CS50 - Week 7 | infuerno.github.io

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IP - Internet Protocol

Method for addressing every computer on the internet. DNS - Domain Name Server - has mapping of URLs to IP addresses and vice versa. Use nslookup to query DNS.

TCP - Transmission Communication Protocol

Method for sending information between computers on the internet with reliabiity that whole message will be delivered. It will also divide the message into multiple segments.

HTTP

Protocol which web browers use to talk to web servers.

GET / HTTP/1.1 Host: www.google.com ...

HTTP/1.1 200 OK Content-Type: text/html ...

PHP Syntax

Use the following to demarquate php code to be interpreted by the php interpreter.
All variables start with a dollar sign e.g. $x = 4; $s = "hello";
Types are implicit, not explicit
Functions are declared using the function keyword e.g. function f() { ... }
Functions do not specify a return type and can return different types at run time as required
Declare arrays using e.g. $a = array(1, 2, 3); $a = [1, 2, 3]; $a = [];
Declare associative arrays using e.g. $h = ["a" => "apple", "b" => "banana"]; $h = [];
Add a new element to an array using e.g. $a[] = 4;
Add a new element to an associate array using e.g. $h["c"] = "cat";
The function count returns the number of elements in an array e.g. count($a);
Single or double quotes can be used for creating strings
The dot operator concatenates strings e.g. $n = 5; $s = 'n is '.$n;
Single or double quotes can be used in the example above.
Variables are interpreted when used in double quoted strings e.g. "n is $n" will become "n is 5"
Variables are not interpreted when using single quotes e.g. 'n is $n' still remain "n is $n"
To include values of arrays in strings use {} e.g. "{h['c']}" would give "cat"
foreach loops are useful in php. Note the order of elements in the declaration
require_once copies the contents of one file into another file

require_once(DIR . "/math.php"); print(add(1, 2) . "\n");
DIR is a special constant for the current directory, so this file is in the same directory as math.php

CSS

A CSS rule is usually a selector followed by declarations to determine the style
Combinators let you join multiple simple selectors
Simple selectors include:
- ID selectors i.e. #
- tag selectors e.g. h1
- class selectors e.g. .box
- attribute selectors e.g. [type="radio"] to target e.g.
- pseudoselectors for specificing styling when interations occur e.g. h1:hover
Combinators include:
- whitespace for children
- commas to distinguish selectors
- > for direct children only
- ~ for sibling elements
- + for the one sibling which comes directly after another element

HTML

Not a programming language, a markup language.

PHP

Find in your environment the interpreter for php

#!/usr/bin/env php

Print out post variables using

<?php print_r($_POST); ?>

Where print_r is a recursive print function and $_POST is a special variable, a super global, which holds the post variables. They can be indexed into using square brackets e.g. $_POST["name"]. The function empty can be used to check if a variable has a function e.g. empty($_POST["name"])

To send email use:

require("libphp-phpmailer/class.phpmailer.php");

$mail = new PHPMailer();
$mail->IsSMTP();
$mail->Host ="smtp.fas.harvard.edu";
$mail->Port = 587;
$mail->SMTPSecure = "tls";

$mail->SetFrom("jharvard@cs50.harvard.edu");
$mail->AddAddress("jharvard@cs50.harvard.edu");
$mail->Subject("Registration");
$mail->Body = 
    "This person just registered:\n\n" .
    "Name: " . $_POST["name"] . "\n";

if ($mail->Send() == false)
{
    die($mail->ErrInfo);
}

Superglobals

$_COOKIE $_GET $_POST $_SERVER $_SESSION ...

Sessions

Sessions need to be started before they can be used.

session_start();
if (isset($_SESSION["counter"]))
{
    $counter = $_SESSION["counter"];
}
else
{
    $counter = 0;
}

// increment counter
$_SESSION["counter"] = $counter + 1;

Parameters to functions

You can pass in arbitrary key value pairs to functions where the key indicates the name of an argument and the value indicates the value to pass for that argument. E.g.

renderHeader(["title" => "CS50"]);

and the square brackets incidate that an associative array is being passed in, so multiple values are possible. E.g.

renderHeader(["title" => "CS50", "week" => 0]);

The definition of this function may be:

function renderHeader($data = [])
{
    extract($data); 
}

where extract($data) takes all the keys and creates variables for them.

Misc

htmlspecialchars($title) is a special function which renders any dangerous characters harmless.
Use <?= ... ?> as a shortcut to <?php print ... ?>
=== compares operands values and types whereas == only compares values. So e.g. an empty array will evaluate to false so [] == false will be true whereas [] === false will be false.

File permissions

PHP files should have 600 so that you (and the webserver) can read and write to it (visitors should not be able to see php files, only the result of executing them) - for security reasons, php web pages are interpreted by "you" (using suPHP and Apache on the appliance)
Non PHP files should have 644 so that you can read and write, others can read and execute
Directories should have 711 so that you can read, write and execute and everyone else can only execute

Stocks

What are stocks?

A stock is a share in the ownership of a company. Stock = Shares = Equity. Being a shareholder means you are one of the many owners of a company and as such have a claim to everything the company owns. You are entitled to your share of the company's earnings as well as any voting rights attached to the share.

Usually holding shares gives you an equivalent number of votes for who should be on the board of directors. Though only having a few share, and therefore a few votes won't affect this too much.

Profits are sometimes paid out in the form of dividends. Your claim on assets is only relevant if the company goes bankrupt. In this scenario, you will only get some money back once all other creditors have been paid. Nevertheless owning shares gives you a claim on the earnings and assets.

Limited liability is another important aspect of shares. You are not personally liable if the company is not able to pay its debts. Partnerships, on the other hand, do hold any partners liable. Shareholders only ever stand to loose their investment in shares.

Companies sell stocks as a way to raise money. Bank loans or issuing bonds (IOUs) are also possible and this is called debt financing. However, issuing stocks to raise money is equity financing. These type of money does not require the company to pay back the money, or pay interest. IPO - initial public offering - the first sale of stock by the company.

Companies do not have to pay dividends and many do not.

Stocks are risky, but the return has historically been good. Over the long term and investment in stocks has historically had an average return of 10-12%.

Different types of stocks

Common stock - as described above.

Preferred stock - doesn't usually have the same voting rights (i.e. one vote per share). Investors are usually guaranteed a fixed dividend forever, and in the event of liquidation preferred shareholders and paid off before common shareholders. Preferred shares have similarity with bonds.

Other classes - other variants can also exist. E.g. a stock which gives voting rights of 10 votes per share.

When there are different classes of stock these are often referred to as Class A and Class B and these are often represented by placing a letter behind the ticker symbol e.g. BRKa and BRKb for a company whose ticker symbol is BRK.

CS50 - Week 7