due 31 December 2014, per schedule

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


  • Introduce you to HTML, CSS, PHP, and SQL.

  • Teach you how to teach yourself new languages.

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.

Learning New Languages

Beyond introducing you to web programming, the overarching goal of this problem set is to teach you—nay, empower you—to teach yourself new languages so that you can stand on your own after term’s end. We’ll guide you through each, but if you nonetheless find yourself Googling and asking lots of questions of classmates and staff, rest assured you’re doing it right!

Incidentally, if you’d like to speed up or slow down any of the videos below using Chrome, join YouTube’s HTML5 trial at http://www.youtube.com/html5 and then re-load this specification. In the bottom-right corner of each video (when hovered upon), you should find a gear icon, via which you should be able to choose 2x and more. And once feeling comfortable, you’re welcome to skip over any of those videos altogether.

  • Let’s first take a look at HTTP (Hypertext Transfer Protocol), the protocol via which you can access the web.

  • Let’s next take a look at HTML (Hypertext Markup Language) in which web pages are written. Here’s Daven!

  • Let’s next take a look at CSS (Cascading Stylesheets) with Joseph.

  • So far so good? Let’s now leverage HTTP, HTML, and CSS to make our own search engine. Well, at least its front end. Take a peek at search-0 through search-4 via the playlist below. (Click PLAYLIST in the player’s top-left corner for a menu as needed.)

  • You are now a web programmer! Okay, not quite. Neither HTML nor CSS are programming languages, but PHP is. Here’s Tommy with a look at PHP’s syntax. You’ll find it’s fairly similar to C’s!

  • Alright, let’s now use PHP to make a website with an actual "back end", server-side code that responds to submissions of HTML forms. Take a look at froshims-0 through froshims-3 via the playlist below.

  • Now let’s look at a common "design pattern" for websites called MVC (Model-View-Controller) that we’ll ultimately use for this problem set. Take a look at mvc-0 through mvc-5 via the playlist below.

  • Finally, let’s hear about SQL (Structured Query Language). Here’s Christopher and cupcakes.

  • Phew, bit of a fire hydrant, no? Not to worry, some fun and more comfort await! Let’s get you started.

Getting Started

  • Start up your appliance and, upon reaching John Harvard’s desktop, open a terminal window (remember how?) and execute


    to ensure that your appliance is up-to-date!

  • Like Problem Set 6, this problem set comes with some distribution code that you’ll need to download before getting started. Go ahead and execute

    cd ~/vhosts

    in order to navigate to your ~/vhosts directory. Then execute

    wget http://cdn.cs50.net/2013/fall/psets/7/pset7/pset7.zip

    in order to download a ZIP (i.e., compressed version) of this problem set’s distro. If you then execute


    you should see that you now have a file called pset7.zip in your ~/vhosts directory. Unzip it by executing the below.

    unzip pset7.zip

    If you again execute


    you should see that you now also have a directory called pset7. You’re now welcome to delete the ZIP file with the below.

    rm -f pset7.zip

    If you next execute

    cd pset7

    followed by


    you should see that pset7 contains three subdirectories: includes, public, and templates. But more on those soon.

  • Next, ensure a few directories are world-executable by executing

    chmod a+x ~
    chmod a+x ~/vhosts
    chmod a+x ~/vhosts/pset7
    chmod a+x ~/vhosts/pset7/public

    so that the appliance’s web server (and you, from a browser) will be able to access your work. Then, navigate your way to ~/vhosts/pset7/public by executing the below.

    cd ~/vhosts/pset7/public

    If you execute


    you should see that public contains four subdirectories and three files. Ensure that the former are word-executable by executing the below.

    chmod a+x css fonts img js

    Finally, ensure that the files within those directories are world-readable by executing the below.

    chmod a+r css/* fonts/* img/* js/*

    If unfamiliar, * is a "wildcard character," so css/*, for instance, simply means "all files within the css directory."

    For security’s sake, don’t make ~/vhosts/pset7/includes or ~/vhosts/pset7/templates world-executable (or their contents world-readable), as they shouldn’t be accessible to the whole world (only to your PHP code, as you’ll soon see).

  • Even though your code for this problem set will live in ~/vhosts/pset7, let’s ensure that it’s nonetheless backed up via Dropbox, assuming you set up Dropbox inside of the appliance. In a terminal window, execute

    ln -s ~/vhosts/pset7 ~/Dropbox

    in order to create a "symbolic link" (i.e., alias or shortcut) to your ~/vhosts/pset7 directory within your ~/Dropbox directory so that Dropbox knows to start backing it up.

  • So why did we put pset7 inside of a directory called vhosts? Well, the appliance is configured to serve "virtual hosts" (i.e., websites) out of the latter. Specifically, if you visit, say, http://pset7/ using Chrome inside of the appliance, the appliance is configured to look in ~/vhosts/pset7/public for that website’s web-accessible files. But for that to work, we also need to associate the appliance’s own IP address with pset7 so that it "resolves" via DNS to it. Rather than set up a whole DNS server to do that, we can actually edit a file called hosts in a directory called etc. Let’s do that.

