CS50's Understanding Technology

Hardware
Internet
Multimedia
Security
Web Development
Programming

Hardware

Binary

computers use the binary number system to represent info
humans typically use decimal numbers (base 10) 123
- rightmost column is the 1s column
- middle, the 10s
- leftmost, the 100s
- we get 100 x 1 + 10 x 2 + 1 x 3 = 100 + 20 + 3 = 123
binary (base 2)
- right most column is the 1s place
- middle, the 2s
- leftmost, the 4s
human (decimal) use powers of 10 for place values
- 10⁰ = 1, 10¹ = 10, 10² = 100, 10³ = 1000, etc.
computer (binary) use powers of 2 for place values
- 2⁰ = 1, 2¹ = 2, 2² = 4, 2³ = 8, etc.
difference between decimal and binary is changing the base
binary number 000 has 4 x 0 + 2 x 0 + 1 x 0 = 0 + 0 + 0 = 0!
even though computers only use binary, they do it with a smaller vocabulary, with just 1 and 0
this is because it's easier to represent two states in the physical world like ligh bulb
- 0 is off
- 1 is on
light bulbs just need electricity to turn on or off
electricity is sufficient to turn a switch on or off
inside a computer these switches exists called transistors
- modern computers have billions
using transistors we can store values, data, compute, and do everything we can with computers
so far all that we can represent is numbers
- a decision needs to be made on what pattern of 1s and 0s to represent letters, words, and paragraphs
- to represent letters, we need a mapping of 0s and 1s to characters
  - ASCII (American Standard Code for Information Interchange) does this
  - 65 -> A, 66 -> B, 67 -> C, etc.
  - 97 -> a, 98 -> b, 99 -> c, etc.
  - ASCII also has mapping for punctuation symbols
- programs like notepad, textedit, and MicroSoft Word decide weather to display patterns of bits as letters or words
- ASCII is limited
  - original ASCII is 7 bits, thus giving 128 characters
  - extended ASCII is 8 bits, yielding 256 characters
  - many symbols are not represented
- UNICODE is a bigger set of characters that includes written languages other than English and even emoji
  - all are still represented by a pattern of bits
16 bits or 2 bytes (1 byte = 8 bits)

CPU

if you have heard that your computer has "Intel Inside", it has an Intel processor in it
- the backside of the processor has pins that connect into the motherboard
- motherboard is a circuit board made of silicon
CPU is the brain of the computer
- does all the thinking
- performs computation
- helps display numbers on a screen
- adds or deletes numbers
CPUs now can have multiple cores
- cores are the devices inside the CPU that can perform mathematical operations, load info from memory, save info to memory, etc.
- the more cores, the more tasks a CPU can do at once
CPUs now also support hyper-threading
- where a single core will present itself as multiple cores to a computer's operating system
Systems on a Chip (SoaC) are when a CPU and more are all interconnected at once rather than attached to a motherboard
- popular in phones, tables, and game consoles
- Raspberry Pi

RAM (Random Access Memory)

circuit board with chips that slides into a slot on the motherboard
the chips store data
- only stores data when power is on (volatile)
files and programs are loaded onto these chips when ran
fast memory
check RAM and other specs on Windows Task Manager:
- CPU chart shows when peak usage occurs
- GHz is the number of operations a CPU can perform per second (in billions)
  - 2.05 GHz = 2.05 billion operations per second
- logical processors in this case is 4, which means both cores support hyper-threading
  - each core will do two things at once as if 4 cores exist

Hard Drives

when we turn a computer off, we need a place to store data (non-volatile)
hard disk drive (HDD) stores this information
RAM may store 1 GB, 2 GB, 4 GB, through 16 GB or so
HDD stores 256 GB, 1024 GB (AKA terabyte or TB), 2 TB
inside a HDD, metal platters physically spin around
- data is stored on these disks
- reading heads move back and forth reading data from the device
- uses tiny magnetic particles where north pole orientation represents 1 and south pole orientation represents 0
  - power is only needed to read or change the data
  - data is preserved when power is off
RAM sends data and instructions to the HDD to store data
hard drive translates that data into voltage fluctuations
- some signals spin the platters, others move the read/write heads
- pulses sent to the read/write head turn on a magnet which creates a field that changes the polarity of a tiny portion of the metal platter's surface
- power is sent in different directions as to change polarity
to read, the particles on the disk use their charge to move the read/write head
pieces of a file can be spread out around the platters
- a special file keeps track of data's location
anytime we have a physical device that moves over a period of time, things may go wrong
- dropping a HDD can corrupt files
platters spin slower than how fast electrons move

Flash Memory

solid-state drive (SSD)
- smaller (3.5 inch width for HDD vs 2.5 inch width for SSD)
  - still fits where old HDDs are
- no moving particles
- inside, it looks a lot like RAM
- much faster than HDD
  - Programs/files load and save more quickly
- SSD theoretically don't last as long as HDD
  - finite number of writes
hybrid drives
- some GB of solid state memory and more GB or TB of HDD space
- stores as much of frequently-needed data on the SSD
- stores less frequently-needed data on HDD
flash memory also exists in the form of USB sticks
- might store 1 GB, 16 GB, or more
- portable
external SSDs exist for more storage
- might store 256 GB or more
- can be used to share data with others without network usage
can also have external HDD

Types of Memory and Funneling

There is a tradeoff between space, money, and speed of data transfer

data is pushed "down the funnel" to our CPU
- from the hard drive, data first goes to the RAM
- theoretically, the CPU never has to wait for data to crunch
- tiny amount of memory (bytes) called registers where numbers are stored for operations
- memory at the bottom is more expensive
- disk is important for the long-term storage
- RAM is important as it stores programs we use simultaneously
- L3, L2, L1 cache are on the motherboard

As an analogy for memory, picture a candy store.

a customer approaches the counter and requests candy
the shop owner then leaves the counter to grab the candy before returning
- not super efficient to walk all the way to the store room to grab candy
- better to have a cache of memory
instead, the shop owner leaves the counter to ready a cache of candy before the customers arrive
when a customer comes, the candy can be distributed quickly
- cache memory similarly helps the CPU in this manner

Display Connectors

sockets all connect to monitors or displays
Mini DisplayPort are used form monitors
HDMI is not only on laptops and computers but also TVs
VGA is older, but still commonly uses on projectors

USB (Universal Serial Bus)

Can plug in a whole range of peripheral devices including printers, keyboards, mice, scanners, etc.
USB A-Type most common
USB B-Type is often used for printers and scanners
USB C-Type is newer and can be plugged in coming from different directions
other variants often exist for phones
older USB connections are slower when transferring data
- external hard drives can connect via USB
  - even if a hard drive is fast, if the USB is slow, the transfer of data will be slow

