• 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 quizzes is not permitted at all. 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 until you solicit and receive approval in writing from the course’s heads. Acts considered not reasonable by the course are handled harshly. If the course refers some matter for disciplinary action and the outcome is punitive, the course reserves the right to impose local sanctions on top of that outcome that may include an unsatisfactory or failing grade for work submitted or for the course itself.

If you commit some act that is not reasonable but bring it to the attention of the course’s heads within 72 hours, the course may impose local sanctions that may include an unsatisfactory or failing grade for work submitted, but the course will not refer the matter for further disciplinary action except in cases of repeated acts.


  • 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 at office hours, elsewhere, or even online, as by viewing, compiling, or running his or her code, even on your own computer.

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

  • Reviewing past semesters' quizzes and solutions thereto.

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

  • Sharing snippets of your own code online 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 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 a solution to a problem set’s problem when it is he or she, and not you, who is struggling to solve it.

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

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

  • 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.

  • Submitting work to this course that you intend to use outside of the course (e.g., for a job) without prior approval from the course’s heads.

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

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


Your work on this problem set will be evaluated along four axes primarily.


To what extent does your code implement the features required by our specification?


To what extent is your code consistent with our specifications and free of bugs?


To what extent is your code written well (i.e., clearly, efficiently, elegantly, and/or logically)?


To what extent is your code readable (i.e., commented and indented with variables aptly named)?

All students, whether or not taking the course for a letter grade, must ordinarily submit this and all other problem sets to be eligible for a satisfactory grade unless granted an exception in writing by the course’s heads.

Getting Ready

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!

But first, a (soon-to-be-relevant) clip from a classic film.

Now consider joining Daven for a tour of HTML again.

And consider reviewing some of these examples from Week 7.

Next consider joining Joseph again for a tour of CSS, the language with which web pages can be stylized.

And consider reviewing some of these examples from Week 7.

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. You’ll find that its syntax is fairly similar to C’s!

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 another look at mvc-0 through mvc-5 from Week 7.

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

Log into CS50 IDE and, in a terminal window, execute

sudo apt-get install -y php5-xdebug

to ensure that your workspace 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 ~/workspace

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

wget http://cdn.cs50.net/2015/fall/psets/7/pset7/pset7.zip
wget http://cdn.cs50.net/2015/fall/psets/7/pset7/pset7.sql

in order to download a ZIP (i.e., compressed version) of this problem set’s distro as well as a MySQL database (that’s been exported to a text file). If you then execute


you should see that you now have a file called pset7.zip in your ~/workspace directory (along with pset7.sql). 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:

config.json  includes/  public/  vendor/  views/

But more on those soon.

Next, ensure that ~/workspace/pset7/public is "world-executable" by executing

chmod a+x ~/workspace/pset7/public

so that CS50 IDE’s web server (Apache) and you (from a browser) will be able to access your work. Then, navigate your way to ~/workspace/pset7/public by executing the below.

cd ~/workspace/pset7/public

If you execute


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

chmod a+x css fonts img js

And 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 ~/workspace/pset7/includes or ~/workspace/pset7/views world-executable (or their contents world-readable), as they shouldn’t be (potentially) accessible to the whole world (only to your PHP code, as you’ll soon see).

Okay, let’s now configure CS50 IDE’s web server (aka Apache) to use ~/workspace/pset7/public as its root. First ensure that Problem Set 6’s server isn’t still running (e.g., in another tab) by executing the below.

killall -9 server

Next, ensure that Apache isn’t already running (with some other root) by executing the below.

apache50 stop

Then (re)start Apache with the below so that it uses ~/workspace/pset7/public as its root.

apache50 start ~/workspace/pset7/public

Next, start CS50 IDE’s database server (MySQL) by executing the below.

mysql50 start

Then open up pset7/config.json, which is a configuration file in JSON (JavaScript Object Notation) format, which essentially means it’s a collection of key-value pairs. JSON is a popular format for configuration files these days, since libraries that can read (i.e., parse) it exist for lots of languages, PHP among them. The curly braces at the top and bottom of this file indicate that the file contains an object, inside of which is one key (database) whose value is another object (per the innermost curly braces). That latter object, meanwhile, has four keys (host, name, password, and username), each of whose values is a string, two of which are TODO! Those values will be used by CS50’s PHP library (which can be found in pset7/vendor) in order to connect to your workspace’s MySQL database. CS50’s PHP library includes a function, query, that will allow you to send queries to that database.

