’ll soon be compiling your first program! For now, though, close gedit (via File > Quit) and, with it, hello.txt.

  • Incidentally, if the need arises, know that you can transfer files to and from the appliance per the instructions at

hello, world

  • Shall we have you write your first program?

    Okay, go ahead and launch gedit. (Remember how?) You should find yourself faced with another Unsaved Document 1. Go ahead and save the file as hello.c (not hello.txt) inside of hacker1, just as before. (Remember how?) Once the file is saved, the window’s title should change to hello.c (~/Dropbox/hacker1) - gedit, and the tab’s title should change to hello.c. (If either does not, best to close gedit and start fresh! Or ask for help!)

    Go ahead and write your first program by typing these lines into the file (though you’re welcome to change the words between quotes to whatever you’d like):

    #include <stdio.h>
    int main(void)
        printf("hello, world\n");

    Notice how gedit adds "syntax highlighting" (i.e., color) as you type. Those colors aren’t actually saved inside of the file itself; they’re just added by gedit to make certain syntax stand out. Had you not saved the file as hello.c from the start, gedit wouldn’t know (per the filename’s extension) that you’re writing C code, in which case those colors would be absent.

    Do be sure that you type in this program just right, else you’re about to experience your first bug! In particular, capitalization matters, so don’t accidentally capitalize words (unless they’re between those two quotes). And don’t overlook that one semicolon. C is quite nitpicky!

    When done typing, select File > Save (or hit ctrl-s), but don’t quit. Recall that the leading asterisk in the tab’s name should then disappear. Click anywhere in the terminal window beneath your code, and its prompt should start blinking. But odds are the prompt itself is just

    jharvard@appliance (~):

    which means that, so far as the terminal window’s concerned, you’re still inside of John Harvard’s home directory, even though you saved the program you just wrote inside of ~/Dropbox/hacker1 (per the top of gedit's window). No problem, go ahead and type

    cd Dropbox/hacker1


    cd ~/Dropbox/hacker1

    at the prompt, and the prompt should change to

    jharvard@appliance (~/Dropbox/hacker1):

    in which case you’re where you should be! Let’s confirm that hello.c is there. Type


    at the prompt followed by Enter, and you should see both hello.c and hello.txt? If not, no worries; you probably just missed a small step. Best to restart these past several steps or ask for help!

    Assuming you indeed see hello.c, let’s try to compile! Cross your fingers and then type

    make hello

    at the prompt, followed by Enter. (Well, maybe don’t cross your fingers whilst typing.) To be clear, type only hello here, not hello.c. If all that you see is another, identical prompt, that means it worked! Your source code has been translated to 0s and 1s that you can now execute. Type


    at your prompt, followed by Enter, and you should see whatever message you wrote between quotes in your code! Indeed, if you type


    followed by Enter, you should see a new file, hello, alongside hello.c and hello.txt.

    If, though, upon running make, you instead see some error(s), it’s time to debug! (If the terminal window’s too small to see everything, click and drag its top border upward to increase its height.) If you see an error like expected declaration or something no less mysterious, odds are you made a syntax error (i.e., typo) by omitting some character or adding something in the wrong place. Scour your code for any differences vis-à-vis the template above. It’s easy to miss the slightest of things when learning to program, so do compare your code against ours character by character; odds are the mistake(s) will jump out! Anytime you make changes to your own code, just remember to re-save via File > Save (or ctrl-s), then re-click inside of the terminal window, and then re-type

    make hello

    at your prompt, followed by Enter. (Just be sure that you are inside of ~/Dropbox/hacker1 within your terminal window, as your prompt will confirm or deny.) If you see no more errors, try running your program by typing


    at your prompt, followed by Enter! Hopefully you now see precisely the below?

    hello, world

    If not, reach out to CS50 Discuss for help!

    Incidentally, if you find gedit's built-in terminal window too small for your tastes, know that you can open one in its own window via Menu > Programming > Terminal. You can then alternate between gedit and Terminal as needed, as by clicking either’s name along the appliance’s bottom.

    Woo hoo! You’ve begun to program!

This is CS50 Check.

  • Now let’s see if the program you just wrote is correct! Included in the CS50 Appliance is check50, a command-line program with which you can check the correctness of (some of) your programs.