    In a terminal window, execute

    sudo gedit /etc/hosts

    in order to run gedit as the appliance’s "superuser" (aka "root") so that you can edit what’s otherwise a read-only file. Carefully add this line at the bottom of that file, which will associate pset7 with the appliance’s "loopback" address (which won’t ever change): pset7

    Then save the file and quit gedit. Then enjoy http://xkcd.com/149/.

    If you encounter a Gtk-WARNING with gedit, try restarting the appliance, as via Menu > Log Out > Restart (or via VMware’s own Restart option), then try again; the warning appears to be a bug in Fedora (the appliance’s operating system).

  • Alright, time for a test! Open up Chrome inside of the appliance and visit http://pset7/.

    You should find yourself redirected to C$50 Finance! (If you instead see Forbidden, odds are you missed a step earlier; best to try all those chmod steps again.) If you try logging into C$50 Finance with a username of, oh, skroob and a password of 12345, you should encounter an error about an Unknown database. That’s simply because you haven’t created it yet! Let’s create it.

    Head to http://pset7/phpMyAdmin using Chrome inside of the appliance to access phpMyAdmin, a Web-based tool (that happens to be written in PHP) with which you can manage MySQL databases. (MySQL is a free, open-source database that CS50, Facebook, and lots of other sites use.) Log in as John Harvard if prompted (with a username of jharvard and a password of crimson). You should then find yourself at phpMyAdmin’s main page. In phpMyAdmin’s top-left corner, you should see No databases. Normally, you can create a database by clicking phpMyAdmin’s Databases tab, but you can also execute some SQL commands manually.

    Go ahead and visit http://cdn.cs50.net/2013/fall/psets/7/pset7/pset7.sql using Chrome inside of the appliance, and then open the file in gedit, as by clicking its name in Chrome’s bottom-left corner or by selecting File > Open… in gedit and then navigating your way to Downloads. You should ultimately see a whole bunch of SQL (i.e., database queries) within pset7.sql. Highlight it all, then select Edit > Copy (or hit ctrl-c), then return to phpMyAdmin. Click phpMyAdmin’s SQL tab, and paste everything you copied into that page’s big text box. Skim what you just pasted to get a sense of the commands you’re about to execute, then click Go. You should then see a green banner, proclaiming Your SQL query has been executed successfully. In phpMyAdmin’s top-left corner, you should now see link to a database called pset7, beneath which is a link to a table called users. But more on those later.

    Return to http://pset7/ using Chrome inside of the appliance and reload that page. Then try to log in with a username of skroob and a password of 12345. This time, you should see some construction.

  • Okay, time for a heads-up. Anytime you create a new file or directory in ~/vhosts/pset7 or some subdirectory therein for this problem set, you’ll want to set its permissions with chmod. Thus far, we’ve relied on a+r and a+x, but let’s empower you with more precise control over permissions.

    Henceforth, for any PHP file, file, that you create, execute

    chmod 600 file

    so that it’s accessible only by you (and the appliance’s webserver). After all, we don’t want visitors to see the contents of PHP files; rather, we want them to see the output of PHP files once executed (or, rather, interpreted) by the appliance’s web server.

    For any non-PHP file, file, that you create (or upload), execute

    chmod 644 file

    so that it’s accessible via a browser (if that’s indeed your intention).

    And for any directory, directory, that you create, execute

    chmod 711 directory

    so that its contents are accessible via a browser (if that’s indeed your intention).

    What’s with all these numbers we’re having you type? Well, 600 happens to mean rw-------, and so all PHP files are made readable and writable only by you; 644 happens to mean rw-r--r--, and so all non-PHP files are to be readable and writable by you and just readable by everyone else; and 711 happens to mean rwx--x--x, and so all directories are to be readable, writable, and executable by you and just executable by everyone else. Wait a minute, don’t we want everyone to be able to read (i.e., interpret) your PHP files? Nope! For security reasons, PHP-based web pages are interpreted "as you" (i.e., under John Harvard’s username) in the appliance. For the curious, we’re using suPHP with Apache.

    Okay, still, what’s with all those numbers? Well, think of rw-r--r-- as representing three triples of bits, the first triple of which, to be clear, is rw-. Imagine that - represents 0, whereas r, w, and x represent 1. And, so, this same triple (rw-) is just 110 in binary, or 6 in decimal! The other two triples, r-- and r--, then, are just 100 and 100 in binary, or 4 and 4 in decimal! How, then, to express a pattern like rw-r--r-- with numbers? Why, with 644.

    Actually, this is a bit of a white lie. Because you can represent only eight possible values with three bits, these numbers (6, 4, and 4) are not actually decimal digits but "octal." So you can now tell your friends that you speak not only binary, decimal, and hexadecimal, but octal as well.


  • If you’re not quite sure what it means to buy and sell stocks (i.e., shares of a company), surf on over to http://www.investopedia.com/university/stocks/ for a tutorial.

    You’re about to implement C$50 Finance, a Web-based tool with which you can manage portfolios of stocks. Not only will this tool allow you to check real stocks' actual prices and portfolios' values, it will also let you buy (okay, "buy") and sell (fine, "sell") stocks! (Per Yahoo’s fine print, "Quotes delayed [by a few minutes], except where indicated otherwise.")