Anyhow, let’s tackle those TODOs! Click the ⓘ icon toward CS50 IDE’s top-right corner. You should see your MySQL Username and your MySQL Password. Copy and paste those values, one at a time, into the appropriate places in config.json, then save and close that file.

It’s now time for a test! Visit https://ide50-username.cs50.io/, where username is your own username. You should find yourself redirected to C$50 Finance’s login page! (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.

In a separate tab, head to https://ide50-username.cs50.io/phpmyadmin (where username is, again, your own username) 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 with the same username and password that you pasted into config.json. You should then find yourself at phpMyAdmin’s main page.

Within CS50 IDE, now, open up pset7.sql, which you downloaded earlier (via wget). You should see a whole bunch of SQL statements. Highlight them all, select Edit > Copy (or hit control-c), then return to phpMyAdmin. Click phpMyAdmin’s SQL tab, and paste everything you copied into that page’s big text box (which is below Run SQL query/queries on server ""). 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 greenish banner indicating success (i.e., 1 row affected). 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. (If you don’t, try reloading the page.) But more on those later.

Return to https://ide50-username.cs50.io/ and reload that page. Then try to log in again with a username of skroob and a password of 12345. 0:-)


Okay, time for a heads-up. Anytime you create a new file or directory in ~/workspace/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 workspace’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 workspace’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 your username) in the workspace.

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 heard about this great "penny stock," whose symbol (ironically) is FREE!

Let’s get in on this opportunity now. Head on over to Yahoo! Finance at http://finance.yahoo.com/. Type the symbol for FreeSeas Inc., FREE, 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.

FreeSeas Inc.

Wow, only $0.0661 per share! That must be a good thing. Anyhow, 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 (albeit delayed). 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, 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 FreeSeas' 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. Unfortunately, Yahoo sometimes returns prices in currencies other than US dollars (without telling you which), but we’ll just assume everything’s in US dollars for simplicity.

If curious as to what they mean, here’s a handy reference.

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.

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 ~/workspace/pset7/public and open up index.php. Know that index.php is the file that’s loaded by default when you visit a URL like https://ide50-username.cs50.io/. 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 helpers.php that can also be found inside of includes) in order to render (i.e., output) a view (i.e., template) called portfolio.php (which is in a directory called views 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 https://ide50-username.cs50.io/ (or, equivalently, https://ide50-username.cs50.io/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 view that this controller ultimately renders.


Navigate your way to ~/workspace/pset7/views and open up portfolio.php. Ah, there’s some HTML. Of course, it’s not much HTML, but it does explain why you saw (and heard!) what you did after you logged in as President Skroob.


Now navigate your way to ~/workspace/pset7/includes and open up config.php. 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, helpers.php and CS50.php, the latter of which is CS50’s PHP library, inside of which is that function, query, that will allow you to query your workspace’s database. Notice how we configure (i.e., initialize) CS50’s library by passing its init method (i.e., function) the path to config.json. (That :: syntax just means that init is "inside of" an "object" called CS50. An object in PHP, meanwhile, is similar in spirit to, but more featureful then, a struct in C. Meanwhile, __DIR__ is a constant that represents the current file’s parent directory.) 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 takes care to redirect the user to login.php anytime he or she visits some page other than login.php, logout.php, and register.php, assuming $_SESSION["id"] isn’t yet set. In other words, config.php 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 helpers.php. It looks like helpers.php 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. Below that 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 $view followed by footer.php, effectively outputting all three.

In fact, navigate your way back to ~/workspace/pset7/views and open up header.php and footer.php. 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 view simply by separating such key/value pairs with a comma, as in the below.