Wireless

Wi-Fi is wireless internet
Bluetooth allows devices such as wireless keyboards and headphones to connect to our computer
- limited range (this is ok, as it is used for us to connect to our own devices)

Operating System (OS)

software that ensures all devices work and can intercommunicate
MacOS and Windows are popular OS
can be installed by the user, but is typically done by a manufacturer
- installed on HDD or SDD so that it exists persistently without power
when computer is power on, OS is loaded into RAM from disk
- gives graphical interface that we can see
- talk to our keyboard and mouse
- display info on the screen
- move things around in memory
all thanks to device drivers installed with the OS
- special software designed to talk to certain model of printer, camera, scanner, etc.
when an OS doesn't recognize a device, perhaps because it's too new, we can download new device drives from the device manufacturer
- teaches Window, MacOS, or Linux about that new hardware
- future-proofing structure
it's this intersection of hardware and software that makes computers powerful

Internet

Introduction

Internet is a very large network of computers connected to each other.
we use the internet on a daily basis and have constant access and connectivity
cable modem, DSL modem, or FIOS device
- connects to the internet
- pay monthly for an ISP (Internet Service Provider)
  - Verizon, Comcast, etc.
- could have built-in wireless connectivity for our devices
  - my need an additional home router
  - devices connect to a router via cables or Wi-Fi

IP

IP address is a combination of numbers that acts like a unique identifier for a computer connected to the Internet, e.g., 91.198.174.192
every computer on the internet has an IP (Internet Protocol) address
- of the form #.#.#.#
  - four numbers separated by dots of the values 0-255
  - other IP address formats exist today as well
- like postal addresses, they uniquely identify computers on the internet
  - any device connected to the internet has an IP address
    - allows other computers to talk to it
ISPs assign an IP address to our computer (router)
- used to be physically configured
- DHCP (Dynamic Host Configuration Protocol)
  - software that ISPs provides to allow our computer to request an IP address
  - DHCP servers respond with a specific IP address for our Home
- multiple devices can connect to our home network
  - home router supports DHCP and assigns IP addresses to our devices

DNS

we access websites using domain names (facebook.com, google.com, etc.), but it turns out that these sites too have IP addresses
DNS (Domain Name System) servers convert domain names into IP addresses
difference with IP addresses is that domains are easier to read for humans

Packets

computers communicate by sending packets, which are like virtual envelopes sent between computers
- still 0s and 1s
analogy: suppose we want to find a cat image on the internet
so, we send a request to a server, say Google, like "get cat.jpg"
- place this request in an envelope
on the envelope, we list out IP as the return address
however, for the recipient of the request, we don't know the IP address for Google
- have to rely on DNS
- send a request to our ISPs DNS server for Google's IP address
  - if the ISP's DNS server doesn't know a website's IP address, it has been configured to ask another DNS server
  - there exist root servers that know where to look to for an IP address if it exists
after sending the request off, we'll get a response ms later
the cat will be sent back in one or more packets
- If the cat image is too large for a single envelope, sending it in one packet could take up internet traffic
- to solve this, Google will divide the cat image into smaller fragments
  - put the fragments into different envelopes
  - write information on the envelopes
    - return address: Google's IP address
    - delivery address: our IP address
    - list the number of packets on each envelope (1 of 4, 2 of 4, etc.)

TCP/IP

IP goes beyond addresses
- set of conventions computers and servers follow to allow intercommunication
fragmentation like in the envelope example are supported by IP
- if missing a packet, we can logically infer which packet we're missing based on the ones received
- however, IP doesn't tell computers what to do in this case
TCP (Transmission Control Protocol) ensures packets can get to their destination
- commonly used with IP (TCP/IP)
- supports sequence numbers that help data get to its destination
  - when missing a packet, a computer can make a request for the missing packet
  - the computer will put packets together to get a whole file
- also includes conventions for requesting services (port identifiers)
  - to make sure Google knows we're requesting a webpage and not an email or other service

Ports

per TCP, the world has standardized numbers that represent different services
if 5.6.7.8 is Google's IP address, 5.6.7.8:80 (port 80) lets use know that we want a webpage
- 80 means http (hypertext transfer protocol)
  - the language that web servers speak
- Google will send the request to their web server via http
many websites use secure connections with SSL or HTTPS, which uses the port 443
email uses port 25
other ports exist as well

Protocols

protocols are just sets of rules like a language, in which computers communicate with each other
- humans use these all the time, such as the protocol for meeting people: handshakes
when a request is made to Google for an image, HTTP tells Google how to respond appropriately

UDP

UPD (User Datagram Protocol)
- doesn't guarantee delivery
- used for video conferencing such as FaceTime
  - packets can be dropped for the sake of keeping the conversation flowing
- used anytime we want to keep data coming without waiting for a buffer to fill

IPs in More Detail

IP addresses are limited
- in the format #.#.#.#, each number is 8 bits, so 32 bits total
  - yields 2³² or about 4 billion possible addresses
    - we're running out of addresses for all computers
- current version of addresses is IPv4
- moving towards IPv6
  - uses 128 bits, yielding 2¹²⁸ possible addresses
private addresses exist
- 10.#.#.#, 192.168.#.#, or 172.16.#.#
- only with special configuration can someone talk to our computer
- personal device is not a server, so people should not need to access them directly
  - our device needs to request data from servers
- even email is stored on a server such as Gmail and our device makes a request to that server to access that email
in advanced settings
- subnet mask is used to decide if another computer is on the same network
- router (aka Gateway) has its own address
  - routes data in different directions
- shows DNS servers as well

Routers

routers have bunches if wires coming and going out of them
- they have a big table with IP addresses and where data should be routed to get to that destination
  - often, the data is routed to some next router
routers purpose is to send data in the direction of a destination
- the next router will send it to another until it reaches a destination
internet is a network of networks (with their own routers)
- often multiple ways to go from A to B
  - based in US Military logic to prevent downtime if a particular router goes down
  - when multiple packets are sent, like cat.jpg from Google, they may each take a different path, still getting to their destination eventually
    - sometimes the internet is busy and the quickest path changes

Traceroute

how long does it take for this process of data transfer to take on the internet?
traceroute is a program that sends packets to each router on a path to a destination, reporting the time it takes to reach that router
from Sanders Theatre to Google.com:
- 1-2: few unnamed routers at Harvard
- 3-4: more Harvard routers
- 5-6: Level3 is a ISP
- 7+: routers are denying the request
from Sanders Theatre to berkeley.edu
- 6: Northern Crossroads
- 7-14: above fast connection
  - 8-9: Chicago
  - 10-11: Denver
  - 12-13: Las Vegas
  - 14: Los Angeles
- 19 is where it arrives at Berkeley in 80 ms!
from Sanders Theatre to mit.edu
- 6-7: goes to New York connectivity
- 8: MIT's website is outsourced to Akamai's NYC servers
from Sanders Theatre to CNN.jp
- 9-10 jumps from Seattle to Osaka past an ocean
  - using undersea cabling