  • Just the other day, I received the stock tip below in my inbox!

    "Discovery Ventures Signs Letter Of Intent To Acquire The Willa Gold Deposit"

    Let’s get in on this opportunity now. Head on over to Yahoo! Finance at http://finance.yahoo.com/. Type the symbol for Discovery Ventures, DVN.V, into the text field in that page’s top-left corner and click Get Quotes. Odds are you’ll see a table like the below, which no one has apparently yet Liked!

    Discovery Ventures

    Wow, only 22.5 cents per share! That must be a good thing. Anyhow, notice how Yahoo reports a stock’s most recent (i.e., "Last Trade") price ($0.27) and more. Moreover, scroll down to the page’s bottom, and you should see a toolbox like the below.


    Looks like Yahoo lets you download all that data. Go ahead and click Download Data to download a file in CSV format (i.e., as comma-separated values). Open the file in Excel or any text editor (e.g., gedit), and you should see a "row" of values, all excerpted from that table. It turns out that the link you just clicked led to the URL below.

    Notice how Discovery Ventures' symbol is embedded in this URL (as the value of the HTTP parameter called s); that’s how Yahoo knows whose data to return. Notice also the value of the HTTP parameter called f; it’s a bit cryptic (and officially undocumented), but the value of that parameter tells Yahoo which fields of data to return to you. If curious as to what they mean, head to http://www.gummy-stuff.org/Yahoo-data.htm.

    It’s worth noting that a lot of websites that integrate data from other websites do so via "screen scraping," a process that requires writing programs that parse (or, really, search) HTML for data of interest (e.g., air fares, stock prices, etc.). Writing a screen scraper for a site tends to be a nightmare, though, because a site’s markup is often a mess, and if the site changes the format of its pages overnight, you need to re-write your scraper. (See https://manual.cs50.net/scraping/ if curious as to how it can be done nonetheless.)

    Thankfully, because Yahoo provides data in CSV, C$50 Finance will avoid screen scraping altogether by downloading (effectively pretending to be a browser) and parsing CSV files instead. Even more thankfully, we’ve written that code for you!

    In fact, let’s turn our attention to the code you’ve been given.

  • Navigate your way to ~/vhosts/pset7/public and open up index.php with gedit. (Remember how?) Know that index.php is the file that’s loaded by default when you visit a URL like http://pset7/. Well, it turns out there’s not much PHP code in this file. And there isn’t any HTML at all. Rather, index.php "requires" config.php (which is in a directory called includes in index.php's parent directory). And index.php then calls render (a function implemented in a file called functions.php that can also be found inside of includes) in order to render (i.e., output) a template called portfolio.php (which is in a directory called templates in index.php's parent directory). Phew, that was a mouthful.

    It turns out that index.php is considered a "controller," whereby its purpose in life is to control the behavior of your website when a user visits http://pset7/ (or, equivalently, http://pset7/index.php). Eventually, you’ll need to add some more PHP code to this file in order to pass more than just title to render. But for now, let’s take a look at portfolio.php, the template that this controller ultimately renders.

    Navigate your way to ~/vhosts/pset7/templates and open up portfolio.php with gedit. Ah, there’s some HTML. Of course, it’s not very interesting HTML, but it does explain why your website is "under construction," thanks to the GIF referenced therein.

    Now navigate your way to ~/vhosts/pset7/includes and open up config.php with gedit. Recall that config.php was required by index.php. Notice how config.php first enables display of all errors (and warnings and notices, which are less severe errors) so that you’re aware of any syntactical mistakes (and more) in your code. Notice, too, that config.php itself requires two other files: constants.php and functions.php. Next, config.php calls session_start in order to enable $_SESSION, a "superglobal" variable via which we’ll remember that a user is logged in. (Even though HTTP is a "stateless" protocol, whereby browsers are supposed to disconnect from servers as soon as they’re done downloading pages, "cookies" allow browsers to remind servers who they or, really, you are on subsequent requests for content. PHP uses "session cookies" to provide you with $_SESSION, an associative array in which you can store any data to which you’d like to have access for the duration of some user’s visit. The moment a user ends his or her "session" (i.e., visit) by quitting his or her browser, the contents of $_SESSION are lost for that user specifically because the next time that user visits, he or she will be assigned a new cookie!) Meanwhile, config.php uses a "regular expression" (via a call to preg_match) to redirect the users to login.php anytime they visit some page other than login.php, logout.php, and register.php, assuming $_SESSION["id"] isn’t yet set. In other words, that block of code requires users to log in if they aren’t logged in already (and if they aren’t already at one of those three pages).