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


Navigate your way back to ~/workspace/pset7/public and open up login.php, another controller. 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 queries your workspace’s MySQL database using CS50’s query function. That function essentially simplifies use of PDO (PHP Data Objects), a library with which you can query MySQL (and other) databases. Much like printf, query 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. Whereas printf uses %i, %s, and the like for placeholders, though, query simply relies on question marks, no matter the type of value. And so the effect of

CS50::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 quoted and 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. If query is passed a SELECT statement, it will return an array with 0 or more rows. If query is instead passed a DELETE, INSERT, or UPDATE statement, it will return a non-negative integer that represents the number of rows deleted, inserted, or updated, respectively. For instance, consider the below, which you might find helpful when it’s time to implement register.php. Note our use of IGNORE, which ensures that this statement will return 0 if username already exists (because of that column’s UNIQUE constraint, per pset7.sql); without IGNORE, this statement might otherwise trigger an error.

$rows = CS50::query("INSERT IGNORE INTO users (username, hash, cash) VALUES(?, ?, 10000.0000)",
    $_POST["username"], password_hash($_POST["password"], PASSWORD_DEFAULT));
if ($rows !== 1)
    // the INSERT failed, presumably because username already existed

Anyhow, notice that login.php verifies a user’s password with password_verify. See http://php.net/manual/en/function.password-verify.php for details. And 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 ~/workspace/pset7/views. 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 ~/workspace/pset7/public to see how it logs out a user.


Alright, now navigate your way to ~/workspace/pset7/public/css and open up styles.css. 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 ~/workspace/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 you created earlier (by executing the statements in pset7.sql in phpMyAdmin’s SQL tab). Head back to https://ide50-username.cs50.io/phpmyadmin/ to access phpMyAdmin. Log in as before if prompted. You should then find yourself at phpMyAdmin’s main page, in the top-left corner of which is a database called pset7 that has (if you click the +) a 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/5.5/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!

What To Do


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

Return to a terminal window, navigate your way to ~/workspace/pset7/views 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 and change the value of form’s action attribute from login.php to register.php. Next add an additional field of type password called confirmation to the HTML form 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 create a new file called register.php with the contents below, taking care to save it in ~/workspace/pset7/public.


    // configuration

    // if user reached page via GET (as by clicking a link or via redirect)
    if ($_SERVER["REQUEST_METHOD"] == "GET")
        // else render form
        render("register_form.php", ["title" => "Register"]);

    // else if user reached page via POST (as by submitting a form via POST)
    else if ($_SERVER["REQUEST_METHOD"] == "POST")
        // TODO


Alright, let’s take a look at your work! Bring up https://ide50-username.cs50.io/login.php and click that page’s link to register.php. You should then find yourself at https://ide50-username.cs50.io/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 should call

    CS50::query("INSERT IGNORE INTO users (username, hash, cash) VALUES(?, ?, 10000.0000)", $_POST["username"], password_hash($_POST["password"], PASSWORD_DEFAULT));

    though we leave it to you to decide how much cash, if not $10,000, your code should give to new users. If curious, meanwhile, as to how password_hash works, see http://php.net/manual/en/function.password-hash.php.

  • Recall that query will return 0 if your INSERT fails (as can happen if, say, username already exists).

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

    $rows = CS50::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 https://ide50-username.cs50.io/register.php 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 workspace 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://finance.cs50.net/.

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 views, 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 helpers.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 views 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 (e.g., portfolios) 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? Odds we should have a field called id that uniquely identifies rows (as the table’s PRIMARY key). And we probably want a field for users' IDs so that we can cross-reference holdings with entries in users. Best to call that field user_id, to make clear that it’s a "foreign key" (i.e., another table’s PRIMARY key). 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 four fields (id, user_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 four fields (namely id, user_id, symbol, and shares), realize that user_id should not be defined as a UNIQUE key in this table, else each user could own no more than one company’s stock since his or her id could not appear (as user_id) in more than one row). Realize, too, that you shouldn’t let some user_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 after creating your table is to add a "joint key." After saving your table, click phpMyAdmin’s Structure tab for the table, then check both user_id and symbol, then click Unique to the right of With selected. That way, INSERT will fail if you try to insert more than one row for some pair of user_id and symbol.) We leave it to you, though, to decide your fields' types. (Just know that user_id in this table should have a type that’s identical to id 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.) 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.

