We think you have liked this presentation. If you wish to download it, please recommend it to your friends in any social system. Share buttons are a little bit lower. Thank you!
Presentation is loading. Please wait.
Published byLena Matson
Modified about 1 year ago
ICS 253: Discrete Structures I Dr. Nasir Al-Darwish Computer Science Department King Fahd University of Petroleum and Minerals Spring Semester 2014 (2013-2) Propositional Logic Section 1.1
KFUPM: Dr. Al-Darwish © Grading Lecture attendance: -1% per 2 unexcused absences Assignments & Quizzes: 20% Two Major Exams: 25% per exam Final Exam: 30%
KFUPM: Dr. Al-Darwish © Expectations This is really a fun course! The course covers some of the most useful math you’ll ever learn. It teaches abstraction, describe (or model) a given problem using precise formal notation Hints for success Read the textbook. Lectures really do help! Do the homework.
KFUPM: Dr. Al-Darwish © Propositional Logic Traditionally, logic distinguishes valid and invalid arguments (2-valued logic). A claim is either true or false. There are other types of logic, e.g. fuzzy logic The building blocks of logic are propositions. A proposition (claim) is a declarative statement that is either true or false.
KFUPM: Dr. Al-Darwish © What’s a proposition? PropositionsNot Propositions = 32Bring me coffee! Math is my favorite subject Every cow has 4 legs.Do you like Cake? There is other life in the universe. I will go fishing tomorrow. X – 2 = 10 (depends on X) A proposition is a declarative statement that’s either TRUE or FALSE (but not both, not conditional). Propositions versus Not Propositions
KFUPM: Dr. Al-Darwish © Atomic versus Compound Propositions Logical OperationSymbol Negation (NOT) Conjunction (AND) Disjunction (OR) Exclusive OR Implication (imply) Biconditional (2-way imply) A proposition is either atomic (simple) or compound A compound proposition is formed by combining simple propositions using logical operators (operations)
KFUPM: Dr. Al-Darwish © Logical Operations Let p & q be propositions, then the following are compound propositions: Negation: p = not p Conjunction: p q = p AND q Disjunction: p q= p OR q Exclusive OR: p q= p XOR q Implication: p q= if p then q Biconditional: p q= p if and only if q
KFUPM: Dr. Al-Darwish © Precedence of Logical Operators In a parenthesized expression, the inner most parenthesized part is computed first Binary operators with same precedence are applied left-to-right; unary are applied right-to-left The following is the order of operators by precedence , , v, , This means for example (assuming parenthesis are not present) (((p ( q)) v p v q) q) ((p ( q)) v p)
KFUPM: Dr. Al-Darwish © Truth Tables A truth table shows the truth values of a compound proposition in relation to the truth values of its constituent propositions Normally, the truth table will have as many rows as the different possible truth value of its constituent propositions. For example, a compound proposition involving two simple propositions P, Q will have 4 rows (4 different T/F combinations for P and Q).
KFUPM: Dr. Al-Darwish © Propositional Logic - negation Suppose p is a proposition. The negation of p is written p and has meaning: “It is not the case that p.” Ex. P = Pizza is my favorite food. P = It is not the case that pizza is my favorite food. More simply, pizza is NOT my favorite food. Truth table for negation p pp FTFT TFTF Note that p is a proposition!
KFUPM: Dr. Al-Darwish © Propositional Logic - conjunction Conjunction corresponds to English “and” p q is true exactly when p is true and q is true (or both are true). Ex. Amy is curious AND clever. Truth table for conjunction pqp q FFTTFFTT FTFTFTFT FFFTFFFT
KFUPM: Dr. Al-Darwish © Propositional Logic - disjunction Disjunction corresponds to English “or” p q is true when p is true or q is true (or both are true). Ex. Michael is brave OR nuts. Truth table for disjunction pqp q FFTTFFTT FTFTFTFT FTTTFTTT
KFUPM: Dr. Al-Darwish © The implication The implication p q corresponds to English: “if p then q” or “p implies q” P Q can be considered as a contract that relates a condition P to a conclusion Q P = build me a house, Q = pay you $1 million If you build me a house then I will pay you $1 million The contract is violated if the house is built but the payment is not made p q is false only in the case where p is true and q is false If p then 2+2=4. (This is true. Why?) Truth table for implication pqp q FFTTFFTT FTFTFTFT TTFTTTFT
KFUPM: Dr. Al-Darwish © Propositional Logic - logical equivalence How many different logical binary operations could we define? To answer, we need the notion of “logical equivalence.” 16 How many different logical operations do we need? ?? p is logically equivalent to q if their truth tables are the same. Logical equivalence is denoted by p q (or p q).