Cable Modem

coaxial cable to plug into the wall
phone jacks (RJ11) as many services are bundled together these days
four jacks for ethernet cables (RJ45)
- devices can plug into these for internet connectivity
modem has wifi support built in

Network Switch

switch is a device that we can plug into our router to allow more connections for all our other devices

Home Router

home routers can have wifi, firewall, and switching capabilities

Network Cable

inside a network cable are 8 wires of different colors
- some are for transmitting data, others for receiving data
- others still are for insulation and cancellation of interference

Closing Thoughts and Homework

homework: find a device that looks like a modem or router and take a look at the connectors on the back of it
- if brave, play around with unplugging cables
  - Note: Our internet may go down in the process, but can be easily restarted with the cables properly reconnected
- if you have a spare ethernet cable, take a look inside yourself
  - these are a bit harder to put back together

Multimedia

Audio

computers are good at recording, playing back, and generating audio
uses different file formats
- file formats are just a way of storing 0s and 1s on disk, so that certain software knows how to interpret it
MIDI
- way of storing musical notes for certain songs
- can do this for different instruments
  - programs can render the notes for these instruments
GarageBand
- included with macOS
- Star Wars theme in MIDI
  - doesn't sound quite as good as the actual version
  - computer synthesizes the notes
    - not an actual recording
    - computer interprets notes in the MIDI file
MIDI is common in the digital workspace among musicians who wish to share music with each other
humans typically like to hear music performed and recorded by humans
- file formats for recorded music include:
  - AAC
  - MIDI
  - MP3
  - WAV
WAV is an early sound format, but still used
- uncompressed data storage allowing high quality
MP3
- file format for audio that uses compression
  - significantly reduce how many bits are necessary to store a song
  - discards 0s and 1s that humans can't necessarily hear
    - true audiophiles may disagree
  - trade off between optimizing storage space and sacrificing quality
  - compression is said to be lossy
    - losing the quality in the compression process
AAC
- similar to MP3
- may see when downloading a song from iTunes
streaming services such as Spotify don't transfer a file to us but rather stream bits of information to us
how do we think about the quality of these formats?
- sampling frequency
  - number of times per seconds we take a digital snapshot of what a person would hear
- bit depth
  - number of bits used for these individual snapshots
- sampling frequency x bit depth = number of bits necessary to store one second of music
- audio file formats allow us to modify what these parameters are

Graphics

graphics, what we see with multimedia, is really just a bunch of pixels both horizontal and vertical
- all file formats are rectangular in nature, though transparent pixels can make images look to take on other shapes
- in the simplest form, each of the dots or pixels is a bunch of 0s and 1s
- to create a file format, we just need to determine a mapping
image is only black and white, so how to represent color?

RGB

RGB stands for Red Green Blue
- with information giving an amount of red, an amount of green, and an amount of blue, we can tell a computer how to colorize pixels
- none of the colors yields a black pixel
- all of the colors yields a white pixel
- in between these two options is where we get all sorts of colors
consider the 3 bytes: 11111111 00000000 00000000
- if we interpret these bytes to represent colors, it appears we want all of the red, none of the green, and none of the blue
- these 24 bits (3 bytes = 3 x 8 bits = 24 bits) represent the color we know as red
  - if a computer wanted to represent this color, it would store these 24 bits
consider the 3 bytes: 00000000 11111111 00000000
- green
consider the 3 bytes: 00000000 0000000 11111111
- blue
consider the 3 bytes: 00000000 0000000 0000000
- black
consider the 3 bytes: 11111111 11111111 11111111
- white
can get many color variations by mixing the above colors in different quantities
when we talk about image formats, we typically don't talk in terms of binary but rather something called hexadecimal (base-16, contains 16 digits)
- 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, a, b, c, d, e, f
- 0 is the smallest number we can represent in single digit
- f is the largest number (value of 15) we can represent in a single digit
consider the 8 bits: 1111 1111
- each hexadecimal digit represents four bits
- one hexadecimal digit can represent the first four bits, another can represent the second four
  - represent something with eight symbols using only two!
- 1111 is the decimal number 15, which is f
- Therefore, 1111 1111 in hexadecimal is ff
red can thus be represented in hexadecimal as ff 00 00
green can be represented in hexadecimal as 00 ff 00
blue can be represented in hexadecimal as 00 00 ff
a lot of graphical editing software such as Photoshop use hexadecimal to represent colors

Bitmap Format

Windows XP background was a bitmap file (.bmp)
- a map or grid of bits much like the smiley face
zooming in on the image show that it is just a grid of dots
- pixelation will be notice
much like with audio, so in the world of images do we have discretion over how many bits to use
- how many bits to represent each pixel's color?
- resolution is another factor
  - image that is only 100 pixels scaled up only duplicates the existing limited information, resulting in a blotchy image
  - would be better to start with image that has a higher resolution (more pixels)
a lot of repeated colors, so it seems silly to represent each color with the same number of bits

Image Compression

graphical file formats can often be compressed
can be done lossy or losslessly
- with audio, we threw away audio information that the human ear can't necessarily hear
  - this is lossy compression; throwing information away
- using fewer bits to represent the same information is lossless compression
there is a lot of repeated blue in the Windows image
- using the same 24 bits to represent each pixel
the apple image is compressed and not what a user would see
- the first column contains the color that the rest of the row (scan line) should have
  - image contains instructions on how to repeat the color in a particular row
- when a color is encountered that isn't in the first column, the instructions would list the colors for each non-repeated pixel
- this uses less bits but makes the original information recoverable
lossy compression
sunflower image is a .jpg photograph that is somewhat compressed, but not easy to tell
let's say we want to compress this image further so that we can share it without going over a social media platform's limit
it contains more complicated patterns of colors, so when we try a lossy compression it will become very pixelated
lossy compression means that we won't be able to get that original image back
- compression throws away bits of information
  - "Does the sky really need this many shades of blue?"
  - "Does this leaf really need this many shades of green"
    - replaces bits with only a few colors giving an approximation
    - we will not be able to know how clear the sky used to be from this information

Image File Formats

BMP
- originally used in Windows
- not super common these days
GIF
- low quality images
  - only supports 8-bit color
- often used for memes
  - can be animated
  - like a video file with only a few images
JPEG
- supports 24-bit color
- losslessly compresses
  - can minimize amount of compression to create high quality photos
PNG
- high quality graphics
- supports 24-bit color
all these formats ultimately have an limited amount of information
- ultimate just store pixels and colors of when the image was taken

