Lecture 23 Everything Else You Should Know but

Lecture 23: Everything Else You Should Know (but won’t see on Exam 2) CS 201 j: Engineering Software University of Virginia 20 November 2003 Computer Science CS 201 J Fall 2003 David Evans http: //www. cs. virginia. edu/evans

Menu • Course Evaluations • 101 Things Every Computer Scientist should know • Course Charge 20 November 2003 CS 201 J Fall 2003 2

Course Pledge You Signed I will provide useful feedback. I realize this is an experimental course and it is important that I let the course staff know what they need to improve the course. I will not wait until the end of the course to make the course staff aware of any problems. I will provide feedback either anonymously (using the course feedback form) or by contacting the course staff directly. I will fill out all course evaluation surveys honestly and thoroughly. 20 November 2003 CS 201 J Fall 2003 3

Two Surveys Required • Official SEAS Survey – You receive email about it – Administrators read it to determine whether or not to fire me • CS 201 J Specific Survey: handed out today – Specific questions to help improve future editions of CS 201 • I do read all of the surveys completely • Department head and curriculum committee will read SEAS survey 20 November 2003 CS 201 J Fall 2003 4

101 Questions 000 What is Computer Science? 001 What problem did the first electronic programmable computer solve? 010 What are the world’s most complex programs? 011 How do Computer Scientists manage complexity? 100 What is and is not computable? 101 Who invented the Internet? 20 November 2003 CS 201 J Fall 2003 5

0. What is Computer Science? 20 November 2003 CS 201 J Fall 2003 6

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) 20 November 2003 CS 201 J Fall 2003 7

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 20 November 2003 CS 201 J Fall 2003 8

What is the difference between Euclid and Ada? “It depends on what your definition of ‘is’ is. ” Bill Gates (at Microsoft’s antitrust trial) 20 November 2003 CS 201 J Fall 2003 9

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 20 November 2003 CS 201 J Fall 2003 10

Was CS 201 J a Computer Science Course? • No! – Only about 30% of CS 201 J is computer science. – Most of it is software engineering. If you want to take a Computer Science course, Consider taking CS 200 in the Spring 20 November 2003 CS 201 J Fall 2003 11

1. What problem did the first electronic programmable computer solve? 20 November 2003 CS 201 J Fall 2003 12

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 20 November 2003 CS 201 J Fall 2003 13

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 20 November 2003 CS 201 J Fall 2003 14

Why perfectly secure? For any given ciphertext, all plaintexts are equally possible. Ciphertext: Key: Plaintext: 20 November 2003 0100111110101 1 1100000100110 B 1000111010011 = “CS” 0 CS 201 J Fall 2003 15

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 20 November 2003 CS 201 J Fall 2003 16

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 20 November 2003 CS 201 J Fall 2003 17

2. What are the world’s most complex programs? 20 November 2003 CS 201 J Fall 2003 18

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? 20 November 2003 CS 201 J Fall 2003 19

Human Genome Produces 60 Trillion Cells (6 * 1013) 50 Million die every second! 20 November 2003 CS 201 J Fall 2003 20

DNA • Sequence of nucleotides: adenine (A), guanine (G), cytosine (C), and thymine (T) • Two strands, A must attach to T and G must attach to C 20 November 2003 CS 201 J Fall 2003 G C T A 21

Codons • Three nucleotides encode an amino acid • Sequence of amino acids makes a protein • But, there are only 20 amino acids, so there may be several different ways to encode the same one From http: //web. mit. edu/esgbio/www/dogma. html 20 November 2003 CS 201 J Fall 2003 22

Central Dogma of Biology (Crick, 1957) Translation Transcription DNA RNA Protein Image from http: //www. umich. edu/~protein/ • DNA makes RNA • RNA makes proteins • Proteins make us 20 November 2003 CS 201 J Fall 2003 23