    Okay, now open up functions.php with gedit. Interesting, it looks like functions.php requires constants.php. More on that file, though, in a moment. It looks like functions.php also defines a bunch of functions, the first of which is apologize, which you can call anytime you need to apologize to the user (because they made some mistake). Defined next is dump, which you’re welcome to call anytime you want to see the contents (perhaps recursively) of some variable while developing your site. That function is only for diagnostic purposes, though. Be sure to remove all calls thereto before submitting your work. Next in the file is logout, a function that logs users out by destroying their sessions. Thereafter is lookup, a function that queries Yahoo Finance for stocks' prices and more. More on that, though, in a bit. Up next is query, a function that executes a SQL query and then returns the result set’s rows, if any. Below it is redirect, a function that allows you to redirect users from one URL to another. Last in the file is render, the function that index.php called in order to render portfolio.php. The function then "extracts" those values into the local scope (whereby a key of "foo" with a value of "bar" in $values becomes a local variable called $foo with a value of "bar"). And it then requires header.php followed by $template followed by footer.php, effectively outputting all three.

    In fact, navigate your way back to ~/vhosts/pset7/templates and open up header.php and footer.php in gedit. Ah, even more HTML! Thanks to render, those files' contents will be included at the top and bottom, respectively, of each of your pages. As a result, each of your pages will have access to Twitter’s Bootstrap library, per the link and script tags therein. And each page will have at least four div elements, three of which have unique IDs (top, middle, and bottom), if only to make styling them with CSS easier. Even more interestingly, though, notice how header.php conditionally outputs $title, if it is set. Remember how index.php contained the below line of code?

    render("portfolio.php", ["title" => "Portfolio"]);

    Well, because render calls extract on that second argument, an array, before requiring header.php, header.php ends up having access to a variable called $title. Neat, eh? You can pass even more values into a template simply by separating such key/value pairs with a comma, as in the below.

    render("portfolio.php", ["cash" => 10000.00, "title" => "Portfolio"]);

    Okay, now open up constants.php in ~/vhosts/pset7/includes (which, recall, config.php required). Suffice it to say, this file defines a bunch of constants, but you shouldn’t need to change any of them.

    Navigate your way back to ~/vhosts/pset7/public and open up login.php, another controller, with gedit. This controller’s a bit more involved than index.php as it handles the authentication of users. Read through its lines carefully, taking note of how it how it queries the appliance’s MySQL database using that query function from functions.php. That function (which we wrote) essentially simplifies use of PDO (PHP Data Objects), a library with which you can query MySQL (and other) databases. Per its definition in functions.php, the function accepts one or more arguments: a string of SQL followed by a comma-separated list of zero or more parameters that can be plugged into that string, not unlike printf. Whereas printf uses %d, %s, and the like for placeholders, though, query simply relies on question marks, no matter the type of value. And so the effect of

    query("SELECT * FROM users WHERE username = ?", $_POST["username"]);

    in login.php is to replace ? with whatever username has been submitted (via POST) via an HTML form. (The function also ensures that any such placeholders' values are properly escaped so that your code is not vulnerable to "SQL injection attacks.") For instance, suppose that President Skroob tries to log into C$50 Finance by inputting his username and password. That line of code will ultimately execute the SQL statement below.

    SELECT * FROM users WHERE username='skroob'

    Beware, though. PHP is weakly (i.e., loosely) typed, and so functions like query can actually return different types. Indeed, even though query usually returns an array of rows (thanks to its invocation of PDO’s fetchAll), it can also return false in case of errors. But, unlike SELECTs, some SQL queries (e.g., DELETEs, UPDATEs, and INSERTs) don’t actually return rows, and so the array that query returns might sometimes be empty. When checking the return value of query for false, then, take care not to use ==, because it turns out than an empty array is == to false because of implicit casting. But an empty array does not necessarily signify an error, only false does! Use, then, PHP’s === (or !==) operator when checking return values for false, which compares its operands' values and types (not just their values), as in the below.

    $result = query("INSERT INTO users (username, hash, cash) VALUES(?, ?, 10000.00)", $_POST["username"], crypt($_POST["password"]));
    if ($result === false)
        // the INSERT failed, presumably because username already existed

    Anyhow, notice too that login.php "remembers" that a user is logged in by storing his or her unique ID inside of $_SESSION. As before, this controller does not contain any HTML. Rather, it calls apologize or renders login_form.php as needed. In fact, open up login_form.php in ~/vhosts/pset7/templates with gedit. Most of that file is HTML that’s stylized via some of Bootstrap’s CSS classes, but notice how the HTML form therein POSTs to login.php. Just for good measure, take a peek at apology.php while you’re in that directory as well. And also take a peek at logout.php back in ~/vhosts/pset7/public to see how it logs out a user.

    Alright, now navigate your way to ~/vhosts/pset7/public/css and open up styles.css with gedit. Notice how this file already has a few "selectors" so that you don’t have to include style attributes the elements matched by those selectors. No need to master CSS for this problem set, but do know that you should not have more than one div element per page whose id attribute has a value of top, more than one div element per page whose id attribute has a value of middle, or more than one div element per page whose id attribute has a value of bottom; an id must be unique. In any case, you are welcome to modify styles.css as you see fit.

    You’re also welcome to poke around ~/vhosts/pset7/public/js, which contains some JavaScript files. But no need to use or write any JavaScript for this problem set. Those files are just there in case you’d like to experiment.

    Phew, that was a lot. Help yourself to a snack.