"Enhance"

common for popular culture abuses of what it means to be a multimedia format
- "Enhancing" means to make an image as clear as possible no matter what format it was saved in
David shows a clip of characters "enhancing" an image
- the characters zoom into a pixelated frame of a video and somehow clear it up to see a reflection
  - video is just a whole bunch of images being shown to us quickly (24 frames per second, etc.)
- the pixelated image only contains information for those pixels
  - there is no way to obtain a clear image unless the original image was already at a high resolution
David contrasts this with an aware clip of Futurama

Video Compression

think of a video format as similar to a flip book
video formats are just a bunch of images shown quickly in succession to create the illusion of motion
- not necessarily all information stored as png, jpg, gif, or even images
- algorithms and mathematics can help go from one frame to another
opportunities for compression
- can leverage same image compression techniques for each frame (intra-frame coding)
- background of multiple frames can contain redundant information
compare current frame and next frame of video and determine what has changed
- store these differences
- key frames store a snapshot of time to remember what the video looks like
- in each subsequent frame remember what has changed
  - using algorithms and math, background is drawn
- key frames are stored multiple times to guarantee that frames can be recovered

Video File Formats

in the world of video, there are more solutions on how to store information
video file formats are containers
- containers are digital container in which we can put multiple types of data
- can include a video track, audio track, a secondary audio track (for different languages), closed captions, …
AVI
- commonly used in Windows
DivX
Matroska
- open source container meant to be more versatile
MP4
- pretty much universal in all browsers
QuickTime
- commonly used in MacOS
Codecs
- ways of storing and encoding information
- for video:
  - H.264
  - MPEG-4 Part-2
  - ...
- for audio:
  - can be stand alone files or tracks in a container
  - AAC
  - MP3
  - ...

3D Video

increasingly, 3D formats are becoming more common
- 360 degree image of Sanders Theatre
  - a spherical image
  - looks distorted in 2D
  - like flattening a globe
images can contain metadata
- information that viewers can't see
- tells programs, applications, and browsers how to display the image
with sensors on a headset, users can experience virtual reality
more file formats are still on the horizon, but ultimately all of them boil down to storing 0s and 1s and why

Security

Introduction

Our data is under constant threat, but how can we defend ourselves?

Privacy

keeping people away from things we don't want them to see
computers are among the lease secure devices we own
- data or files are stored on them as 0s and 1s
  - can be financial info, photos, etc.

Deleting Files

what does it mean to delete a file off of a hard drive?
- visually, it disappears from a desktop or folder
files are stored on a computer as 0s and 1s
some space needs to be allocated for the file
operating system has a file that keeps track of files an their location on disk
graphically, when a file is deleted, it moves to the trash (recycle bin)
- it can still be easily revived from here, until we empty the trash
however, an operating system doesn't actually delete it from the hard drive
- it simply forgets the location and existence of the file
- one can theoretically recover data by looking for familiar patterns of bits
how do we delete more securely?
- re-saving a file with overridden information actually could not override the old bits but rather create more 0s and 1s stored on a hard drive
- special software can wipe data off of a hard drive
who do computers have this obvious flaw with deleting?
- what if we accidentally delete a file?
  - this structure allows for recovery
- wiping data also takes a lot of time, so it's much faster to just forget locations of data

Cookies

feature supported by HTTP
little values a web server puts on a user's browser
used to remember if a user has visited a website before
- allows us to not have to log in every time we visit or refresh a page
  - when we log into a web server, a cookie is planted on our browser
- stored in a database
- browser will send value to web server to remind of previous login
when we make a request we send:
```
GET / HTTP/1.1
Host: example.com
```

we receive:

HTTP/1.1 200 OK
Set-Cookie: session=29823bf3-075a-433a-8754-707d05c418ab

server gives us a cookie, a key-value pair (aka Session)

a cookie is like an ink-based hand stamp for an amusement park or club
wireless information can be intercepted
- what if a hacker could obtain the cookie
  - session hijacking attack
  - if we have already logged in, hacker can pretend to be us
encryption scrambles this value so hackers cannot easily use it
browser history remembers everywhere we've been and everything we've done there
- convenient if we want to recall a website we've visited
  - but, so can anyone else with access to our browser
can clear browser history and cookies
- history likely not securely scrubbed
- will protect us from nosey friends
- websites will forget we visited as the cookies will be deleted as well

Incognito Mode

can open up a typically different colored browser window
use if we want history automatically removed
useful when building a website as sometimes we want a browser to forget old iterations of our website build

Authentication

all of this assumes we log in
if we don't use a passcode to protect our device, anyone can pretend to be us
- what if we lose our phone or device?

Passwords

on a phone could only be a few digits
- not super secure
- - with numbers, each space has 10 options
  - 10 x 10 x 10 x 10 = 10,000 possibilities
  - 0000-9999
on many smartphones, we will have to wait for an amount of time if we have entered a bad passcode
- slows down the process of someone guessing
add more digits or letters of the alphabet
using a-z, A-Z, 0-9
each space now has 62 options (26 + 26 + 10)
62 x 62 x 62 x 62 = 14,776,336 possibilities
maybe we're super secure and we have a 20-char password
- we could forget it
- annoying to type in repetitively
no one fits all
- short = bad, longer = good
- don't use popular words and phrases
  - hackers will look for words or common phrases
most common Passwords
1. 123456
2. 123456789
3. qwerty
4. 12345678
5. 111111
6. 1234567890
7. 1234567
8. password
9. 123123
10. 987654321
hackers have dictionaries of bad passwords that they can search through and try
random passwords
- usually have to confirm so it can be hard to replicate
using numbers to represent letter is common
- 1 for l
- 4 for A
it's suggested to mix uppercase, lowercase, and and throw in numbers
- good to use misspellings
don't put post-it with our password on our monitor
constant password changes can be a net negative
- can encourage easier passwords to help with memorization

Password Resetting

what if we forget our password?
- often can click on a link to reset our password
  - asks us to type email address or username
- typically, we get an email with a link
  - hopefully this goes back to the same website
  - it likely has a random value in the URL
- once back at the website, we update our password
the website has a database
- it generated a random number and stored it with a note indicating password recovery
- the website assumes that anyone who has access to this value and to the user's email is us
typically, tech staff can't tell us what our password is
- odds are our password is encrypted (scrambled) or, more technically, hashed in their database
getting a password in email means that the password are not hashed or encrypted
- also, sending a password over email opens that email to interception
- this is a red flag if a website does this

Using The Same Password

we may have a favorite password that we reuse
- upside is that it's convenient
however, what if one of the websites are hacked?
- a hacker may try to use the password on other websites to see what she or he can get into

