Khan Academy Computers Study List

Tess is creating an alarm clock using an LCD display like this one: Each digit will be 5 pixels wide and 7 pixels tall. She decides to save space by storing the pixels for each digit using run-length encoding (RLE). RLE is a data compression technique that replaces "runs" (sequences of bits with the same value) with a number representing the length of the run. Here's the RLE for one of the digits: 1,3,1 0,1,3,1 0,1,3,1 1,3,1 0,1,3,1 0,1,3,1 1,3,1 In her RLE scheme, the first number in each row represents a run of "off" pixels, the second number represents a run of "on" pixels, and subsequent numbers alternate between representing runs of "off" and "on" pixels. Which digit is represented by that encoding?

8

ABRACADABRA ALAKAZAM In a standard uncompressed representation, each letter would likely be represented with its ASCII encoding (A as 0100 0001, B as 0100 0010, etc.) The Huffman coding compression algorithm reduces the amount of bits required to represent data by choosing new bit codes for each source symbol, and choosing short codes for the most frequent source symbols. If the computer applies Huffman coding to the wizard spell, which letter is the most likely to be represented with a shorter bit code?

A

Marquis needs to upload a photo of himself to a website, and the website requires that photos be in JPEG format. However, his only photo is in the GIF format. Which of these approaches should he take?

He can open the file in an image editing application, export it as a JPEG, and upload the exported JPEG file.

Shir wants to know how powerful her computer is, and she finds this display of information: It says her computer has 16 GB of memory. What can her computer do with that memory amount?

Her computer can remember 16GB worth of instructions and data at any time, shared by all the open applications.

A computer is built up from layers of components, like an integrated circuit, logic gates, and transistors. Which of these is the best description of a transistor?

It converts an electrical signal into a 0 or 1 based on the amount of voltage going through it.

A computer is built up from layers of components, like a CPU, logic circuits, logic gates, and transistors. Which of these is the best description of a CPU?

It receives input data, processes & stores it, and produces output.

The following processes are used by video compression algorithms. Categorize each process as either lossless or lossy: inter-frame coding: Instead of storing every pixel for every frame, the computer stores the differences between the pixels. entropy coding: The computer replaces frequently occurring patterns of bits with shorter representations, remembering the replacements in a table. subjective field of interest: The computer detects when a person is the focus of the video frame, and stores less information about the background.

Lossless Lossless Lossy

Remy is setting up a cooking blog and is deciding whether to use lossy compression algorithms on her photos of yummy meals. What's a good explanation of the benefits and drawbacks to lossy compression algorithms?

Lossy compression algorithms reduce the file size, but the compression is irreversible, so you need to store the original images if you ever want to display the photos at a higher quality.

Nora is learning to use image editing applications and doesn't understand when to lower the quality setting for JPEG saving. What's a good use case for using a lower quality setting?

Saving a thumbnail that will link to a full-sized version

Cora is planning to take photos of her cat and print posters of each photo to hang on her walls. What advice will help her the most in creating high-quality posters?

She should save photos with a lossless compression algorithm, to ensure that the posters are of highest quality.

Which of these are examples of metadata for a photo? 👁️Note that there may be multiple answers to this question. Choose all answers that apply:

The number of bytes of data in the file The people who have permission to edit the file The type of camera that took the photo The timestamp when the photo was downloaded

Byte pair encoding is a compression algorithm that replaces the most common pairs of characters in a string with a character that isn't in the data, and creates a table of replacement mappings. Here's the output from a byte pair encoding: One ZY Two ZY Red ZY Blue ZY What was the original string?

The original string can't be reconstructed

Lailah and Alonso are setting up a home security system for their house. The system can currently store 2 months of video on an 8GB hard drive. They are going away for 3 months and want video for all 3 months. What are the best options for them to consider? 👁️Note that there are 2 answers to this question.

They could replace the hard drive with a higher capacity hard drive. They could save the videos using a lossy compression algorithm.

Alejandro wants to explain the importance of file extensions to his friend. Which of these explanations is the best?

When a file has an extension, the computer can decide what application to open it in.

Pixar is a company that creates a huge amount of images, audio recordings, and videos, and they need to decide what compression algorithms to use on all those files. When would Pixar most likely use lossless compression?

When saving the rendered 3D images that will become the film.

Peter wants to make a computer, but doesn't have the money to buy a CPU. He only has the money for a keyboard, RAM (Random Access Memory), and monitor. Why is the CPU important?

Without a CPU, the computer can't process the data that comes from the keyboard.

Consider these two files: A 1-minute video of a cat pouncing ferociously on a twisty tie. A download of "The Lion King", a 1.5-hour animated film Is it possible for "The Lion King" to have a smaller file size than the shorter cat video?

Yes, if "The Lion King" video is compressed with a lossy compression algorithm.

Consider these files: A 3-second audio recording of a baby's first word. A 30-second video recording of a baby's first steps. Is it possible for the 30-second video recording to have a smaller file size than the 3-second audio recording?

Yes, if the 30-second video is compressed with a lossy compression algorithm.

Consider these two files: A still screenshot of the Apple homepage A 5-second video of the same Apple homepage Is it possible for the video to take up less space than the image?

Yes, if the video is compressed with a lossy compression algorithm.