  • Alright, let’s talk about that database we keep mentioning. So that you have someplace to store users' portfolios, the appliance comes with a MySQL database (called pset7). We’ve even pre-populated it with a table called users (which is why you were able to log in as President Skroob). Let’s take a look.

    Head back to http://pset7/phpMyAdmin/ using Chrome inside of the appliance to access phpMyAdmin. Log in as John Harvard if prompted (with a username of jharvard and a password of crimson). You should then find yourself at phpMyAdmin’s main page, in the top-left corner of which is that table called users. Click the name of that table to see its contents. Ah, some familiar folks. In fact, there’s President Skroob’s username and a hash of his password (which is the same as the combination to his luggage)!

    Now click the tab labeled Structure. Ah, some familiar fields. Recall that login.php generates queries like the below.

    SELECT id FROM users WHERE username='skroob'

    As phpMyAdmin makes clear, this table called users contains three fields: id (the type of which is an INT that’s UNSIGNED) along with username and hash (each of whose types is VARCHAR). It appears that none of these fields is allowed to be NULL, and the maximum length for each of each of username and hash is 255. A neat feature of id, meanwhile, is that it will AUTO_INCREMENT: when inserting a new user into the table, you needn’t specify a value for id; the user will be assigned the next available INT. Finally, if you click Indexes (above Information), you’ll see that this table’s PRIMARY key is id, the implication of which is that (as expected) no two users can share the same user ID. Recall that a primary key is a field with no duplicates (i.e., that is guaranteed to identify rows uniquely). Of course, username should also be unique across users, and so we have also defined it to be so (per the additional Yes under Unique). To be sure, we could have defined username as this table’s primary key. But, for efficiency’s sake, the more conventional approach is to use an INT like id. Incidentally, these fields are called "indexes" because, for primary keys and otherwise unique fields, databases tend to build "indexes," data structures that enable them to find rows quickly by way of those fields.

    Make sense?

  • Okay, let’s give each of your users some cash. Assuming you’re still on phpMyAdmin’s Structure tab, you should see a form with which you can add new columns. Click the radio button immediately to the left of After, select hash from the drop-down menu, as in the below, then click Go.


    Via the form that appears, define a field called cash of type DECIMAL with a length of 65,4, with a default value of 0.0000, and with an attribute of UNSIGNED, as in the below, then click Save.


    If you pull up the documentation for MySQL at http://dev.mysql.com/doc/refman/5.6/en/numeric-types.html, you’ll see that the DECIMAL data type is used to "store exact numeric data values." A length of 65,4 for a DECIMAL means that values for cash can have no more than 65 digits in total, 4 of which can be to the right of the decimal point. (Ooo, fractions of pennies. Sounds like Office Space.)

    Okay, return to the tab labeled Browse and give everyone $10,000.00 manually. (In theory, we could have defined cash as having a default value of 10000.000, but, in general, best to put such settings in code, not your database, so that they’re easier to change.) The easiest way is to click Check All, then click Change to the right of the pencil icon. On the page that appears, change 0.0000 to 10000.0000 for each of your users, then click Go. Won’t they be happy!

  • It’s now time to code! Let’s empower new users to register.

    Return to a terminal window, navigate your way to ~/vhosts/pset7/templates and execute the below. (You are welcome, particularly if among those more comfortable, to stray from these filename conventions and structure your site as you see fit, so long as your implementation adheres to all other requirements.)

    cp login_form.php register_form.php

    Then open up register_form.php with gedit and change the value of form’s action attribute from login.php to register.php. Next add an additional field of type password to the HTML form called confirmation so that users are prompted to input their choice of passwords twice (to discourage mistakes). Finally, change the button’s text from Log In to Register and change

    or <a href="register.php">register</a> for an account


    or <a href="login.php">log in</a>

    so that users can navigate away from this page if they already have accounts.

    Then, using gedit, create a new file called register.php with the contents below, taking care to save it in ~/vhosts/pset7/public.

        // configuration
        // if form was submitted
        if ($_SERVER["REQUEST_METHOD"] == "POST")
            // TODO
            // else render form
            render("register_form.php", ["title" => "Register"]);

    Alright, let’s take a look at your work! Bring up http://pset7/login.php in Chrome inside of the appliance and click that page’s link to register.php. You should then find yourself at http://pset7/register.php. If anything appears awry, feel free to make tweaks to register_form.php or register.php. Just be sure to save your changes and then reload the page in the browser.

    Of course, register.php doesn’t actually register users yet, so it’s time to tackle that TODO! Allow us to offer some hints.

    • If $_POST["username"] or $_POST["password"] is empty or if $_POST["password"] does not equal $_POST["confirmation"], you’ll want to inform registrants of their error.

    • To insert a new user into your database, you might want to call

      query("INSERT INTO users (username, hash, cash) VALUES(?, ?, 10000.00)", $_POST["username"], crypt($_POST["password"]));

      though we leave it to you to decide how much cash your code should give to new users.

    • Know that query will return false if your INSERT fails (as can happen if, say, username already exists). Be sure to check for false with === and not ==.