Password Managers

difficult to remember all these passwords
software called password managers exist that store on our phone or hard drive all usernames and passwords in an encrypted way
- we have a master password that logs us into everywhere
  - store it physically in somewhere like a safety deposit box
password managers create long random passwords and will log in for us
- all websites have different passwords
however, if we lose the master password, we cannot get the accounts back

Two Factor Authentication

first factor is a password
- historically, something "only" the user knows
- can be guessed
second factor should be fundamentally different
- should be something we have
  - an RSA device displays a unique value that is synced with a server
    - this number needs to be typed in too
    - as long as this device isn't stolen by someone with our password, they can't get in as easily
  - phones now run software that allows us to get a code and type them in
should think about what websites we care about the most and enable two factor authentication
- some companies can use sms (text messages)

Network security

many of our current networks are wireless
- we probably been conditioned to look for free wifi
  - sometimes still might not connect for various reasons
if the wireless connection has not padlock (no password to log in) the connection is not secure
- we may still visit https or secure websites
- however, everything we do on http sites can be seen
what to do?
- don't use that network
- use a VPN (Virtual Private Network)
  - connection to internet is encrypted
    - with an unsecured connection, anyone can access our data

VPN

first establish encrypted connection to a server and let this server communicate for us
- connection between the VPN server and website can still be insecure
because we are encrypting data through an algorithm, using a VPN can slow down speed

Firewall

physical firewall is a wall between connected buildings that prevents the spread of fire
in the world of computer science, a firewall is software that looks at IP addresses and helps keep bad guys out and user data inside
helps prevent people from accessing our computer

Encryption

suppose we want to send a secret message for "HI"
- HI ➟ IJ
- change each letter by 1
- the recipient needs to know how it changed to revert
Plaintext ➟ Cyphertext ➟ Plaintext
- HI ➟ IJ ➟ HI
this is called a caesar cypher
- rotational cyphers are not that secure
  - can be guessed easily
  - not used for internet encryption
- for this to work, recipient needs the key
  - to know the key, we need to agree in advance
    - can't send it encrypted as well as they need the key

Public Key Cryptography

the last example with a caesar cypher is secret-key cryptography
- only one key
in public key cryptography there are two keys, one public and one private
- mathematical relationship between them
- use public key to encrypt, private key to decrypt
- Bob's private key can undo the effects of his public key
- when Bob responds...
- Bob sends a message using Alice's public key
our browser has its own public and private keys
- so does websites like Google and Amazon
  - this allows them to communicate securely with us
often this processes is used to exchange a secret key

Phishing

Phishing attacks are when an adversary sends a somewhat official-looking email
- may contain a link asking for a password or account info
- the email may contain an elaborate backstory "justifying" the request
- the malicious email is trying to obtain information from us
odds are that the link provided doesn't go to the website being claimed
- can go to a website that looks legit
  - people can just copy HTML
results in giving up private information
it's healthy to distrust most email we get
- don't follow links, type in the address for the company ourselves
- sketchy emails may have typographical errors

Malware

Malicious software can also be sent via email
Windows is particularly vulnerable
software can be injected into our browser and our computer to erase our hard drive, make our computer send spam, or hold our data hostage
some malware encrypts our data and asks for large sums of money to get the key to decrypt it
- key could not even work
- this is called ransomware
Malware can ultimately do anything on our computer

Trust

at the end of the day, all of security and privacy boils down to trust
- people around us
- algorithms/software
- manufacturers
we've downloaded software with trust that it will only do what it claims
- word could log our key strokes
- Chrome could monitor us even when not on Google's website
- Snapchat could not delete posts after being seen
there have been cases where software was written to cover tracks of being monitored!
who's to say the software we're using is actually doing what we say?
it's east to curl up into a ball and worry, but we need to decide who to trust
security measures make it more difficult for someone to be malicious, but ultimately they can't guarantee privacy
we have to decide what data we're comfortable with storing, what we view on the internet, who to trust, and how much to trust them

Web Development

Internet

all the computers on the internet are interconnected that supports HTTP and TCP/IP
the internet is an infrastructure to get data from a server to a client
- supports emails, video conferencing, etc.
web is one specific application or service that runs atop the internet
- assumes an internet exists to get data from point A to point B
  - layers functionality that allows us to click, browse, etc.

Web Browser

web browsers are found on phones, computers, and game consoles
have a space to enter a URL (Uniform Resource Locator)
- prefixed by http:// or https://
when typing in a URL, we're sending a request from our device to some remote server
- the server looks at our request and figures out how to respond

Web Server

a computer that has CPU, RAM, and a hard drive
rack servers
- sized so they can be stacked
odds are companies has many of these if they have a web server

HTTP

we send requests to web servers
- language of these requests is HTTP (Hypertext Transfer Protocol)
request: GET /cat.jpg HTTP/1.1
- 1.1 refers to HTTP language 1.1
response by the server: HTTP/1.1 200 OK
- literally means that everything was okay with the request

HTTP Status Codes

Code	Status	Meaning
200	OK	Everything is OK
301	Moved Permanently	Browser should redirect to new location
302	Found	Browser should redirect to new location
304	Not Modified	Browser will cache files if things don't change to save time/bytes from requests
401	Unauthorized	Not authorized to view content; Could require login
403	Forbidden	Not able to view content
404	Not Found	The requested data could not be found because it doesn't exist on the server
500	Internal Service Error	Not your fault; The server erred

HTML

in addition to the HTTP headers that include status codes, the bits representing an image or website will be sent to us
the language that builds websites is HTML (Hypertext Markup Language)
- sent as a response for a request for a web page

HTML isn't a programming language but rather a markup language

allows us to format, but doesn't have control flow such as loops and conditions

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>hello, world</title>
  </head>
  <body>
    hello, world
  </body>
</html>

to implement webpages, we need to write HTML
- editors like Atom and Sublime Text exist to help write HTML
  - all we need is a computer, a keyboard, and some way of typing out text
<!DOCTYPE html>
- lets the browser know the following file is written in HTML 5
<html lang="en">
- specifies that the webpage is written in English
<head></head>
- example of open and close tags
first tag opened is the last tag closed
- HTML is a tag-based markup language
  - tags have attributes
standard extension for a webpage is .html
opening html file and loads the page
- in the top corner of the browser tab is the title
  - comes from the head
- body contains 99% of the webpage's content
- page is a local document, so the address starts with file:// and located where the file is saved
  - not on a web server, so no one else can access it
web hosts exist to serve up websites we write
- we can also buy our own domain name

