CS101 Lecture 13:
Text Representation
and Data Compression
John Magee
15 July 2013
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Overview/Questions
– How do computers store text information?
– Why do some characters show up as �s on
my browser?
– What is compression, and why is it important?
– How can data be compressed?
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Binary Representations
Recall: a single bit can be either a 0 or a 1
What if you need to represent more than 2
choices?
n bits can represent 2n possible combinations
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Representing Text
There are finite number of characters to represent,
so list them all and assign each a binary pattern.
Character set
A list of characters and the binary codes used to
represent each one.
Computer manufacturers agreed to standardize in
the early 1960s.
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The ASCII Character Set
ASCII stands for American Standard Code
for Information Interchange
ASCII originally used seven bits to represent
each character, allowing for 128 unique
characters
Later extended ASCII evolved so that all
eight bits were used.
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The ASCII
Character
Set
(7 bits)
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The
Extended
ASCII
Character
Set
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Can't You Take a Joke? :-)
Carnegie Mellon professor Scott E. Fahlman
Proposed ASCII emoticons, Sept. 19, 1982.
Source: http://www.wired.com/science/discoveries/news/2008/09/dayintech_0919
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ASCII Art
Text-based systems
had no graphics…
…so people created art
and graphics out of
ASCII text!
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The Unicode Character Set
Extended ASCII is not enough for
international use.
Unicode uses 16 bits per character
How many characters can UNICODE
represent?
Unicode is a superset of ASCII.
The first 256 characters correspond exactly
to the extended ASCII character set
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The Unicode Character Set
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Recall: Morse Code
Invented by
Samuel Morse for the
telegraph in 1840s
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Text Compression
Problem: Assigning 16 bits to each character in a
document uses a heck of a lot of space.
We need ways to store and transmit text
efficiently. Why?
Common compression techniques:
keyword encoding
run-length encoding
Huffman encoding
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Keyword Encoding
Replace frequently used words with a single
character
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Keyword Encoding Example
Given the following paragraph,
We hold these truths to be self-evident, that all men are created
equal, that they are endowed by their Creator with certain
unalienable Rights, that among these are Life, Liberty and the
pursuit of Happiness. That to secure these rights, Governments
are instituted among Men, deriving their just powers from the
consent of the governed, That whenever any Form of
Government becomes destructive of these ends, it is the Right of
the People to alter or to abolish it, and to institute new
Government, laying its foundation on such principles and
organizing its powers in such form, as to them shall seem most
likely to effect their Safety and Happiness.
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Keyword Encoding Example
The encoded paragraph is
We hold # truths to be self-evident, $ all men are
created equal, $ ~y are endowed by ~ir Creator with
certain unalienable Rights, $ among # are Life,
Liberty + ~ pursuit of Happiness. — $ to secure #
rights, Governments are instituted among Men,
deriving ~ir just powers from ~ consent of ~
governed, — $ whenever any Form of Government
becomes destructive of # ends, it is ~ Right of ~
People to alter or to abolish it, + to institute new
Government, laying its foundation on such principles
+ organizing its powers in such form, ^ to ~m shall
seem most likely to effect ~ir Safety + Happiness.
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Keyword Encoding
Compression ratio
The size of the compressed data divided by the size
of the original data (0 < c.r. <= 1)
What did we save?
Original paragraph:
Encoded paragraph:
Characters saved:
Compression ratio:
656 characters
596 characters
60 characters
596/656 = 0.9085
Could we use this substitution chart for all text?
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Run-Length Encoding
Consider a single character which is repeated over
and over again in a long sequence.
Replace a repeated sequence with
– a flag character
– repeated character
– number of repetitions
Example: *n8
– * is the flag character
– n is the repeated character
– 8 is the number of times n is repeated
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Run-Length Encoding Example
Original text
bbbbbbbbjjjkllqqqqqq+++++
Encoded text
*b8jjjkll*q6*+5 (Why isn't l encoded? J?)
The compression ratio is 15/25 or .6
Encoded text
*x4*p4l*k7
Original text
xxxxpppplkkkkkkk
This type of repetition isn’t very helpful for English text;
can you think of a situation where it might be helpful?
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Huffman Encoding
Why should the character “X" and "z" take
up the same number of bits as "e" or " "?
Huffman codes use variable-length bit
strings to represent each character.
More frequently used letters have shorter
strings to represent them.
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Huffman Encoding Example
ballboard would be
1010001001001010110001111011
compression ratio is
28/72 or 0.39
as compared to ASCII
Try to encode roadbed
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Huffman Encoding
Prefix Property
No character's bit string is the prefix of any
other character's bit string.
To decode
look for match left to right, bit by bit
record letter when a match is found
begin next character where you left off
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Huffman Encoding Example
Try it!
Decode
1011111001010
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Huffman Encoding
The technique for creating codes guarantees the
prefix property of the codes.
There is no single “Huffman code” -- each depends
on the application. Two types of Huffman codes:
– general, based on use of letters in English, Spanish,
….
– specialized, based on text itself or specific types of
text
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Take-Away Points
– Character Sets, ASCII, Unicode
– Data Compression
– Key encoding
– Run-length encoding
– Huffman encoding
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Student To Dos
–Readings:
 Reed ch 5, pp 83-95
– HW03 (HTML) due Monday 11:59pm.
– HW04 (Alice) due Thursday 11:59pm.
– HW05 (Networking) to be posted.
Practice, Practice, Practice!
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