CSRU 1100 Structures of Computer Science

1. CSRU 1100 Structures of Computer Science Chapter 2 Logic CS1100 1

2. Smullyan’s Island Puzzle • You meet two inhabitants of Smullyan’s Island (where each one is either a liar or a truth-teller). • A says, “Either B is lying or I am” • B says, “A is lying” • Who is telling the truth ? CS1100 2

3. Symbolic logic Numerical Algebra Identify important parts of the numerical problem, assigning them to variables. Use algebraic laws and formulas to solve the problem. Symbolic Logic Identify important parts of a logical argument, assign them to logical variables. Use laws and formulas of symbolic logic to determine whether or not the logical argument is valid. CS1100 3

4. Propositional Logic • A proposition is a statement which is either true or false • For example: • Ten is less than seven. • There are life forms on other planets in the universe. • A set of cardinality n has 2n subsets. • The followings are not propositions: • x2 = 16 • How are you ? • x+y<10 CS1100 4

5. Proposition • Truth value of a proposition is either true or false. • We use logical variables, usually a lower case letter, to represent a proposition • p = “Ten is smaller than seven” • p has truth value of false, i.e., F. CS1100 5

6. Logical Operation : Negation • Negation ¬ applies to a single proposition • If p is true, then ¬ p is false • If p is false, then ¬ p is true Truth Table of Negation Operator CS1100

7. Logical Operation : Negation • Example p = It will rain tomorrow. ¬ p = It will not rain tomorrow. • If it’s true that it will rain tomorrow, it is false that it will not rain tomorrow. • If it’s false that it will rain tomorrow, it is true that it will not rain tomorrow. CS1100 7

8. Logical Conjunction (AND) The conjunction is true if and only if both of two simple propositions are true; otherwise it’s false. We use Λ to connect the two simple propositions: CS1100 8

9. Logical Conjunction Example I am a female andyou are a male. • p = I am a female. • q = You are a male. • p Λ q = I am a female and you are a male. CS1100

10. Logical Disjunction (OR) The conjunction is true whenever one or more of two simple propositions are true; otherwise it’s false. The conjunction is false if and only if both of two simple propositions are false. We use ν to connect the two simple propositions: CS1100 10

11. Logical Disjunction Example The computer is fast or the printer is fast. • p = The computer is fast • q = The printer is fast • p ν q = The computer is fast or the printer is fast. CS1100

12. Inclusive OR vs. Exclusive OR(XOR) Inclusive OR is true when both of two simple compositions are true. Exclusive OR (XOR) is true if and only if two simple propositions are not true or false at the same; otherwise it’s false. We use ⊕ to connect the two simple propositions: CS1100 12

13. Exclusive OR Example You may have coffee or you may have tea. • p = You may have coffee. • q = You may have tea. • p ⊕ q = You may have coffee or you may have tea. CS1100

14. From English to Propositions • English words connecting the propositions might be “but”, “nevertheless”, “unfortunately”, …. For example: • Although the villain is French, the movie is good. • The hero is not American, but the villain is French. • As long as it means that both simple propositions are true, it’s a conjunction, i.e., AND. In proposition forms: CS1100 14

15. Exercises • Introduce letters to stand for simple propositions, and write the following statements as complicated propositions: • Either the food is good or the service is excellent. p v q • He does the homework himself, or his girl friend helps him to do the homework. p ⊕ q CS1100

16. Complicated Proposition • One can connect propositions using “and”, “or”, “not”, “only if” …to form complicated proposition: • It will not rain tomorrow. • Fishes are jumping and the cotton is high. • Truth value of compound proposition depends on the truth value of the simple propositions involved. CS1100 16

17. Complicated Propositions Connectives (¬, Λ,ν, etc.) can be applied to build complicated propositions, e.g.: ¬(p Λ q), (¬p) ν (p Λ q) CS1100 17

18. Order of Evaluation • Parenthesized sub-expressions are evaluated first. • Operations have a precedence hierarchy: • Negations (¬) are done first. • Logical conjunction (Λ) are done next. • Inclusive OR and Exclusive OR(XOR) (ν, ⊕) are done next. • Other operations are done last. • For tie, follow left-to-right order. CS1100

19. Truth Table for Complicated Composition • First fill in all possible input values • For 2 logical variables, p, q, there are 4 possible input values • Next, create a column for each complicated propositions following evaluation order. • Third, fill in the columns one by one, starting from simple ones CS1100 19