Atom

TextEdit is not designed for web page development
free alternatives made for web development exist
Atom is an example of editors
in Atom, we can open multiple files at once
colors are added for readability and don't effect how the webpage will render
HTML supports comments
- To help colleagues who look at our code know our intentions of the code

Links

links can make pages more dynamic by linking to other pages

<a></a> are anchor tags that can be used for links

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>link</title>
  </head>
  <body>
    Visit <a href="http://www.harvard.edu/">Harvard</a>.
  </body>
</html>

href (hyper reference) set to where we want the link to go
blue, underlined text traditionally represents a link on a webpage
hovering over the link shows the destination of a link at the bottom left corner on Chrome
traditionally, link becomes purple if we've already followed that link
- browser remembers where we've been
  - potential privacy concern

Images

the web is filled with images

<img/> is the image tag

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>image</title>
  </head>
  <body>
    <img alt="Grumpy Cat" src="cat.jpg"/>
  </body>
</html>

src (source) attribute is set to the address of the file
alt (alternative text) attribute is what displays if the page can't be seen
closes itself as one tag

Paragraphs

even if we add spaces to format paragraphs, HTML will render without them!
when looking at our webpage we can "view page source" on our browser to see the original HTML with our spaces, but the webpage still doesn't have these spaces
the browser will only do what HTML tells it to do
the browser needs instructions in the form of HTML tags
paragraph tags (<p></p>) tell the browser to create a paragraph of text

Headings

<h1></h1> are the heading 1 tags
there also exists <h2></h2>, <h3></h3>, <h4></h4>, <h5></h5>, and <h6></h6>
headings get smaller the larger the number
these make the font larger for usage similar to marking chapters in a book

Lists

unordered lists use bullets
- <ul></ul>
ordered lists use numbers
- <ol></ol>

<li></li> are list item tags

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>unordered and ordered list</title>
  </head>
  <body>
    <ul>
      <li>foo</li>
      <li>bar</li>
      <li>baz</li>
    </ul>
    <!-- <ol></ol> -->
  </body>
</html>

Tables

<table></table> are table tags that create a table

<tr></tr> are table row tags
<td></td> are table data tag
- Like columns or cells

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>table</title>
  </head>
  <body>
    <table>
      <tr>
        <td>7</td>
        <td>8</td>
        <td>9</td>
      </tr>
      <tr>
        <td>4</td>
        <td>5</td>
        <td>6</td>
      </tr>
      <tr>
        <td>1</td>
        <td>2</td>
        <td>3</td>
      </tr>
      <tr>
        <td>*</td>
        <td>0</td>
        <td>#</td>
      </tr>
    </table>
  </body>
</html>

Implementing Google

when we type google.com browser adds "https://www." to the beginning of the URL
- as is needed to surf the web
curl is a command ran in the terminal that behaves much like a browser
- it sends a request like a browser and shows what html is returned
  - capital letter tags are a bit dated
  - shows google.com is located at http://www.google.com
  - -I flag tells curl to return HTML headers
    - includes status codes and other info humans normally don't see
  - google's server has been configured to redirect users to http://www.google.com
  - UTF-8 is unicode characters
- curl http://www.google.com returns a webpage that includes HTML and JavaScript
searching for cats changes the URL to https://www.google.com followed by a large sequence of characters
- distilling this URL to https://www.google.com/search?q=cats leads to the same results
  - We can "create" a search engine using this info!

Forms

<form></form> are form tags that take attributes for an action and a method

<!DOCTYPE html>
<html>
  <head>
    <title>search</title>
  </head>
  <body>
    <form action="https://www.google.com/search" method="get">
      <input name="q" type="text"/>
      <input type="submit" text="Search"/>
    </form>
  </body>
</html>

action="https://www.google.com/search" method="get" means "get me https://www.google.com/search"
inside the form, we can have <input/> tags
- these can have name, type, value, and text attributes
this implementation punts the searching to Google
The browser uses the HTML form to assemble a URL
- https://www.google.com/search?q=cats
? in the URL means "Hey Server! Here comes my HTTP parameters!"
- a URL may have multiple parameters separated by &

css0.html

CSS (Cascading Style Sheets) allows us to style our webpages

in contrast, HTML allows us to structure our webpages

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>css0</title>
  </head>
  <body>
    <header style="font-size: large; text-align: center;">
      John Harvard
    </header>
    <main style="font-size: medium; text-align: center;">
      Welcome to my home page!
    </main>
    <footer style="font-size: small; text-align: center;">
      Copyright &%169; John Harvard
    </footer>
  </body>
</html>

inside body, we have three tags: <header></header>, <main></main>, and <footer></footer>
- includes style attributes written in CSS
  - written as key-value pairs
  - in CSS, there is a property called font-size
    - CSS supports small, medium, large, and exact sizes such as 16px
  - text-align: center; centers the text
this example has some redundancy

DOM

CSS supports the notion of a hierarchy
- rectangles here represent HTML tags or elements
- ovals represent text values
- Computer Science this is call called tree, much like a family tree
when a browser receives a webpage, it builds a tree-like data structure in our computer's RAM
header, main, and footer are all child nodes of of the parent node body

css1.html

we can put the text-align: center; attribute on the body element so it will pass it on to its children (header, main, and footer)

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>css1</title>
  </head>
  <body style="text-align: center;">
    <header style="font-size: large;">
      John Harvard
    </header>
    <main style="font-size: medium;">
      Welcome to my home page!
    </main>
    <footer style="font-size: small;">
      Copyright &%169; John Harvard
    </footer>
  </body>
</html>

this is better design as we can change all the text alignment at once

css2.html

combining HTML and CSS is generally frowned upon

makes it hard to collaborate

one person can work on content (HTML)
the other on style (CSS)

<!DOCTYPE html>
<html lang="en">
  <head>
    <style>
      .centered { text-align: center; }
      .large { font-size: large; }
      .medium { font-size: medium; }
      .small { font-size: small; }
    </style>
    <title>css2</title>
  </head>
  <body class="centered">
    <header class="large">
      John Harvard
    </header>
    <main class="medium">
      Welcome to my home page!
    </main>
    <footer class="small">
      Copyright &%169; John Harvard
    </footer>
  </body>
</html>

<style> can be a tag as well as an attribute
.centered defines a class named centered
- anything with this class with have the style attribute text-align: center;

css3.html

we can even get rid of class attributes to further separate style from content

<!DOCTYPE html>
<html lang="en">
  <head>
    <style>
        body { text-align: center; }
        header { font-size: large; }
        main { font-size: medium; }
        footer { font-size: small; }
    </style>
    <title>css3</title>
  </head>
  <body>
    <header>
      John Harvard
    </header>
    <main>
      Welcome to my home page!
    </main>
    <footer>
      Copyright &%169; John Harvard
    </footer>
  </body>
</html>

tags can be given CSS directly
looks identical, but better design

css4.html

what if we remove the style altogether and store it elsewhere?