KFUPM: Dr. Al-Darwish © Propositional Logic - logical equivalence Challenge: Try to find a proposition that is equivalent to p q, but that uses only the connectives , , and . pqp q FFTTFFTT FTFTFTFT TTFTTTFT pq p p q FFTTFFTT FTFTFTFT TTFFTTFF TTFTTTFT
KFUPM: Dr. Al-Darwish © Propositional Logic – a proof of one famous Distributivity: p (q r) (p q) (p r) pqr q rq rp (q r)p qp qp rp r(p q) (p r) FFFFFFFF FFTFFFTF FTFFFTFF FTTTTTTT TFFFTTTT TFTFTTTT TTFFTTTT TTTTTTTT All truth assignments for p, q, and r. This is a “proof” of “law of distributivity.”
KFUPM: Dr. Al-Darwish © Expressing Implications The implication p q essentially says that we cannot have p true yet q is false, (p q). Thus, p q (p q) (1). (The equivalence can be shown by truth table) Simplifying the RHS (use DeMorgan’s law), gives p q p q (2) It is important to observe that p q also means (because we cannot have p and not q): not q not p It is easy to show that p q q p (3) This last equivalence is known as the ContraPositive (negative of positive). I liken it to “the cub is half full the cub is half empty”
KFUPM: Dr. Al-Darwish © Expressing Implications – cont. Definition: p is sufficient for q if whenever p is true, q is true. Thus, p q means that p is sufficient for q. Definition: p is necessary for q if whenever p is false, q is false. Thus, p q means that q is necessary for q. (recall from previous slide that p q q p)
KFUPM: Dr. Al-Darwish © Expressing Implications – cont. The following are some of the ways of expressing p q p implies q if p then q if p, q q if p (stating the conclusion first) p only if q* (see next two slides for justification) p is sufficient for q q is necessary for p q whenever (when) p q follows from p q unless ¬p* ** These seem confusing
KFUPM: Dr. Al-Darwish © Expressing Implications – cont. p q can be expressed as: p only if q: p cannot be true if q is not true This corresponds to the contrapositive of p q q unless p If p is false, then q must be true Example: If You fail the final exam then you will get F You will get F unless you do not fail the final exam
KFUPM: Dr. Al-Darwish © Propositional Logic - biconditional The biconditional proposition p ↔ q (read as, p if and only if q) states that p and q are always equal (either they are both true or both false) The biconditional proposition p ↔ q is equivalent to “(p q) and (q p)” Because in the expression “p if and only if q”, “p if q” is q p, it follows that p q is “p only if q” Question: Is p ↔ q (p q) ( p q)? pqp ↔ q FFTTFFTT FTFTFTFT TFFTTFFT Truth table for biconditional
KFUPM: Dr. Al-Darwish © Propositional Logic - some definitions Contrapositives: p q and q p Ex. “If it is noon, then I am hungry.” “If I am not hungry, then it is not noon.” Converses: p q and q p Ex. “If it is noon, then I am hungry.” “If I am hungry, then it is noon.” Inverses: p q and p q Ex. “If it is noon, then I am hungry.” “If it is not noon, then I am not hungry.” Note: p q q p
KFUPM: Dr. Al-Darwish © Propositional Logic - more definitions… A tautology is a proposition that’s always TRUE. A contradiction is a proposition that’s always FALSE. p ppp pp pp pp p TFTF FTTF
KFUPM: Dr. Al-Darwish © Translating English Sentences English (and every other human language) is often ambiguous. Translating sentences into compound propositions removes the ambiguity. Example: How can this English sentence be translated into a logical expression? “You can access the Internet from campus only if you are a computer science major or you are not a freshman.” Solution: Let a, c, and f represent “You can access the Internet from campus”, “You are a computer science major”, and “You are a freshman” The sentence can be translated into: a → (c ν ¬f).