    • If, though, your INSERT succeeds, know that you can find out which id was assigned to that user with code like the below.

      $rows = query("SELECT LAST_INSERT_ID() AS id");
      $id = $rows[0]["id"];
    • If registration succeeds, you might as well log the new user in (as by "remembering" that id in $_SESSION), thereafter redirecting to index.php.

    Here’s Zamyla with some additional hints:

  • All done with register.php? Ready to test? Head back to http://pset7/register.php using Chrome inside of the appliance and try to register a new username. If you reach index.php, odds are you done good! Confirm as much by returning to phpMyAdmin, clicking once more that tab labeled Browse for the table called users. May that you see your new user. If not, it’s time to debug!

    Be sure, incidentally, that any HTML generated by register.php is valid, as by ctrl- or right-clicking on the page in Chrome, selecting View Page Source, highlighting and copying the source code, and then pasting it into the W3C’s validator at http://validator.w3.org/#validate_by_input and then clicking Check. Ultimately, the Result of checking your page for validity via the W3C’s validator should be Passed or Tentatively passed, in which case you should see a friendly green banner. Warnings are okay. Errors (and big red banners) are not. Note that you won’t be able to "validate by URI" at http://validator.w3.org/#validate_by_uri, since your appliance isn’t accessible on the public Internet!

  • Do bear in mind as you proceed further that you are welcome to play with and learn from the staff’s implementation of C$50 Finance at https://www.cs50.net/finance.

    In particular, you are welcome to register with as many (fake) usernames as you would like in order to play. And you are welcome to view our pages' HTML and CSS (by viewing our source using your browser) so that you might learn from or improve upon our own design. If you wish, feel free to adopt our HTML and CSS as your own.

    But do not feel that you need copy our design. In fact, for this problem set, you may modify every one of the files we have given you to suit your own tastes as well as incorporate your own images and more. In fact, may that your version of C$50 Finance be nicer than ours!

  • Okay, now it’s time to empower users to look up quotes for individual stocks. Odds are you’ll want to create a new controller called, say, quote.php plus two new templates, the first of which displays an HTML form via which a user can submit a stock’s symbol, the second of which displays, minimally, a stock’s latest price (if passed, via render, an appropriate value).

    How to look up a stock’s latest price? Well, recall that function called lookup in functions.php. Odds are you’ll want to call it with code like the below.

    $stock = lookup($_POST["symbol"]);

    Assuming the value of $_POST["symbol"] is a valid symbol for an actual stock, lookup will return an associative array with three keys for that stock, namely its symbol, its name, and its price. Know that you can use PHP’s number_format function (somehow!) to format price to at least two decimal places but no more than four decimal places. See http://php.net/manual/en/function.number-format.php for details.

    Of course, if the user submits an invalid symbol (for which lookup returns false), be sure to inform the user somehow. Be sure, too, that any HTML generated by your templates is valid, per the W3C’s validator.

    Here’s Zamyla again:

  • And now it’s time to do a bit of design. At present, your database has no way of keeping track of users' portfolios, only users themselves. (By "portfolio," we mean a collection of stocks (i.e., shares of companies) that some user owns.) It doesn’t really make sense to add additional fields to users itself in order to keep track of the stocks owned by users (using, say, one field per company owned). After all, how many different stocks might a user own? Better to maintain that data in a new table altogether so that we do not impose limits on users' portfolios or waste space with potentially unused fields.

    Exactly what sort of information need we keep in this new table in order to "remember" users' portfolios? Well, we probably want a field for users' IDs (id) so that we can cross-reference holdings with entries in users. We probably want to keep track of stocks owned by way of their symbols since those symbols are likely shorter (and thus more efficiently stored) than stocks' actual names. Of course, you could also assign unique numeric IDs to stocks and remember those instead of their symbols. But then you’d have to maintain your own database of companies, built up over time based on data from, say, Yahoo. It’s probably better (and it’s certainly simpler), then, to keep track of stocks simply by way of their symbols. And we probably want to keep track of how many shares a user owns of a particular stock. In other words, a table with three fields (id, symbol, and shares) sounds pretty good, but you’re welcome to proceed with a design of your own. Whatever your decision, head back to phpMyAdmin and create this new table, naming it however you see fit. To create a new table, click pset7 in phpMyAdmin’s top-left corner, and on the screen that appears, input a name for your table and some number of columns below Create table, then click Go. On the screen that appears next, define (in any order) each of your fields.

    If you decide to go with three fields (namely id, symbol, and shares), realize that id should not be defined as a primary key in this table, else each user could own no more than one company’s stock (since his or her id could not appear in more than one row). Realize, too, that you shouldn’t let some id and some symbol to appear together in more than one row. Better to consolidate users' holdings by updating shares whenever some user sells or buys more shares of some stock he or she already owns. A neat way to impose this restriction while creating your table is to define a "joint primary key" by selecting an Index of PRIMARY for both id and symbol. That way, INSERT will fail if you try to insert more than one row for some pair of id and symbol. We leave it to you, though, to decide your fields' types. (If you include id in this table, know that its type should match that in users. But don’t specify AUTO_INCREMENT for that field in this new table, as you only want auto-incrementation when user IDs are created for new users. And don’t call your table tbl.) When done defining your table, click Save!