<!DOCTYPE html>
<html lang="en">
  <head>
    <link href="css4.css" rel="stylesheet"/>
    <title>css4</title>
  </head>
  <body>
    <header>
      John Harvard
    </header>
    <main>
      Welcome to my home page!
    </main>
    <footer>
      Copyright &%169; John Harvard
    </footer>
  </body>
</html>

pruned html down to its essence
- no usage of style tags
Note: <link/> tag with href attribute of css4.css and a rel (relationship) attribute of stylesheet
- specifying in the head of the webpage "Hey Browser! Please link css4.css file to this page!"

have this stylesheet in the same directory

body { text-align: center; }
header { font-size: large; }
main { font-size: medium; }
footer { font-size: small; }

factored out the style to its own file
- easier for collaboration and sharing
- can use on multiple html pages
- can create different themes

Closing Thoughts

web development is about writing code
- in the language called HTML, which builds the structure of a webpage
- CSS allows to fine tune the webpage's aesthetics
we can use these building blocks to further learn about web development on our own
underlying concepts are more important than details
- a webpage is nothing more than a text file written in HTML, CSS, and maybe some JavaScript
  - this file can be uploaded to a server to put on the internet
    - can sign up for a web host with data centers
    - all files will go in a folder on the server so that the webpage can be accessed on the internet
  - can also buy a domain name and configure it to point to the web host
these building blocks are what allow us to put our content on the internet

Programming

Scratch is a graphical programming language created by MIT's Lifelong Kindergarten Group
the language not only helps get kids excited about programming, but it's also very instructive

Software

programing is about making software
- software is what runs on our hardware
  - could run on a desktop, phone, etc.

Finding Mike Smith

code is just a technical implementation of algorithms
- algorithms are step by step instructions for solving problems
consider a phonebook full of thousands of names and phone numbers
how to lookup someone like Mike Smith?
- could start at the first page, move to the next, and so on until he's find
  - correct algorithm, as we will find Mike Smith eventually
  - however, it's inefficient
- could start at the first page and count by 2s
  - would find Mike Smith twice as quickly
  - however, this alone is not correct as we could miss Mike Smith if his name is sandwiched between two pages
  - we could fix this by checking the previous page if we go past where Mike Smith should be
more likely, we'd probably go to the middle of the phonebook and find ourselves in the "M" section
- as Smith is after M, he must be in the latter (right) half of the book
  - ignore the other half
- after removing the other half, we are left with half of the book, representing the same problem we started with fundamentally
  - we can keep repeating this process until we're down to one page with Mike's number on it
this leverages the fact that the book is sorted alphabetically
dividing and conquering the problem again and again
- 1000 pages → 500 pages → 250 pages → 125 pages…

Phonebook Algorithm

1 pick up phone book
2 open to middle of phone book
3 look at names
4 if Smith is among names
5     call Mike
6 else if Smith is earlier in book
7     open to middle of left half of book
8     go back to step 3
9 else if Smith is later in book
10    open to middle of right half of book
11    go back to step 3
12 else
13    quit

Pseudocode

Pseudocode algorithm is code, not written in a programming language, but rather English
- code-like syntax written in English
- numbered lines to maintain order and reference lines
- pick up, open to, look at, call, open, and go back are functions
- if, if else, and else are conditions
- Smith is among names, Smith is earlier in book, and Smith is later in book are Boolean expressions
  - can be either true or false
  - if these are true, the indented code below is executed
- both line 8 and 11 say to go back to step 3
  - this creates a loop
  - doing the sane thing again and again

Programming Constructs

functions
loops
conditions
Boolean expressions
variables
threads
events
and more are common across all programming languages

C

C is one of the oldest programming languages that someone might still write in
```
#include <stdio.h>

int main(void)
{
    printf("hello, world/n");
}
```
some of this syntax may look cryptic, but we can likely guess what it does
- it prints "hello, world" to the screen
- the other details can be learned
  - just like with written human languages that are foreign, we just haven't learned the patterns yet
many programming languages have similarities, so it becomes easier to learn new ones with knowledge under your belt
programming is about writhing software to control hardware to solve a problem
however, computers only understand binary (0's and 1's)
- source code what we humans write and it can be converted into machine code (0's and 1's)
  - achieved by using a program called a compiler
  - allows a human to write the code and a machine to read and run it

C++

#include <iostream>

int main()
{
    std::out << "hello, world" << std::endl;
}

program written in C++ still prints "hello, world"
many programming languages do the same things differently
- we can solve the same problem using any different number of languages
- could be easier to use one programming language for a specific problem
  - different languages were invented to tackle different kinds of problems

Python

print("hello, world")

straightforwardly, prints "hello, world"
Python is a different type of language as you don't type source code and manually convert it into machine code
- a special program called an interpreter converts the source code into an intermediate language called byte code
  - which is not machine code (0's and 1's)
```
2      0 LOAD_GLOBAL           0 (print)
       3 LOAD_CONST            1 ('hello, world')
       6 CALL_FUNCTION         1 (1 positional, 0 keyword pair)
       9 POP_TOP
      10 LOAD_CONST            0 (None)
      13 RETURN_VALUE
```
- interpreter reads one line at a time
Note: There are different ways of doing the same thing in many different languages
- and languages get used in different ways
- sometimes you need to compile code, sometimes you need to interpret code
consumer interacts with the software in the same way
- don't need to know what language it's written in as long is it run on their computer
  - programs are often packaged differently for different operating systems

Other Programming Language

Java

class Hello
{
    public static void main(String [] args)
    {
        System.out.println("hello, world");
    }
}

Ruby
```
put "hello, world"
```
Lisp
```
(print "hello, world")
```
JavaScript
```
console.log("hello, world")
```

Introducing Scratch

each of languages have a lot more features, but they have commonalities
- functions, loops, conditions, Boolean expressions, variables, threads, etc.
focusing on these ideas in a graphical manner, using Scratch
Oscartime example, trash was moving down the screen
- a screen is just a grid of pixels
- to make animation, we move an image on it slightly
  - if done quickly, it looks like movement
to make the trash stop at the bottom of the screen, we use some Boolean expression and condition
- "If you're touching the bottom of the screen, stop moving"
- or "Only if you're not touching the bottom of the screen, keep moving"
when trash is put in the can, the lid lifts and Oscar counts the pieces of trash disposed of
- Oscar sprite is using a variable
  - initialized (set initially) to 0
  - increments (adds 1 to the variable) for each piece of trash
- a condition is also used here
  - "If a piece of trash is added, then increment the variable for trash pieces"
music was playing in some kind of loop
even though this program is complex enough to take 8 hours to make, it's ultimately built with the same fundamental building blocks of conditions, loops, etc.