SELECT * FROM portfolios WHERE user_id = 9

Assuming 9 is President Skroob’s user ID, that query should return all rows from portfolios that represent the president’s holdings. If the only fields in table are, say, id, user_id, symbol, and shares, then know that the above is actually equivalent to the below.

SELECT id, user_id, symbol, shares FROM portfolios WHERE user_id = 9

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

SELECT shares FROM portfolios WHERE user_id = 9 AND symbol = 'FREE'

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 (i.e., an empty array).

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 view) 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 view (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) view; you should only call lookup in controllers. Even though views (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://finance.cs50.net/ 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 views. 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 portfolios WHERE user_id = 9 AND symbol = 'FREE'

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 = 9

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/5.5/en/sql-syntax-transactions.html if curious.

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 views. (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://finance.cs50.net/ 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, free and FREE 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 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 portfolios (user_id, symbol, shares) VALUES(9, 'FREE', 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 views, 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:

extra feature

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 to deposit additional funds.

Here’s Zamyla with a few final thoughts:

Sanity Checks

Before you consider this problem set done, best to ask yourself these questions and then go back and improve your code as needed! Do not consider the below an exhaustive list of expectations, though, just some helpful reminders. The checkboxes that have come before these represent the exhaustive list! To be clear, consider the questions below rhetorical. No need to answer them in writing for us, since all of your answers should be "yes!"

  • Is the HTML generated by all of your PHP files valid according to http://validator.w3.org/?

  • Do your pages detect and handle invalid inputs properly?

  • Are you recording users' histories of transactions properly?

  • Did you add one (1) additional feature of your own?

  • Did you choose appropriate data types for your database tables' fields?

  • Are you displaying any dollar amounts to at least two decimal places but no more than four?

  • Are you storing stocks' symbols in your table(s) in uppercase?

How to Submit

Step 1 of 2

  1. When ready to submit, "export" your MySQL database (i.e., save it into a text file) by executing the commands below, where username is your own username, pasting your MySQL password when prompted for a password. (Recall that you can see your MySQL password by clicking the ⓘ icon toward CS50 IDE’s top-right corner.) For security, you won’t see the password as you paste it.

    cd ~/workspace/pset7
    mysqldump -u username -p pset7 > pset7.sql

    If you type ls thereafter, you should see that you have a new file called pset7.sql in ~/workspace/pset7. (If you realize later that you need to make a change to your database and re-export it, you can delete pset7.sql with rm pset7.sql, then re-export as before.)

  2. Toward CS50 IDE’s top-left corner, within its "file browser" (not within a terminal window), control-click or right-click your pset7 folder and then select Download. You should find that your browser has downloaded pset7.tar.gz, a "gzipped tarball" that’s similar in spirit to a ZIP file.

  3. In a separate tab or window, log into CS50 Submit, logging in if prompted.

  4. Click Submit toward the window’s top-left corner.

  5. Under Problem Set 7 on the screen that appears, click Upload New Submission.

  6. On the screen that appears, click Add files…. A window entitled Open Files should appear.

  7. Navigate your way to pset7.tar.gz. Odds are it’s in your Downloads folder or wherever your browser downloads files by default. Once you find pset7.tar.gz, click it once to select it, then click Open (or the like).

  8. Click Start upload to upload all of your files at once to CS50’s servers.

  9. On the screen that appears, you should see a window with No File Selected. If you move your mouse toward the window’s lefthand side, you should see a list of the files you uploaded. Click each to confirm the contents of each. (No need to click any other buttons or icons.) If confident that you submitted the files you intended, consider your source code submitted! If you’d like to re-submit different (or modified) files, simply return to CS50 Submit and repeat these steps. You may re-submit as many times as you’d like; we’ll grade your most recent submission, so long as it’s before the deadline.

Step 2 of 2

This was Problem Set 7.