Shortest (Known) Life Program • Nanoarchaeum equitans – 490, 885 bases (522 genes) = 490, 885 * ¼ * 21/64 = 40, 268 bytes – Parasite: no metabolic capacity, must steal from host – Complete components for information processing: transcription, replication, enzymes for DNA repair http: //www. mediscover. net/Extremophiles. cfm KO Stetter and Dr Rachel Reinhard • Size of compiling C++ “Hello World”: Windows (bcc 32): 112, 640 bytes Linux (g++): 11, 358 bytes 20 November 2003 CS 201 J Fall 2003 24

The Make-Human Program • 3 Billion Base Pairs – Each nucleotide is 2 bits (4 possibilities) – 3 B bases * 1 byte/4 pairs = 750 MB CD = 650 MB – Highly redundant encoding (21/64) ~ 250 MB • Only 5% is transcribed (exons) ~ 12 MB – 95% junk (intons): genomes from viruses reverse transcribed into human genome, but inactive Wal-Mart’s database is 280 Terabytes 20 November 2003 CS 201 J Fall 2003 25

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 20 November 2003 CS 201 J Fall 2003 26

3. How do Computer Scientists manage complexity? 20 November 2003 CS 201 J Fall 2003 27

Abstraction Adapted from Gerard Holzmann’s FSE Slides 20 November 2003 CS 201 J Fall 2003 28

Abstraction in CS 201 J • Abstraction by Specification – Abstract away how details by saying what a procedure does • Data Abstraction – Abstract away representation details by specifying what you can do with something • Subtyping – Abstract away actual type details by allowing many types to be used in the same way (? ) • Concurrency – Abstract away when details (? ) 20 November 2003 CS 201 J Fall 2003 29

4. What is and is not computable? 20 November 2003 CS 201 J Fall 2003 30

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? 20 November 2003 CS 201 J Fall 2003 31

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! 20 November 2003 CS 201 J Fall 2003 32

Learned Discussion on Computability (Video) 20 November 2003 CS 201 J Fall 2003

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? 20 November 2003 CS 201 J Fall 2003 34

20 November 2003 CS 201 J Fall 2003

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 20 November 2003 CS 201 J Fall 2003 36

5. Who Invented the Internet? skip 20 November 2003 CS 201 J Fall 2003 37

What is a Network? A group of three or more connected entities communicating indirectly Ancient Greeks had beacon chain networks on Greek island mountaintops 20 November 2003 CS 201 J Fall 2003 38

Chappe’s Semaphore Network First Line (Paris to Lille), 1794 20 November 2003 Mobile Semaphore Telegraph Used in the Crimean War 1853 -1856 CS 201 J Fall 2003 39

internetwork A collection of multiple networks connected together, so messages can be transmitted between nodes on different networks. 20 November 2003 CS 201 J Fall 2003 40

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 20 November 2003 CS 201 J Fall 2003 41

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. 20 November 2003 CS 201 J Fall 2003 42

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. 20 November 2003 J. C. R. Licklider and Robert W. Taylor, The Computer as a Communication Device, April 1968 CS 201 J Fall 2003 43

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”) 20 November 2003 CS 201 J Fall 2003 44

Charge 1 • Exam 2 – Out today – Due Friday at 3: 55 PM – Turn in to folder outside my office 20 November 2003 CS 201 J Fall 2003 45

Charge 2 • Course Evaluations – SEAS Survey: follow the email instructions – Course Specific survey • Handed out today • Turn in to folder outside my office when you turn in your exam 20 November 2003 CS 201 J Fall 2003 46

Charge 3 Change the World! Caveat: before worrying about changing the world, make sure you turn in your exam Friday! 20 November 2003 CS 201 J Fall 2003 47

Small, Simple Programs that Changed the World e. Bay (P. Omidyar) World. Wide. Web Visi. Calc (Tim Berners-Lee) (Dan Bricklin and Bob Frankston) Smalltalk (Adele Goldberg, Alan Kay, Dan Ignalls) Altair BASIC (Bill Gates and Paul Allen) Napster Tetris (Alexey Pazhitnov) 20 November 2003 (Shawn Fanning and Sean Parker) CS 201 J Fall 2003 ? (CS 201 J Student) 48

Lawn Lighting: 7 pm Thursday Never doubt that a small group of thoughtful, committed people can change the world. Indeed, it is the only thing that ever has. Margaret Mead