KFUPM: Dr. Al-Darwish © Example: How can this English sentence be translated into a logical expression? “You cannot enter into the pool if you are under 4 feet tall unless you are older than 16 years old.” Solution: Let p, r, and s represent “You can enter into the pool”, “You are under 4 feet tall”, and “You are older than 16 years old” The sentence can be translated into: (r Λ ¬ s) → ¬p. Translating English Sentences
KFUPM: Dr. Al-Darwish © Propositional Logic – why?… We’re primarily using propositional logic as a foundation for formal proofs. Later we will study different proof methods and these are needed for developing (or ensuring correctness) of algorithms. Propositional logic is a key to writing correct program code…you can’t do any kind of conditional (if) statement without understanding the condition you’re testing. The logical operations we’ve discussed are also found in hardware and are called “logic gates.”
Chapter 1: The Foundations: Logic and Proofs Discrete Mathematics and Its Applications CSE 211 Department of Computer Science and Engineering, CUET 1.
University of Aberdeen, Computing Science CS2013 Mathematics for Computing Science Adam Wyner Slides adapted from Michael P. Frank ’ s course based on.
Artificial Intelligence Dr. Eng. Ahmed Moustafa Elmahalawy Computer Science and Engineering Department.
Exercises for CS1512 Weeks 7 and 8 Propositional Logic 1 (questions + solutions)
Propositional Logic Lecture 10: Oct 29. Familiar? Obvious? c b a Pythagorean theorem.
Introduction to Proofs A proof is a valid argument that establishes the truth of a statement. Previous section discussed formal proofs Informal proofs.
2012: J Paul GibsonTSP: Mathematical FoundationsMAT7003/Logic.1 MAT 7003 : Mathematical Foundations (for Software Engineering) J Paul Gibson, A207
Logic The study of correct reasoning. Propositions A proposition is a statement that is either true or false Examples Today is Monday All humans respire.
Logic Dr.A.Kannan Professor Dept of CSE Anna University Chennai-25.
Reason and Argument Chapter 6 (1/4). Common misperceptions about logic: …The science of Deduction and Analysis is one which can only be acquired by long.
Logic & Critical Reasoning Translation into Propositional Logic.
Formal Logic Mathematical Structures for Computer Science Chapter 1 Copyright © 2006 W.H. Freeman & Co.MSCS SlidesFormal Logic.
An ISO 9001:2008 Certified Organization PCTI Group Artificial Intelligence.
The Game of Algebra or The Other Side of Arithmetic The Game of Algebra or The Other Side of Arithmetic © 2007 Herbert I. Gross by Herbert I. Gross & Richard.
Determine if each statement is true or false. 1. The measure of an obtuse angle is less than 90°. 2. All perfect-square numbers are positive. 3. Every.
Ch 3.2: Solutions of Linear Homogeneous Equations; Wronskian Let p, q be continuous functions on an interval I = ( , ), which could be infinite. For.
Introduction to Logic and Prolog Sabu Francis, B.Arch (Hons)
© 2002 Franz J. Kurfess Logic and Reasoning 1 CPE/CSC 481: Knowledge-Based Systems Dr. Franz J. Kurfess Computer Science Department Cal Poly.
Types and Programming Languages Lecture 1 Simon Gay Department of Computing Science University of Glasgow 2006/07.
FeatureLesson Geometry Lesson Main 1. Complete the sentence: To find the centroid of a triangle, you need to draw at least ? median(s). 2. FGH has vertices.
Predicate Logic Colin Campbell. A Formal Language Predicate Logic provides a way to formalize natural language so that ambiguity is removed. Mathematical.
1 Sets CS 202, Spring 2007 Epp, chapter 5 Aaron Bloomfield.
Completeness and Expressiveness. Consistency: a syntactic definition, related to the proof system. A set of formulas is consistent if there is no formula.
CS1502 Formal Methods in Computer Science Lecture Notes 3 Consequence Rules Boolean Connectives.
With examples from Number Theory (Rosen 1.5, 3.1, sections on methods of proving theorems and fallacies)
Cognitive Computing 2012 The computer and the mind KNOWLEDGE REPRESENTATION (1) Mark Bishop.
Page 14 When Can We Plug It in and When We Cant? Are there any cases we cant just plug it in? Yes and No. Yessometimes the simply-plug-it-in method will.
Logic Day Two The Biconditional is a compound statement that combines 2 conditionals and the connector and (p q) (q p). That is (p implies q) and (q.
CS1022 Computer Programming & Principles Lecture 9.1 Boolean Algebra (1)
© 2016 SlidePlayer.com Inc. All rights reserved.