Byte pair encoding is a compression algorithm that replaces repeated pairs of characters in a string with a character that isn't in the data, and creates a table of replacement mappings. Here's a quote from Dr. Seuss: "Think left and think right and think low and think high. Oh, the thinks you can think up if only you try!" Which of these character pairs would the algorithm replace? 👁️Note that there may be multiple answers to this question.

in th ig

Erica is trying to describe how a computer works to her 7-year-old cousin, by relating each part to how humans work. Fill in the words that make the most sense for this metaphor:

input devices microphone CPU output devices speaker

Nikolas is looking at the files for a project he's working on: Which file was modified the longest time ago? Which file is the largest? Which file type appears the most in this listing?

pause.svg tests.zip png

Jermaine is storing files on his laptop for a project about his family ancestry. Sort their files by size, from largest (at the top) to smallest (at the bottom):

reunion.mov 15GB interview.mov 1.2 GB grandfather.jpg 2 MB grandma_bw.jpg 900KB familytree.ged 32KB notes.txt 450B

Here's the output from a byte pair encoding: Ze mXe Zat yB rFd, Ze mXe Zings yB wiJ know. Ze mXe Zat yB lFrn, Ze mXe places yB'J go Here's the replacements table: original replacement th Z or X ou B ea F ll J Decode the compressed string to discover the original string, a quote from Dr. Seuss:

the more that you read the more things you will know. the more that you learn the more places youll go

A truth table shows the output values for a logic gate, based on a set of input values. Complete this truth table for an OR gate by entering either 0 or 1 in the output column:

0 0 0 0 1 1 1 0 1 1 1 1

You're playing around with an old computer that only accepts binary numbers as input. How do you enter the decimal number 73 into the computer?

1001001

Web developers can specify colors for web pages using the "RGB" color system, where each color is specified in terms of its red, green, and blue components. Here are a few examples: RGB color FF0000 red 00FF00 green 0000FF blue What's the maximum number of unique colors that can be represented with this RGB notation?

16^6 unique colors

The Apple iPhone includes a settings menu where you can toggle various accessibility options on or off: Mono Audio -- Off Phone Noise Cancellation -- On Hearing Aid Compatibility -- Off What is the minimum number of bits that can represent those 3 options?

3

Audacity is an open source audio editing application that provides a variety of effects that you can apply to audio files. Which of these effects sounds like a lossy operation?

Equalization: Adjusts the volume levels of particular frequencies.

A computer is built up from layers of components. The lowest level components are responsible for tiny computations, and the highest level components are responsible for sophisticated multi-step computations. Sort these component from high-level (at the top) to low-level (at the bottom):

Computer Integrated Circuit Logic Gate transistor

In natural languages, there are big differences in what letters are the most common across words. In the Spanish language, the letter "e" is the most common, occurring 12.2% of the time, and the letter "k" is the least common, occurring only 0.01% of the time. That's why the Huffman coding algorithm can help to compress natural language documents. Huffman coding reduces the amount of bits required to represent data by choosing new bit codes for each source symbol, and choosing short codes for the most frequent source symbols. Here's a Spanish word encoded with Huffman coding: 1101101111111110 These are the codes for the letters in the word, a subset of the table of codes for all the letters in the Spanish alphabet: letter bit code a 011 c 11011 o 1111 s 1110 Decode the word according to the bit codes in the table. What is the original Spanish word represented by that encoding?

caos

Kevin is entering the JS1K contest, a contest to write an interesting program in just 1K of JavaScript code. He's hoping he can rewrite a program that's currently 1MB and enter that for the contest. If he succeeds, how much smaller will his new program be?

Around 1/1000 of the original size.

Landyn has a smart phone that can store 1 GB of data. He sees a commercial for a new phone that promises storage for 1 TB of data. What's the best estimate of how much more data the new phone will allow him to store?

Around 1000 times more data.

The diagram below shows a circuit with three gates, where the output from two OR gates are sent into a third OR gate. The circuit has four inputs (A, B, C, D), two for each OR gate. a--> b-->OR--> OR--> c-->OR--> d--> If the final output is 0, what is a possible state of the inputs A/B/C/D?

A = 0, B = 0, C = 0, D = 0

Compression algorithms vary in how much they can reduce the size of a document. Which of the following would likely be the easiest to compress?

A black & white icon of a unicorn

Compression algorithms vary in how much they can reduce the size of a document. Which of the following would likely be the hardest to compress?

A document created by 10 monkeys banging randomly on typewriters for 10 hours

8 1,2,2,2,1 1,2,2,2,1 8 8 2,4,2 1,1,4,1,1 8 In this RLE scheme, the numbers represent alternating runs of black and white pixels and the first number in each line represents a run of white pixels. What does this icon look like?

A frowning face

Run-length encoding (RLE) is a data compression technique that replaces "runs" (sequences of bits with the same value) with a number representing the length of the run. RLE can be used to compress color images, especially when the colors are from a limited palette. The following is a run-length encoding of a 8x8 icon with 3 colors. "R" signifies a red run, "G" signifies a green run, and "Y" signifies a yellow run. 3Y,2R,3Y 2Y,1R,2G,1R,2Y 1Y,1R,4G,1R,1Y 1R,6G,1R 1R,6G,1R 1Y,1R,4G,1R,1Y 2Y,1R,2G,1R,2Y 3Y,2R,3Y What does this icon look like?

A green diamond

Sort these component from high-level (at the top) to low-level (at the bottom):

CPU logic circuit logic gate transistor

How does a computer store numbers in memory?

Computers represent numbers in memory using the binary number system.