10002 Things Every SelfRespecting Computer Scientist Should Know

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10002 Things Every Self-Respecting Computer Scientist Should Know University of Richmond 10 September 2003

10002 Things Every Self-Respecting Computer Scientist Should Know University of Richmond 10 September 2003 David Evans [email protected] virginia. edu http: //www. cs. virginia. edu/evans/talks/richmond/ 10 November 2003 1

1000 Questions 0000 What is Computer Science? 0001 What problem did the first electronic

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? 0110 Who was the first object-oriented programmer? 0111 Who invented the Internet? 10 November 2003 2

0. What is Computer Science? 10 November 2003 3

0. What is Computer Science? 10 November 2003 3

Let AB and CD be the two given numbers not relatively prime. It is

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 (300 BC) 10 November 2003 4

The note on the inflected line is only difficult to you, because it is

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 10 November 2003 5

What is the difference between Euclid and Ada? “It depends on what your definition

What is the difference between Euclid and Ada? “It depends on what your definition of ‘is’ is. ” Bill Gates (at Microsoft’s antitrust trial) 10 November 2003 6

Geometry vs. Computer Science • Geometry (mathematics) is about declarative knowledge: “what is” •

Geometry vs. Computer Science • Geometry (mathematics) is about declarative knowledge: “what is” • Computer Science is about imperative knowledge: “how to” – Ways of describing imperative Language processes (computations) – Ways of reasoning about (predicting) Logic what imperative processes will do 10 November 2003 7

1. What problem did the first electronic programmable computer solve? 10 November 2003 8

1. What problem did the first electronic programmable computer solve? 10 November 2003 8

Colossus First Programmable Computer • Bletchley Park, 1943 • Designed by Tommy Flowers •

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 1970 s • 2 years before ENIAC – calculating artillery tables 10 November 2003 9

Colossus’ Problem • Decode Nazi high command messages from Lorenz Machine • XOR encoding:

Colossus’ Problem • Decode Nazi high command messages from Lorenz Machine • XOR encoding: Ci = M i K i – Perfect cipher, if K is random and secret 10 November 2003 10

Why perfectly secure? For any given ciphertext, all plaintexts are equally possible. Ciphertext: Key:

Why perfectly secure? For any given ciphertext, all plaintexts are equally possible. Ciphertext: Key: Plaintext: 10 November 2003 0100111110101 1 1100000100110 B 1000111010011 = “CS” 0 11

Breaking Lorenz • Operator and receiver need same keys • Generate key bits using

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 10 November 2003 12

Colossus • Read ciphertext and Lorenz wheel patterns from tapes • Tried each alignment,

Colossus • Read ciphertext and Lorenz wheel patterns from tapes • Tried each alignment, calculated correlation with German • Decoded messages (63 M letters by 10 Colossus machines) that enabled Allies to know German troop locations to plan D-Day 10 November 2003 13

2. Why was the first personal computer built? 10 November 2003 14

2. Why was the first personal computer built? 10 November 2003 14

Apollo Guidance Computer, 1961 -69 1 cubic foot, 70 pounds 4 KB of read/write

Apollo Guidance Computer, 1961 -69 1 cubic foot, 70 pounds 4 KB of read/write magnetic core memory 64 KB of read-only memory Why did they need to fit the guidance computer in the rocket? 10 November 2003 15

AGC History • Needed all guidance to be on board in case Soviets jammed

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 60 s! – Spurred industry growth 10 November 2003 16

3. Science, Engineering or Other? 10 November 2003 17

3. Science, Engineering or Other? 10 November 2003 17

Science? • Understanding Nature through Observation – About real things like bowling balls, black

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 10 November 2003 18

Engineering? “Engineering is design under constraint… Engineering is synthetic - it strives to create

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 10 November 2003 19

Computing Power 1969 -2002 (in Apollo Control Computer Units) Moore’s Law: computing power doubles

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! (1 GB/4 KB = 262144) 10 November 2003 20

Constraints Computer Scientists Face • Not like those for engineers: – Cost, weight, physics,

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 10 November 2003 21

So, what is computer science? • Science – No: its about fake things like

So, what is computer science? • Science – No: its about fake things like numbers, not about observing and understanding nature • Engineering – No: we don’t have to deal with engineeringtype constraints Must be a Liberal Art! 10 November 2003 22

The Liberal Arts e g ua g n la num ber s Trivium (3

The Liberal Arts e g ua g n la num ber s Trivium (3 roads) Quadrivium (4 roads) Grammar Rhetoric Logic Arithmetic Music Geometry 10 November 2003 Astronomy 23

Liberal Arts Quadrivium Trivium Yes, we need to understand • Grammar: study of meaning