20. Exercises • Write the following statement as complicated propositions and build the truth table for it: • Neither the food is good nor the service is excellent. (¬p) Λ (¬q) CS1100

21. Conditional Operation • Complicated proposition • If it rains, then the ground is wet. • Logic implication • If p, then q • If part: hypothesis; Then part: conclusion • If the hypothesis happens, then the conclusion happens. • We use ⇒ to denote conditional operation. p ⇒ q CS1100 21

22. Conditional Operation “If I am elected, then I will lower the taxes next year”. • p: I am elected. • q: I will lower the taxes next year. • When p ⇒ q is T : • If p is true, then q must be true. • If p is false, we can derive anything ! CS1100 22

23. Understand Conditional Operation • Under what conditions, the promise is broken, i.e., the whole statement is false ? • When he is elected, but did not lower the taxes next year! • For all other scenarios, he keep promise, i.e. above statement is true. • He is elected, and lower the taxes next year • He is not elected, the taxes are lowered next year. • He is not elected, the taxes are not lowered next year. CS1100 23

24. p ⇒ q in English • If p, then q. • If “it rains”, then “the ground is wet”. • p only if q. • “It rains” only if “the ground is wet”. • p implies q. • “It rains” implies “the ground is wet”. • p is sufficient for q. • “It rains” is a sufficient condition for “the ground is wet”. • q is necessary for p. • “The ground is wet” is necessary for “It rains”. CS1100 24

25. Exercises: Write following in propositional forms A good diet is a necessary condition for a healthy cat. p⇒ q The central switch goes down is a sufficient condition for network failure. p⇒ q CS1100 25

26. Order of Evaluating Complicated Composition • Parenthesized sub-expressions are evaluated first. • Operations have a precedence hierarchy: • Negations (¬) are done first. • Logical conjunction (Λ) are done next. • Inclusive OR and Exclusive OR(XOR) (ν, ⊕) are done next. • The conditional-type operations (⇒) are done last. • For tie, follow left-to-right order, except for the conditional operator ⇒, which is done in a right-to-left order. That is, p⇒ q ⇒r is interpreted as p ⇒ (q⇒ r) CS1100

27. Exercise • Using letter f, b, s for the simple propositions, translate following into propositional forms: • Plentiful fish is a sufficient condition for bears to be happy. • Bears are happy only if there are plentiful fish. • Unhappy bears is a necessary condition for heavy snow. CS1100 27

28. From English to Propositional Forms • Translate into English: 1. 2. Where A: the villain is French. B: the hero is American. C: the heroine is British D: the movie is good. CS1100

29. Equivalent Propositions • Two propositional forms p and q are logically equivalent, we write p Ξ q. • p is true if and only if q is true. • Two equivalent forms have same truth value no matter what values the variables take. • To demonstrate two propositions are equivalent, show they have same truth-table. CS1100

30. Logic Equivalences (1) • Double Negation • Negation • Idempotent CS1100

31. Logic Equivalences (2) • Commutative • Associative CS1100 31

32. Logic Equivalences (3) • Distributive • DeMorgan’s Laws CS1100 32

33. Smullyan’s Island Puzzle • You meet two inhabitants of Smullyan’s Island (where each one is either a liar or a truth-teller). • A says, “Either B is lying or I am” • B says, “A is lying” • Who is telling the truth ? CS1100 33

34. How to solve such puzzle ? • The key: each inhabitant is either a liar or a truth-teller. A liar always lies; a truth-teller always tells the truth. • Introduce simple propositions: • a: A is a truth-teller, i.e., A is telling the truth • b: B is a truth-teller CS1100 34

35. Express statements as propositional forms • a: A is a truth-teller, i.e., A is telling the truth • b: B is a truth-teller • A says, “Either B is lying or I am” (¬b) ⊕ (¬a) • B says, “A is lying” ¬a CS1100 35

36. Examine all possibilities A is lying. Either B is lying or I am. Which scenarios, if any, are consistent, i.e., truth-tellers told the truth, and liars lied ? * A is truth-teller, B is a liar. CS1100 36

37. Smullyan’s Island Puzzle • You meet one inhabitant of Smullyan’s Island (where each inhabitant is either a liar or a truth-teller). • He says, “I am lying.” • Is he a truth-teller ? CS1100 37