Representing Text

Question 1

What is a character set?

Answer 1

A list of characters and the codes used to

represent each one

Question 2

What does ASCII stand for? How many bits are in each character and how many characters total?

Answer 2

• ASCII: American Standard Code for Information Exchange

- Seven bits for each character, 128 unique character

Question 3

How many bits was ASCII later evolved to?

Answer 3

• ASCII evolved so that all eight bits were used

- Represent lines, symbols, and letters with accents

Question 4

How many are control characters?

Answer 4

• First 32 and last one are control characters or hidden characters, they control how text appears, but do not
  appear as text

Question 5

How to find the code for uppercase/lowercase letters?

Answer 5

Uppercase letters start 65
• Code for J (10th letter) is 65 + (10 – 1) = 74
Lowercase letters start at 97
• Code for j (10th letter) is 97+ (10 – 1) = 106

Question 6

What is the issue with the ASCII set?

Answer 6

• Limited, a lot of symbols are missing and is not enough for international use

Question 7

What is Unicode character set?

Answer 7

• A super set of ASCII

- The first 128 characters in the Unicode character set correspond exactly to the ASCII character set

Question 8

How many bits per character and characters are represented in the Unicode character set?

Answer 8

• Uses 16+ bits per character and can represent more than 1 million characters

Question 9

What is lossless compression?

Answer 9

• A compression technique that does not lose any data in the compression process

Question 10

What is data compression?

Answer 10

• A reduction in the amount of space needed to store a piece of data

Question 11

What is the compression ratio?

Answer 11

• Size of the compressed data/size of the original data

Question 12

What are the two types of data compression?

Answer 12

• Lossless: A compression technique that does not lose any data in the compression process
• Lossy: Some information may be lost in the process
  of compaction.

Question 13

What are the three types of loseless techniques?

Answer 13

1. Keyword Encoding
2. Run-Length Encoding
3. Huffman Encoding

Question 14

What is keyword encoding?

Answer 14

• Frequently used words are replaced with a single character
• The characters used to encode cannot be part of the original text

Question 15

What type of technique is keyword encoding usually used with?

Answer 15

-Huffman encoding

Question 16

What is Run-length encoding?

Answer 16

• A single character may be repeated over and over again in a long sequence
• A sequence of repeated characters
  is replaced by
  1. A flag character,
  2. Followed by the repeated character,
  3. Followed by a single digit that indicates how many times the character is repeated
• you do not need to process 1-3 characters

Question 17

What does Huffman Encoding use to represent each

character?

Answer 17

• Uses variable-length bit strings

Question 18

What is an advantage of Huffman Encoding

Answer 18

-saving lots of storage space by making shorter sequences (less bits) for the frequent letters and longer sequences (more bits) for the non-frequent letters

Question 19

What is the least effective compression?

Answer 19

-Keyword Encoding

Question 20

What is the most effective compression?

Answer 20

-Huffman Encoding

Question 21

What are the applications of Huffman encoding?

Answer 21

• JPG, MP3, ZIP

Question 22

How to find compression ratio for Huffman encoding?

Answer 22

number of characters*bits/compressed bit length