Liberal Arts Quadrivium Trivium Yes, we need to understand • Grammar: study of meaning in meaning to describe written expression computations • Rhetoric: comprehension of verbal Interfaces between components, discourse and written discourse between programs and users • Logic: argumentative discourse for Logic for controlling and reasoning about discovering truth computations • Arithmetic: understanding numbers Yes • Geometry: quantification of space Yes (graphics) • Music: number in time Yes (read Gödel, Escher, Bach) • Astronomy: laws of the planets and stars Sort of, read Neil De. Grasse Tyson’s essay 10 November 2003 24

4. What are the world’s most complex programs? 10 November 2003 25

4. What are the world’s most complex programs? 10 November 2003 25

Complex Programs • Apollo Guidance Software – ~36 K instructions • F-22 Steath Fighter

Complex Programs • Apollo Guidance Software – ~36 K instructions • F-22 Steath Fighter Avionics Software – 1. 5 M lines of code (Ada) • 5 EEE (phone switching software) – 18 M lines • Windows XP – ~50 M lines (1 error per kloc ~ 50, 000 bugs) • Anything more complex? 10 November 2003 26

Human Genome Produces 60 Trillion Cells (6 * 1013) 50 Million die every second!

Human Genome Produces 60 Trillion Cells (6 * 1013) 50 Million die every second! 10 November 2003 27

How Big is the Make-a-Human Program? • 3 Billion Base Pairs – Each nucleotide

How Big is the Make-a-Human Program? • 3 Billion Base Pairs – Each nucleotide is 2 bits (4 possibilities) – 3 B bases * 1 byte/4 pairs = 750 MB – Highly redundant encoding (21/64) ~ 250 MB – About 95% of it is junk ~ 12 MB 1 CD ~ 650 MB 10 November 2003 Wal-Mart’s database is 280 Terabytes 28

Expressiveness of DNA • Genetic sequence for 2 humans differs in only 2 million

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 10 November 2003 29

5. What is and is not computable? 10 November 2003 30

5. What is and is not computable? 10 November 2003 30

Halting Problem Input: a procedure P Output: true if P halts (finishes execution), false

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? 10 November 2003 31

Halts is not Computable boolean contradict. Halts (Program P) if (halts “contradict. Halts (P);

Halts is not Computable boolean contradict. Halts (Program P) if (halts “contradict. Halts (P); ”) while (true) ; else return true; If contradict. Halts halts, the if test is true if enters the while loop - it doesn’t halt! If contradict. Halts doesn’t halt, the if test if false, and it evaluates to true. It halts! 10 November 2003 32

Learned Discussion on Computability (Video) 10 November 2003 33

Learned Discussion on Computability (Video) 10 November 2003 33

Ali G Multiplication Problem • Input: a list of n numbers • Output: the

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? 10 November 2003 34

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10 November 2003 35

Ali G was Right! • Theory assumes ideal computers: – Unlimited memory – Unlimited

Ali G was Right! • Theory assumes ideal computers: – 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 10 November 2003 36

6. Who was the first Object-Oriented Programmer? “I invented the term Object-Oriented and I

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 10 November 2003 37

What is an Object? • Packaging state and procedures – State: the representation •

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 10 November 2003 38

Object-Oriented Programming • State of mind where you program by thinking about objects •

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. 10 November 2003 39

Who was the first object-oriented programmer? 10 November 2003 40

Who was the first object-oriented programmer? 10 November 2003 40

By the word operation, we mean any process which alters the mutual relation of

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 10 November 2003 41

7. Who Invented the Internet? 10 November 2003 42

7. Who Invented the Internet? 10 November 2003 42

What is a Network? A group of three or more connected entities communicating indirectly

What is a Network? A group of three or more connected entities communicating indirectly Ancient Greeks had beacon chain networks on Greek island mountaintops 10 November 2003 43

Chappe’s Semaphore Network First Line (Paris to Lille), 1794 10 November 2003 Mobile Semaphore

Chappe’s Semaphore Network First Line (Paris to Lille), 1794 10 November 2003 Mobile Semaphore Telegraph Used in the Crimean War 1853 -1856 44

internetwork A collection of multiple networks connected together, so messages can be transmitted between

internetwork A collection of multiple networks connected together, so messages can be transmitted between nodes on different networks. 10 November 2003 45

The First Internetwork • 1800: Sweden and Denmark worried about Britain invading • Edelcrantz

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 10 November 2003 46

First Use of The Internet • October 1969: First packets on the ARPANet from

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. 10 November 2003 47

Licklider and Taylor’s Vision Available within the network will be functions and services to

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. 10 November 2003 J. C. R. Licklider and Robert W. Taylor, The Computer as a Communication Device, April 1968 48

The Modern Internet • Packet Switching: Leonard Kleinrock (UCLA), Donald Davies and Paul Baran,

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”) 10 November 2003 49

Summary • Computer Science is a real intellectual discipline: not like “Automotive Engineering” or

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: http: //www. cs. virginia. edu/evans/talks/richmond/ 10 November 2003 50

Any Questions? 10 November 2003 51

Any Questions? 10 November 2003 51