1. 1000 Things Every Self-Respecting Computer Scientist Should Know2
University of Richmond
10 September 2003
David Evans
10 November 2003

2. 1000 Questions 0000 What is Computer Science?
0001 What problem did the first electronic programmable computer solve?
0010 Why was the first “personal computer” built?
0011 Is Computer Science a science, engineering or other?
0100 What are the world’s most complex programs?
0101 What is and is not computable?
Who was the first object-oriented programmer?
0111 Who invented the Internet?
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3. 0. What is Computer Science?
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4. Let AB and CD be the two given numbers not relatively prime
Let AB and CD be the two given numbers not relatively prime. It is required to find the greatest common measure of AB and CD.
If now CD measures AB, since it also measures itself, then CD is a common measure of CD and AB. And it is manifest that it is also the greatest, for no greater number than CD measures CD.
Euclid’s Elements, Book VII, Proposition 2 (300BC)
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5. The note on the inflected line is only difficult to you, because it is so easy. There is in fact nothing in it, but you think there must be some grand mystery hidden under that word inflected!
Whenever from any point without a given line, you draw a long to any point in the given line, you have inflected a line upon a given line.
Ada Byron (age 19), letter to Annabella Acheson (explaining Euclid), 1834
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6. What is the difference between Euclid and Ada?
“It depends on what your definition of ‘is’ is.”
Bill Gates (at Microsoft’s anti- trust trial)
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7. Geometry vs. Computer Science
Geometry (mathematics) is about declarative knowledge: “what is”
Computer Science is about imperative knowledge: “how to”
Ways of describing imperative processes (computations)
Ways of reasoning about (predicting) what imperative processes will do
Language
Logic
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8. 1. What problem did the first electronic programmable computer solve?
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9. Colossus First Programmable Computer
Bletchley Park, 1943
Designed by Tommy Flowers
10 Colossi in operation at end of WWII
Destroyed in 1960, kept secret until 1970s
2 years before ENIAC – calculating artillery tables
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10. Colossus’ Problem
Decode Nazi high command messages from Lorenz Machine
XOR encoding:
Ci = Mi  Ki
Perfect cipher, if K is random and secret
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11. Why perfectly secure? Key: 1100000100110
For any given ciphertext, all plaintexts are equally possible.
Ciphertext:
Key:
Plaintext: = “CS” 1 B
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12. Breaking Lorenz Operator and receiver need same keys
Generate key bits using rotor machine, start with same configuration
One operator retransmitted a message (but abbreviated message header the second time!)
Enough for Bletchley Park to figure out key – and structure of machine that generated it!
But still had to try all configurations
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13. Colossus Read ciphertext and Lorenz wheel patterns from tapes
Tried each alignment, calculated correlation with German
Decoded messages (63M letters by 10 Colossus machines) that enabled Allies to know German troop locations to plan D-Day
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14. 2. Why was the first personal computer built?
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15. Apollo Guidance Computer, 1961-69
1 cubic foot, 70 pounds
4KB of read/write magnetic core memory
64KB of read-only memory
Why did they need to fit the guidance computer in the rocket?
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16. AGC History
Needed all guidance to be on board in case Soviets jammed signals for Earth
Design began in 1961
Risky decision to use Integrated Circuits (invented in 1958)
Building 4 prototypes used 60% of all ICs produced in the US in the early 60s!
Spurred industry growth
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17. 3. Science, Engineering or Other?
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18. Science? Understanding Nature through Observation
About real things like bowling balls, black holes, antimatter, electrons, comets, etc.
Math and Computer Science are about fake things like numbers, graphs, functions, lists, etc.
Computer Science is a useful tool for doing real science, but not a real science
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19. Engineering?
“Engineering is design under constraint… Engineering is synthetic - it strives to create what can be, but it is constrained by nature, by cost, by concerns of safety, reliability, environmental impact, manufacturability, maintainability and many other such 'ilities.' ...”
William Wulf
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20. Computing Power 1969-2002 (in Apollo Control Computer Units)
Moore’s Law: computing power doubles every 18 months!
If Apollo Guidance Computer power is 1 inch, you have 5 miles!
(1GB/4KB = )
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21. Constraints Computer Scientists Face
Not like those for engineers:
Cost, weight, physics, etc.
If 8 Million times what NASA had in 1969 isn’t enough for you, wait until 2007 and you will have 32 Million times…
More like those for Musicians and Poets:
Imagination and Creativity
Complexity of what we can understand
Cost of human effort
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22. So, what is computer science?
No: its about fake things like numbers, not about observing and understanding nature
Engineering
No: we don’t have to deal with engineering-type constraints
Must be a Liberal Art!
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23. The Liberal Arts Quadrivium (4 roads) Trivium (3 roads) language
numbers
Trivium (3 roads)
Quadrivium (4 roads)
Grammar
Rhetoric
Logic
Arithmetic
Music
Geometry
Astronomy
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24. Liberal Arts Trivium Quadrivium
Grammar: study of meaning in written expression
Rhetoric: comprehension of verbal and written discourse
Logic: argumentative discourse for discovering truth
Arithmetic: understanding numbers
Geometry: quantification of space
Music: number in time
Astronomy: laws of the planets and stars
Yes, we need to understand
meaning to describe
computations
Trivium
Interfaces between
components, discourse
between programs and users
Logic for controlling
and reasoning about
computations
Yes
Yes (graphics)
Quadrivium
Yes (read Gödel,
Escher, Bach)
Sort of, read Neil DeGrasse Tyson’s essay
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25. 4. What are the world’s most complex programs?