    If not already there, navigate your way to ~/Dropbox/hacker1 by executing the command below.

    cd ~/Dropbox/hacker1

    If you then execute


    you should see, at least, hello.c. Be sure it’s indeed spelled hello.c and not Hello.c, hello.C, or the like. If it’s not, know that you can rename a file by executing

    mv source destination

    where source is the file’s current name, and destination is the file’s new name. For instance, if you accidentally named your program Hello.c, you could fix it as follows.

    mv Hello.c hello.c

    Okay, assuming your file’s name is definitely spelled hello.c now, go ahead and execute the below. Note that 2013.hacker1.hello is just a unique identifier for this problem’s checks.

    check50 2014/x/hacker1/hello hello.c

    Assuming your program is correct, you should then see output like

    :) hello.c exists
    :) hello.c compiles
    :) prints "hello, world\n"

    where each green smiley means your program passed a check (i.e., test). You may also see a URL at the bottom of check50's output, but that’s just for staff (though you’re welcome to visit it).

    If you instead see yellow or red smileys, it means your code isn’t correct! For instance, suppose you instead see the below.

    :( hello.c exists
       \ expected hello.c to exist
    :| hello.c compiles
       \ can't check until a frown turns upside down
    :| prints "hello, world\n"
       \ can't check until a frown turns upside down

    Because check50 doesn’t think hello.c exists, as per the red smiley, odds are you uploaded the wrong file or misnamed your file. The other smileys, meanwhile, are yellow because those checks are dependent on hello.c existing, and so they weren’t even run.

    Suppose instead you see the below.

    :) hello.c exists
    :) hello.c compiles
    :( prints "hello, world\n"
       \ expected output, but not "hello, world"

    Odds are, in this case, you printed something other than hello, world\n verbatim, per the spec’s expectations. In particular, the above suggests you printed hello, world, without a trailing newline (\n).

    Know that check50 won’t actually record your scores in CS50’s gradebook. Rather, it lets you check your work’s correctness before you submit your work. Once you actually submit your work (per the directions at this spec’s end), CS50’s staff will use check50 to evaluate your work’s correctness officially.


  • Head to Week 1’s shorts and curl up with Nate’s short on libraries.

    • What’s a pre-processor? How does

      #include <cs50.h>


    • What’s a compiler?

    • What’s an assembler?

    • What’s a linker? How does



  • Curl up with at least two other shorts from Week 1. Some additional questions may be in your future!

Bad Credit

  • Odds are you have a credit card in your wallet. Though perhaps the bill does not (yet) get sent to you! That card has a number, both printed on its face and embedded (perhaps with some other data) in the magnetic stripe on back. That number is also stored in a database somewhere, so that when your card is used to buy something, the creditor knows whom to bill. There are a lot of people with credit cards in this world, so those numbers are pretty long: American Express uses 15-digit numbers, MasterCard uses 16-digit numbers, and Visa uses 13- and 16-digit numbers. And those are decimal numbers (0 through 9), not binary, which means, for instance, that American Express could print as many as 10^(15) = 1,000,000,000,000,000 unique cards! (That’s, ahem, a quadrillion.)

    Now that’s a bit of an exaggeration, because credit card numbers actually have some structure to them. American Express numbers all start with 34 or 37; MasterCard numbers all start with 51, 52, 53, 54, or 55; and Visa numbers all start with 4. But credit card numbers also have a "checksum" built into them, a mathematical relationship between at least one number and others. That checksum enables computers (or humans who like math) to detect typos (e.g., transpositions), if not fraudulent numbers, without having to query a database, which can be slow. (Consider the awkward silence you may have experienced at some point whilst paying by credit card at a store whose computer uses a dial-up modem to verify your card.) Of course, a dishonest mathematician could certainly craft a fake number that nonetheless respects the mathematical constraint, so a database lookup is still necessary for more rigorous checks.

    So what’s the secret formula? Well, most cards use an algorithm invented by Hans Peter Luhn, a nice fellow from IBM. According to Luhn’s algorithm, you can determine if a credit card number is (syntactically) valid as follows:

    1. Multiply every other digit by 2, starting with the number's second-to-last digit, and then add those products' digits together.
    2. Add the sum to the sum of the digits that weren't multiplied by 2.
    3. If the total's last digit is 0 (or, put more formally, if the total modulo 10 is congruent to 0), the number is valid!
    That's kind of confusing, so let's try an example with Nate's AmEx: 378282246310005.
    1. For the sake of discussion, let's first underline every other digit, starting with the number's second-to-last digit:


      Okay, let's multiply each of the underlined digits by 2:

      7•2 + 2•2 + 2•2 + 4•2 + 3•2 + 0•2 + 0•2

      That gives us:

      14 + 4 + 4 + 8 + 6 + 0 + 0

      Now let's add those products' digits (i.e., not the products themselves) together:

      1 + 4 + 4 + 4 + 8 + 6 + 0 + 0 = 27

    2. Now let's add that sum (27) to the sum of the digits that weren't multiplied by 2:

      27 + 3 + 8 + 8 + 2 + 6 + 1 + 0 + 5 = 60

    3. Yup, the last digit in that sum (60) is a 0, so Nate's card is legit!

    So, validating credit card numbers isn’t hard, but it does get a bit tedious by hand. Let’s write a program.

    In credit.c, write a program that prompts the user for a credit card number and then reports (via printf) whether it is a valid American Express, MasterCard, or Visa card number, per the definitions of each’s format herein. So that we can automate some tests of your code, we ask that your program’s last line of output be AMEX\n or MASTERCARD\n or VISA\n or INVALID\n, nothing more, nothing less, and that main always return 0. For simplicity, you may assume that the user’s input will be entirely numeric (i.e., devoid of hyphens, as might be printed on an actual card). But do not assume that the user’s input will fit in an int! Best to use GetLongLong from CS50’s library to get users' input. (Why?)

    Of course, to use GetLongLong, you’ll need to tell clang about CS50’s library. Be sure to put

    #include <cs50.h>

    toward the top of credit.c. And be sure to compile your code with a command like the below.

    clang -o credit credit.c -lcs50

    Note that -lcs50 must come at this command’s end because of how clang works.

    Incidentally, recall that make can invoke clang for you and provide that flag for you, as via the command below.

    make credit

    Assuming your program compiled without errors (or, ideally, warnings) via either command, you can run your program with the command below.


    Consider the below representative of how your own program should behave when passed a valid credit card number (sans hyphens); highlighted in bold is some user’s input.

    jharvard@appliance (~/Dropbox/hacker1): ./credit
    Number: 378282246310005

    Of course, GetLongLong itself will reject hyphens (and more) anyway:

    jharvard@appliance (~/Dropbox/hacker1): ./credit
    Number: 3782-822-463-10005
    Retry: foo
    Retry: 378282246310005

    But it’s up to you to catch inputs that are not credit card numbers (e.g., Lauren’s phone number), even if numeric:

    jharvard@appliance (~/Dropbox/hacker1): ./credit
    Number: 7722574501

    Test out your program with a whole bunch of inputs, both valid and invalid. (We certainly will!) Here are a few card numbers that PayPal recommends for testing:

    Google (or perhaps a roommate’s wallet) should turn up more. (If your roommate asks what you’re doing, don’t mention us.) If your program behaves incorrectly on some inputs (or doesn’t compile at all), time to debug!

    If you’d like to check the correctness of your program with check50, you may execute the below.

    check50 2014/x/hacker1/credit credit.c

    And if you’d like to play with the staff’s own implementation of credit in the appliance, you may execute the below starting Tue 9/17.


Itsa Mario

  • Toward the beginning of World 1-1 in Nintendo’s Super Mario Brothers, Mario must hop over two "half-pyramids" of blocks as he heads toward a flag pole. Below is a screenshot.

    Super Mario Brothers

    Write, in a file called mario.c in your ~/Dropbox/hacker1 directory, a program that recreates these half-pyramids using hashes (#) for blocks. However, to make things more interesting, first prompt the user for the half-pyramids' heights, a non-negative integer no greater than 23. (The height of the half-pyramids pictured above happens to be 4, the width of each half-pyramid 4, with an a gap of size 2 separating them.) Then, generate (with the help of printf and one or more loops) the desired half-pyramids. Take care to left-align the bottom-left corner of the left-hand half-pyramid, as in the sample output below, wherein boldfaced text represents some user’s input.

    jharvard@appliance (~/Dropbox/hacker1): ./mario
    Height: 4
       #  #
      ##  ##
     ###  ###
    ####  ####

    No need to generate the bricks, cloud, numbers, or text in the sky or Mario himself. Just the half-pyramids! And be sure that main returns 0.

    We leave it to you to determine how to compile and run this particular program!

    If you’d like to check the correctness of your program with check50, you may execute the below.

    check50 2014/x/hacker1/mario mario.c

    And if you’d like to play with the staff’s own implementation of mario in the appliance, you may execute the below starting Tue 9/17.