snick  snack CPSC 121: Models of Computation 2011 Winter Term 1 Introduction & Motivation Steve Wolfman, based on notes by Patrice Belleville and others 1

Learning Goals: In-Class By the end of the unit, you should be able to: –Give an example of how we can apply formal reasoning and computers to a simple, real- world task. –Give an example of how a computational solution to a simple task might go wrong. –Describe the two “big stories” of CS121: reasoning about computation and building computers. 2

Outline Introductions Introductions Exercise The CS121 Story Course Administration Next Lecture Notes 3

Introductions Steven Wolfman ICICS 239; office hours listed on the website I also have an open door policy: If my door is open, come in and talk! Also, I will usually be available after class. And, you can make appointments with me. Additionally, you can use TA office hours. 4

Outline Introductions Introductions Exercise The CS121 Story Course Administration Next Lecture Notes 5

More Introductions Introduce yourselves… how? 6

Introduce Yourselves in Groups of 4-ish FIND ~3 people around you, preferably people you’ve never met. Form a group. Have everyone in the group introduce themselves to everyone else in the group. (Count the number of intros this takes.) Tell everyone why you’re here, your favorite career that you’ll never have, and one unusual thing about you. 7

Problem: How Many Introductions? Problem: How many introductions does it take for everyone in a group to meet everyone else in a group? 8

Concept Q: Intros for 4 How many introductions does a group of 4 people take? a.3 b.4 c.6 d.12 e.None of these 9

Problem: How Many Introductions? Problem: How many introductions does it take for everyone in a group to meet everyone else in a group? To solve this problem, we need to model it more formally. 10

How Many Introductions? Model: One “introduction” is one person introducing themselves to another person. (Two people need two introductions to introduce themselves to each other.) A group has “introduced themselves” when every group member has introduced themselves to every other member. (No self-introductions!) Hi, I’m Grace H. Hi Grace, I’m Alan T. Intro #1 Intro #2 11

How Many Introductions? Problem: How many introductions does it take for a group of n people to introduce themselves? 12

How Many Introductions? For 2 people? For 3 people? For 4 people? For 5 people? … For n people? 13

How Many Introductions? For 100 people? For people? For people? 14

Program for Introductions int introductions(int n) { return n * (n - 1); } (define (introductions n) (* n (- n 1))) Do you believe it? 15 (in Java) (in Scheme/Racket)

Program for Introductions: Testing Java version with 100: 9900 Do you believe it? 16

Program for Introductions: Testing Java version with 100: 9900 Java version with : Do you believe it? 17

Program for Introductions: Testing Java version with 100: 9900 Java version with : Java version with : Um.. Do you believe it? Does this fit with your “model” of computation? 18

Program for Introductions: Testing Racket version with 100: 9900 Racket version with : Racket version with : Do you believe it? Will Racket always get the right answer? 19

Outline Introductions Introductions Exercise The CS121 Story Course Administration Next Lecture Notes 20

Questions that CPSC121 Answers How can we prove that our formula for the number of introductions is correct? (induction) What went wrong in our Java implementation? (number representation) How do we model and reason about computational systems at various levels of abstraction? (propositional and predicate logic, proof, sets, functions, DFAs, relations,...) 21

CPSC 121: The Big Stories Theory How do we model computational systems (programs/computers) in order to design and analyze them? Grand goal: Reason about what is and isn’t possible to compute. Hardware How do we build devices that can compute out of real materials (“sand and rocks and water”)? Grand goal: break down a full computer into simple “gates”. Bonus end goal: Develop tools to communicate computational ideas clearly and precisely. 22

Our Working Computer 23 The whole thing (mostly wires connecting boxes).

Our Working Computer (zoomed in on one “box”) 24 Just the “ALU” (arithmetic and logic unit). You’ll see a pared-down version in lab in a couple of weeks.

CPSC 121: The (By?)Products Theory Products: Propositional logic Predicate logic Sets and functions Proof techniques (especially induction!) Finite Automata/Regular Expressions Universal machines Uncomputability Hardware Products: Gates Combinational circuits Binary data representations Sequential circuits A full computer 25

What is CPSC 121 Good For? With CPSC121’s help, you will be able to: model important problems so that they are easier to discuss, reason about, solve, and test. learn new modeling formalisms more easily. communicate clearly and unambiguously with other CS experts on complex topics. characterize algorithms (CS problem solutions), such as proving their correctness or efficacy. critically read proofs: justifying why each step is correct and judging what the proof means. 26

Outline Introductions Introductions Exercise The CS121 Story Course Administration Next Lecture Notes 27

Course Administration Explore the CPSC 121 website: You are required to be familiar with the course website. Ignorance of information on the website may harm you. 28

Additional Administrative Notes The first quiz is “any marks gives full marks”. So, if you get more than 0%, we’ll count it as 100%. Labs, tutorials, and TA office hours in the Learning Centre all start NEXT week. (My office hours start this week!) 29

Outline Introductions Introductions Exercise The CS121 Story Course Administration Next Lecture Notes 30

Learning Goals: In-Class By the end of the unit, you should be able to: –Give an example of how we can apply formal reasoning and computers to a simple, real- world task. –Give an example of how a computational solution to a simple task might go wrong. –Describe the two “big stories” of CS121: reasoning about computation and building computers. 31

Next Lecture Learning Goals: Pre-Class By the start of class, you should be able to: –Translate back and forth between simple natural language statements and propositional logic. –Evaluate the truth of propositional logic statements using truth tables. –Translate back and forth between propositional logic statements and circuits that assess the truth of those statements. 32

Next Lecture Prerequisites Read Sections 1.1 and 1.4/2.1 and 2.4 You should have completed the open-book, untimed quiz on Vista that’s due by 9PM the day before class. (You are responsible for ensuring that you have submitted the quiz by 9PM!) 33 Readings: 3 rd ed in black/4 th ed in red

snick  snack Some Things to Try... (on your own if you have time, not required) 34

What Works is NOT Always Obvious Let’s try out what we’ll call the “Radix” Algorithm. General form: 1.Put all items together in a list in order 2.Make an empty list for each possible value of each “digit” in the items (or letter in a word, etc.). 3.For each digit of the items in the main list, starting with the rightmost digit: a)For each item in the main list, in order: Put it at the back of the list indicated by the current digit in that item b)For each digit list, in order: Put the list at the back of the main list. What does this do? How does it work? 35

What Doesn’t Work is Not Always Obvious (1 of 2) Class Main { public static void main(String[] args) { // Let's add up 4 quarters. System.out.println("4 quarters gives us:"); System.out.println( ); // Let's do something a hundred times. int i = 100; do { // Make i one smaller. i--; } while (i > 0); System.out.println("Done!"); System.out.println("i ended up with the value: " + i); System.out.println("It went down by: " + (100 - i)); } 36

What Doesn’t Work is Not Always Obvious (2 of 2) Class Main { public static void main(String[] args) { // Let's add up 10 dimes. System.out.println("10 dimes gives us:"); System.out.println( ); // Let's try do something a hundred times.. // but accidentally go forever int i = 100; do { // Make i one LARGER. Oops! i++; } while (i > 0); System.out.println("Done!"); System.out.println("i ended up with the value: " + i); System.out.println("It went down by: " + (100 - i)); } 37

Even Bigger Story: “Clear Thought” Computer Science is the science of “clear thought”, but not like philosophy (or religion, or poetry, or...). CS is the science of thoughts so clear in their expression and intent that we can realize them: execute them or test their truth in the world. CPSC121 provides and applies the fundamentals you need to model the world with “clear thought”. 38