  • Before we let users buy and sell stocks themselves, let’s give some shares to President Skroob and friends at no charge. Click, in phpMyAdmin’s left-hand frame, the link to users and remind yourself of your current users' IDs. Then click, in phpMyAdmin’s left-hand frame, the link to your new table (for users' portfolios), followed by the tab labeled Insert. Via this interface, go ahead and "buy" some shares of some stocks on behalf of your users by manually inserting rows into this table. (You may want to return to Yahoo! Finance to look up some actual symbols.) No need to debit their cash in users; consider these shares freebies.

    Once you’ve bought your users some shares, let’s see what you did. Click the tab labeled SQL and run a query like the below, where tbl represents your new table’s name.

    SELECT * FROM tbl WHERE id = 7

    Assuming 7 is President Skroob’s user ID, that query should return all rows from tbl that represent the president’s holdings. If the only fields in table are, say, id, symbol, and shares, then know that the above is actually equivalent to the below.

    SELECT id, symbol, shares FROM tbl WHERE id = 7

    If, meanwhile, you’d like to retrieve only President Skroob’s shares of Discovery Ventures, you might like to try a query like the below.

    SELECT shares FROM tbl WHERE id = 7 AND symbol = 'DVN.V'

    If you happened to buy President Skroob some shares of that company, the above should return one row with one column, the number of shares. If you did not get buy any such shares, the above will return an empty result set.

    Incidentally, via this SQL tab, you could have inserted those "purchases" with INSERT statements. But phpMyAdmin’s GUI saved you the trouble.

    Alright, let’s put this knowledge to use. It’s time to let users peruse their portfolios! Overhaul index.php (a controller) and portfolio.php (a template) in such a way that they report each of the stocks in a user’s portfolio, including number of shares and current price thereof, along with a user’s current cash balance. Needless to say, index.php will need to invoke lookup much like quote.php did, though perhaps multiple times. And know that a PHP script can certainly invoke query multiple times, even though, thus far, we’ve seen it used in a file no more than once. And you can certainly iterate over the array it returns in a template (assuming you pass it in via render). For instance, if your goal is simply to display, say, President Skroob’s holdings, one per row in some HTML table, you can generate rows with code like the below, where $positions is an array of associative arrays, each of which represents a position (i.e., a stock owned).

            foreach ($positions as $position)
                print("<td>" . $position["symbol"] . "</td>");
                print("<td>" . $position["shares"] . "</td>");
                print("<td>" . $position["price"] . "</td>");

    Alternatively, you can avoid using the concatenation operator (.) via syntax like the below:

            foreach ($positions as $position)

    Note that, in the above version, we’ve surrounded the lines of HTML with double quotes instead of single quotes so that the variables within ($position["symbol"], $position["shares"], and $position["price"]) are interpolated (i.e., substituted with their values) by PHP’s interpreter; variables between single quotes are not interpolated. And we’ve also surrounded those same variables with curly braces so that PHP realizes they’re variables; variables with simpler syntax (e.g., $foo) do not require the curly braces for interpolation. (It’s fine to use double quotes inside those curly braces, even though we’ve also used double quotes to surround the entire argument to print.) Anyhow, though commonly done, generating HTML via calls to print isn’t terribly elegant. An alternative approach, though still a bit inelegant, is code more like the below.

    <?php foreach ($positions as $position): ?>
            <td><?= $position["symbol"] ?></td>
            <td><?= $position["shares"] ?></td>
            <td><?= $position["price"] ?></td>
    <?php endforeach ?>

    Of course, before you can even pass $positions to portfolio.php, you’ll need to define it in index.php. Allow us to suggest code like the below, which combines names and prices from lookup with shares and symbols, as might be returned as $rows from query.

    $positions = [];
    foreach ($rows as $row)
        $stock = lookup($row["symbol"]);
        if ($stock !== false)
            $positions[] = [
                "name" => $stock["name"],
                "price" => $stock["price"],
                "shares" => $row["shares"],
                "symbol" => $row["symbol"]

    Note that, with this code, we’re deliberately create a new array of associative arrays ($positions) rather than add names and prices to an existing array of associative arrays ($rows). In the interests of good design, it’s generally best not to alter functions' return values (like $rows from query).

    Now, much like you can pass a page’s title to render, so can you pass these positions, as with the below.

    render("portfolio.php", ["positions" => $positions, "title" => "Portfolio"]);

    Of course, you’ll also need to pass a user’s current cash balance from index.php to portfolio.php via render as well, but we leave it to you to figure out how.

    To be clear, in the spirit of MVC, though, do take care not to call lookup inside of that (or any other) template; you should only call lookup in controllers. Even though templates (aka views) can contain PHP code, that code should only be used to print and/or iterate over data that’s been passed in (as via render) from a controller.

    As for what HTML to generate, look, as before, to https://www.cs50.net/finance/ for inspiration or hints. But do not feel obliged to mimic our design. Make this website your own! Although any HTML and PHP code that you yourself write should be pretty-printed (i.e., nicely indented), it’s okay if lines exceed 80 characters in length. HTML that you generate dynamically (as via calls to print), though, does not need to be pretty-printed.