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26. Complex Programs Apollo Guidance Software
~36K instructions
F-22 Steath Fighter Avionics Software
1.5M lines of code (Ada)
5EEE (phone switching software)
18M lines
Windows XP
~50M lines (1 error per kloc ~ 50,000 bugs)
Anything more complex?
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27. Human Genome Produces 60 Trillion Cells (6 * 1013)
50 Million die every second!
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28. How Big is the Make-a-Human Program?
3 Billion Base Pairs
Each nucleotide is 2 bits (4 possibilities)
3B bases * 1 byte/4 pairs = 750 MB
Highly redundant encoding (21/64) ~ 250 MB
About 95% of it is junk ~ 12 MB
Wal-Mart’s database
is 280 Terabytes
1 CD ~ 650 MB
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29. Expressiveness of DNA
Genetic sequence for 2 humans differs in only 2 million bases
4 million bits = 0.5 MB
1/3 of a floppy disk
<1% of Windows 2000
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30. 5. What is and is not computable?
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31. Halting Problem Input: a procedure P
Output: true if P halts (finishes execution), false otherwise.
Is it possible it implement a procedure that correctly implements halts and always terminates?
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32. Halts is not Computable
boolean contradictHalts (Program P)
if (halts “contradictHalts (P);”)
while (true) ;
else
return true;
If contradictHalts halts, the if test is true if enters the while loop - it doesn’t halt!
If contradictHalts doesn’t halt, the if test if false,
and it evaluates to true. It halts!
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33. Learned Discussion on Computability (Video)
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34. Ali G Multiplication Problem
Input: a list of n numbers
Output: the product of all the numbers
Is it computable?
Yes – a straightforward algorithm
solves it.
Can real computers solve it?
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35. 10 November 2003

36. Ali G was Right! Theory assumes ideal computers: Real computers have:
Unlimited memory
Unlimited power
Unlimited (finite) time
Real computers have:
Limited memory, time, power outages, flaky programming languages, etc.
There are many decidable problems we cannot solve with real computer: the numbers do matter
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37. 6. Who was the first Object-Oriented Programmer?
“I invented the term
Object-Oriented and I can tell you I did not have C++ in mind.”
Alan Kay
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38. What is an Object? Packaging state and procedures
State: the representation
What a thing is
Procedures: methods and constructors
What you can do with it
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39. Object-Oriented Programming
State of mind where you program by thinking about objects
It is difficult to reach that state of mind if your language doesn’t have:
Mechanisms for packaging state and procedures (Java has class)
Subtyping (Java has extends and implements)
Other things can help: dynamic dispatch, implementation inheritance, automatic memory management, Krispy Kremes, etc.
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40. Who was the first object-oriented programmer?
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41. By the word operation, we mean any process which alters the mutual relation of two or more things, be this relation of what kind it may. This is the most general definition, and would include all subjects in the universe. Again, it might act upon other things besides number, were objects found whose mutual fundamental relations could be expressed by those of the abstract science of operations…Supposing, for instance, that the fundamental relations of pitched sounds in the science of harmony and of musical composition were susceptible of such expression and adaptations, the engine might compose elaborate and scientific pieces of music of any degree of complexity or extent.
Ada Augusta Byron, 1842
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42. 7. Who Invented the Internet?
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43. What is a Network?
A group of three or more connected entities communicating indirectly
Ancient Greeks had beacon chain
networks on Greek island mountaintops
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44. Chappe’s Semaphore Network
First Line (Paris to Lille), 1794
Mobile Semaphore Telegraph Used in the Crimean War
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45. internetwork
A collection of multiple networks connected together, so messages can be transmitted between nodes on different networks.
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46. The First Internetwork
1800: Sweden and Denmark worried about Britain invading
Edelcrantz proposes link across strait separating Sweden and Denmark to connect their (signaling) telegraph networks
1801: British attack Copenhagen, transmit message to Sweden, but they don’t help.
Denmark signs treaty with Britain, and stops communications with Sweden
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47. First Use of The Internet
October 1969: First packets on the ARPANet from UCLA to Stanford. Starts to send "LOGIN", but it crashes on the G.
20 July 1969:
Live video (b/w) and audio transmitted from moon to Earth, and to several hundred million televisions worldwide.
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48. Licklider and Taylor’s Vision
Available within the network will be functions and services to which you subscribe on a regular basis and others that you call for when you need them. In the former group will be investment guidance, tax counseling, selective dissemination of information in your field of specialization, announcement of cultural, sport, and entertainment events that fit your interests, etc. In the latter group will be dictionaries, encyclopedias, indexes, catalogues, editing programs, teaching programs, testing programs, programming systems, data bases, and – most important – communication, display, and modeling programs. All these will be – at some late date in the history of networking - systematized and coherent; you will be able to get along in one basic language up to the point at which you choose a specialized language for its power or terseness.
J. C. R. Licklider and Robert W. Taylor, The Computer as a Communication Device, April 1968
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49. The Modern Internet
Packet Switching: Leonard Kleinrock (UCLA), Donald Davies and Paul Baran, Edelcrantz’s signaling network (1809)
Internet Protocol: Vint Cerf, Bob Kahn
Vision, Funding: J.C.R. Licklider, Bob Taylor
Government: Al Gore (first politician to promote Internet, 1986; act to connect government networks to form “Interagency Network”)
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50. Summary
Computer Science is a real intellectual discipline: not like “Automotive Engineering” or “Toaster Science”
Seek out and work on problems that are at the intellectual core of computer science
Most of Computer Science was not invented yesterday
For slides and links to more information:
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51. Any Questions?
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