hello, world in Scratch

display/play the text hello world using function say
forever block runs the code inside it infinitely
repeat block will repeat the code inside it the number of times you speficified
if/else block is used to make a condition
Scratch allow you to program by piecing together puzzle pieces with shapes that imply what to do
we can put an if else inside another if else
green blocks are Boolean Expressions

Scratch Interface

Scratch is not only a language but a programming environment as well
on the left is Scratch the cat in a 2D world with height and width
- can change background and more sprites to this world
in the middle are palettes containing scripts
- blue are motion blocks
- in the costumes tab we can change aesthetics
- the sounds tab can introduces sounds and multimedia
blank slate on the right is where we can drag and drop the puzzle pieces and connect them in order to instruct Scratch to do things
when green flag clicked is equivalent to the start of your program
- green flag button starts, the red stop sign button ends
when we drag blocks together, the edge of the block glows white to signify they connect
the hello, world Scratch program won't stop until we click the red stop sign as we never told Scratch to stop in the script

Sounds

we can also add sounds

Loops

if we want Scratch to do something repeatedly, we can use loops
- can move the sound into a repeat block
  - the containing block will grow to fit
  - this seems to only apply the meow once
  - the sound repeats so quickly they overlap
  - one plays the sound until done before the next cycle in the loop
this processes was an example of a common and frustrating experience when programming: bugs

Animation

move back and forth the cat forever
this moves the Scratch the cat forward (to the right) until he hits the edge
- if we drag the cat back, he'll keep moving forward
Scratch will now rotate 180° if touching the edge of the screen
- but Scratch is flipping upside down (literally rotating 180°)
  - another bug
we can record custom sounds under the sounds tab and add it

Breaking Down Problems

it's much easier to write complex programs if you start out by breaking them down into their component parts
- consider individual milestones for yourself
even companies like Microsoft didn't create Word in a day
- software developers make one small feature at a time
- eventually, this becomes millions of lines of code

pet the cat

reading and understanding code is another side of software development
- teams need to do this to collaborate
when this program starts, nothing happens until the mouse pointer touches the cat, in which the cat meows

don't pet the cat

the script has an if else
will play a lion's roar if the mouse pointer touches the cat, but will meow and wait 2 seconds if not

counting sheep

this first sets a variable called counter to 0
it will forever say counter for 1 second, wait one second, then increment the counter
this will count forever

cough0

we can create our own puzzle pieces
- we can do this in most programming languages
  - where we create functions
- in scratch we can utilize the functionality of existing puzzle pieces
there is an opportunity for better design here
- it looks like we've copied and pasted puzzle pieces

cough1

we can improve this with loops
better design as we can change what the cat is saying or the wait time in one place

cough2

what if I just want a puzzle piece to make any sprite cough?
- gain the ability to share the functionality to use elsewhere
- we've defined a new block called cough
  - we repeat cough 3 times, abstracting away the complexity
we can go even further by passing in a value to your custom block
- this value is called an argument or parameter
  - e.g., say block takes in an argument of "hello, world" or some other phrase
whatever the user passed into cough will replace n
the evolution of this program is an example of what it's like to program and solve problems
- there were opportunities to improve from a correct yet poor design
- to be good at programming is to be able to notice opportunities like this

Threads

in Scratch, we can have multiple sprites, each with their own scripts
- two things will happen simultaneously, called threads
the program has a cat chasing a bird
- here's what guides the bird:
- location in the world can be addressed with coordinates
- will keep moving around if not touching the cat
here's what guides the cat:
- the cat will point in a random direction
- forever checks if touching the bird and moves towards the bird
  - if touching the bird, a lion's roar will play and the script will stop
if we increase the movement speed of the bird to 6 steps, it still gets caught
if we increase the movement speed of the cat to 10 steps, the bird stands no chance

Events

a computer can do multiple things at a time due to multithreading
- now that computers have multiple cores, they can literally do two things at once
- however, computers are so fast that even if two things are technically not happening at the same time, we can't notice the difference
- these threads can also intercommunicate in Scratch with events
the sprite (an orange puppet) will forever check for the spacebar being pressed
- if this happens, the sprite will say "Marco!" for 2 seconds and broadcast event
  - events are messages only the computer can hear
    - if another sprite is configured to listed for event it can respond
the sprite will say "Polo!" for 2 seconds if it hears event
when the green flag is clicked, the orange puppet will wait for the spacebar and then tell the other sprite when to say "Polo!"
the idea allows two sprites to interact in such a way that one sprite does something only if the other does something first

Closing Thoughts

programmers in the real world don't typically program by dragging and dropping code blocks
- they write textural lines of code (C, Java, Python, etc.)
- however, the ideas are fundamentally identical
  - Scratch gets rid of the syntactical distractions
understanding functions, loops, conditions, variables, etc. provides a fundamental understanding of what it's like to program
we focused on imperative or procedural programming, but other types of programming exist as well
- Object oriented programming
- Functional programming
even in all these different ways of programming, we are still utilizing the same basic building blocks we've explored in Scratch
- we can assemble these building blocks to solve problems
Oscartime was a complex game
- zooming in, we see these basic concepts
  - forever loops make the trash fall, an if conditions to raise the lid of the trash, etc.
there are many more languages out there
- https://en.wikipedia.org/wiki/List_of_programming_languages
- there tend to be trends in the industry
- a programmer typically has one or a few languages that the reach for to tackle a problem
- good to introduce yourself to new languages
  - they are easier to learn than spoken or written languages as the ideas persist

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CS50's Understanding Technology

Hardware

Binary

CPU

RAM (Random Access Memory)

Hard Drives

Flash Memory

Types of Memory and Funneling

Display Connectors

USB (Universal Serial Bus)

Wireless

Operating System (OS)

Internet

Introduction

IP

DNS

Packets

TCP/IP

Ports

Protocols

UDP

IPs in More Detail

Routers

Traceroute

Cable Modem

Network Switch

Home Router

Network Cable

Closing Thoughts and Homework

Multimedia

Audio

Graphics

RGB

Bitmap Format

Image Compression

Image File Formats

"Enhance"

Video Compression

Video File Formats

3D Video

Security

Introduction

Privacy

Deleting Files

Cookies

Incognito Mode

Authentication

Passwords

Password Resetting

Using The Same Password

Password Managers

Two Factor Authentication

Network security

VPN

Firewall

Encryption

Public Key Cryptography

Phishing

Malware

Trust

Web Development

Internet

Web Browser

Web Server

HTTP

HTTP Status Codes

HTML

Atom

Links

Images

Paragraphs

Headings

Lists

Tables