    As before, be sure to display stocks' prices and users' cash balances to at least two decimal places but no more than four.

    Incidentally, though we keep using President Skroob in examples, your code should work for whichever user is logged in.

    As always, be sure that the HTML generated by index.php is valid.

    Here’s Zamyla with some additional tips:

  • And now it is time to implement the ability to sell with a controller called, say, sell.php and some number of templates. We leave the design of this feature to you. But know that you can delete rows from your table (on behalf of, say, President Skroob) with SQL like the below.

    DELETE FROM tbl WHERE id = 7 AND symbol = 'DVN.V'

    We leave it to you to infer exactly what that statement should do. Of course, you could try the above out via phpMyAdmin’s SQL tab. Now what about the user’s cash balance? Odds are, your user is going to want the proceeds of all sales. So selling a stock involves updating not only your table for users' portfolios but users as well. We leave it to you to determine how to compute how much cash a user is owed upon sale of some stock. But once you know that amount (say, $500), SQL like the below should take care of the deposit (for, say, President Skroob).

    UPDATE users SET cash = cash + 500 WHERE id = 7

    Of course, if the database or web server happens to die between this DELETE and UPDATE, President Skroob might lose out on all of that cash. You need not worry about such cases! It’s also possible, because of multithreading and, thus, race conditions, that a clever president could trick your site into paying out more than once. You need not worry about such cases either! Though, if you’re so very inclined, you can employ SQL transactions (with InnoDB tables). See http://dev.mysql.com/doc/refman/5.6/en/sql-syntax-transactions.html for reference.

    It’s fine, for simplicity, to require that users sell all shares of some stock or none, rather than only a few. Needless to say, try out your code by logging in as some user and selling some stuff. You can always "buy" it back manually with phpMyAdmin.

    As always, be sure that your HTML is valid!

    And as always, here is Zamyla!

  • Now it’s time to support actual buys. Implement the ability to buy, with a controller called, say, buy.php and some number of templates. (As before, you need not worry about interruptions of service or race conditions.) The interface with which you provide a user is entirely up to you, though, as before, feel free to look to https://www.cs50.net/finance for inspiration or hints. Of course, you’ll need to ensure that a user cannot spend more cash than he or she has on hand. And you’ll want to make sure that users can only buy whole shares of stocks, not fractions thereof. For this latter requirement, know that a call like

    preg_match("/^\d+$/", $_POST["shares"])

    will return true if and only if $_POST["shares"] contains a non-negative integer, thanks to its use of a regular expression. See http://www.php.net/preg_match for details. Take care to apologize to the user if you must reject their input for any reason. In other words, be sure to perform rigorous error-checking. (We leave to you to determine what needs to be checked!)

    When it comes time to store stocks' symbols in your database table, take care to store them in uppercase (as is convention), no matter how they were inputted by users, so that you don’t accidentally treat, say, dvn.v and DVN.V as different stocks. Don’t force users, though, to input symbols in uppercase.

    Incidentally, if you implemented your table for users' portfolios as we did ours (with that joint primary key), know that SQL like the below (which, unfortunately, wraps onto two lines) will insert a new row into table unless the specified pair of id and symbol already exists in some row, in which case that row’s number of shares will simply be increased (say, by 10).

    INSERT INTO table (id, symbol, shares) VALUES(7, 'DVN.V', 10) ON DUPLICATE KEY UPDATE shares = shares + VALUES(shares)

    As always, be sure to bang on your code. And be sure that your HTML is valid!

    Here’s Zamyla with some additional help:

  • Alright, so your users can now buy and sell stocks and even check their portfolio’s value. But they have no way of viewing their history of transactions.

    Enhance your implementations for buying and selling in such a way that you start logging transactions, recording for each:

    • Whether a stock was bought or sold.

    • The symbol bought or sold.

    • The number of shares bought or sold.

    • The price of a share at the time of transaction.

    • The date and time of the transaction.

    Then, by way of a controller called, say, history.php and some number of templates, enable users to peruse their own history of transactions, formatted as you see fit. Be sure that your HTML is valid!

    Here’s Zamyla again:

  • Phew. Glance back at index.php now and, if not there already, make that it somehow links to, at least, buy.php, history.php, logout.php, quote.php, and sell.php (or their equivalents) so that each is only one click away from a user’s portfolio!

  • And now the icing on the cake. Only one feature to go, but you get to choose. Implement at least one (1) of the features below. You may interpret each of the below as you see fit; we leave all design decisions to you. Be sure that your HTML is valid.

    • Empower users (who’re already logged in) to change their passwords.

    • Empower users who’ve forgotten their password to reset it (as by having them register with an email address so that you can email them a link via which to do so).

    • Email users "receipts" anytime they buy or sell stocks.

    • Empower users to deposit additional funds.

    For tips on how to send email programmatically, see https://manual.cs50.net/Sending_Mail.

    Here’s Zamyla with a few final thoughts:

Sanity Checks