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1 DCP 1172 Introduction to Artificial Intelligence Lecture notes for Chap. 7 & 10 [AIMA] Chang-Sheng Chen.

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Presentation on theme: "1 DCP 1172 Introduction to Artificial Intelligence Lecture notes for Chap. 7 & 10 [AIMA] Chang-Sheng Chen."— Presentation transcript:

1 1 DCP 1172 Introduction to Artificial Intelligence Lecture notes for Chap. 7 & 10 [AIMA] Chang-Sheng Chen

2 DCP 1172, Ch. 7 2 Knowledge and reasoning – second part Knowledge representation Logic and representation Propositional (Boolean) logic Normal forms Inference in propositional logic Wumpus world example

3 DCP 1172, Ch. 7 3 Review We studied search because it facilitates the creation of agents that can reason about hypothetical (future) states of the world. But … we haven ’ t said much of anything about how those states should be represented. Or about how these future (successor) states can be generated from current states

4 DCP 1172, Ch. 7 4 Knowledge-Based Agents A knowledge-based agent is one that chooses its actions at least in part on the basis of the contents of its knowledge-base. A knowledge-base is simply a repository of things (i.e., domain-specific) you know represented in some useful way. The knowledge base cannot be a simple table. It has to be set up so that an agent can conclude facts about the world that are not already represented in the knowledge base. In other words, it has to reason about unseen worlds

5 DCP 1172, Ch. 7 5 Knowledge-Based Agent Agent that uses prior or acquired knowledge to achieve its goals Can make more efficient decisions Can make informed decisions Knowledge Base (KB): contains a set of representations of facts about the Agent’s environment Each representation is called a sentence Use some knowledge representation language (KRL), to TELL it what to know e.g., (temperature 72F) ASK agent to query what to do Agent can use inference to deduce new facts from TELLed facts Knowledge Base Inference engine Domain independent algorithms Domain specific content TELL ASK

6 DCP 1172, Ch. 7 6 Generic knowledge-based agent 1.TELL KB what was perceived Uses a Knowledge Representation Language (KRL) to insert new sentences, representations of facts, into KB 2.ASK KB what to do. Uses logical reasoning to examine actions and select best.

7 DCP 1172, Ch. 7 7 Knowledge Representation A knowledge representation is a formal scheme that dictates how an agent is going to represent its knowledge. Syntax ( 語法 ): Rules that determine the possible strings in the language. Semantics( 語意 ): Rules that determine a mapping from sentences in the representation to situations in the world.

8 DCP 1172, Ch. 7 8 Reasoning

9 DCP 1172, Ch. 7 9 Wumpus world example

10 DCP 1172, Ch. 7 10 Wumpus world characterization Deterministic?Yes – outcome exactly specified. Fully Observable?No – only local perception. Static?Yes – Wumpus and pits do not move. Discrete?Yes Episodic?(Yes) – because static.

11 DCP 1172, Ch. 7 11 Exploring a Wumpus world A= Agent B= Breeze S= Smell P= Pit W= Wumpus OK = Safe V = Visited G = Glitter

12 DCP 1172, Ch. 7 12 Exploring a Wumpus world A= Agent B= Breeze S= Smell P= Pit W= Wumpus OK = Safe V = Visited G = Glitter

13 DCP 1172, Ch. 7 13 Exploring a Wumpus world A= Agent B= Breeze S= Smell P= Pit W= Wumpus OK = Safe V = Visited G = Glitter

14 DCP 1172, Ch. 7 14 Exploring a Wumpus world A= Agent B= Breeze S= Smell P= Pit W= Wumpus OK = Safe V = Visited G = Glitter

15 DCP 1172, Ch. 7 15 Exploring a Wumpus world A= Agent B= Breeze S= Smell P= Pit W= Wumpus OK = Safe V = Visited G = Glitter

16 DCP 1172, Ch. 7 16 Exploring a Wumpus world A= Agent B= Breeze S= Smell P= Pit W= Wumpus OK = Safe V = Visited G = Glitter

17 DCP 1172, Ch. 7 17 Exploring a Wumpus world A= Agent B= Breeze S= Smell P= Pit W= Wumpus OK = Safe V = Visited G = Glitter

18 DCP 1172, Ch. 7 18 Exploring a Wumpus world A= Agent B= Breeze S= Smell P= Pit W= Wumpus OK = Safe V = Visited G = Glitter

19 DCP 1172, Ch. 7 19 Other tight spots

20 DCP 1172, Ch. 7 20 Another example solution No perception  1,2 and 2,1 OK Move to 2,1 B in 2,1  2,2 or 3,1 P? 1,1 V  no P in 1,1 Move to 1,2 (only option)

21 DCP 1172, Ch. 7 21 Example solution S and No S when in 2,1  1,3 or 1,2 has W 1,2 OK  1,3 W No B in 1,2  2,2 OK & 3,1 P

22 DCP 1172, Ch. 7 22 Representation and Mappings Two different kinds of entities are usually mentioned in the discussions about AI programs: Facts: truths in some relevant world (e.g., including each agent’s behavior and goals, etc.). Representation of facts in some chosen formalism. These are the things that we will actually be able to manipulate.

23 DCP 1172, Ch. 7 23 Mapping between Facts and Representations Facts Internal Representations Natural Language Representation (e.g., English, Chinese, etc.) Reasoning Programs * * Natural Language generation Natural Language understanding

24 DCP 1172, Ch. 7 24 Representation of Facts Initial facts Final facts Internal representation of initial facts Internal representation of final facts Desired real reasoning Operation of program * Forward representation mapping * Backward representation mapping

25 DCP 1172, Ch. 7 25 Logic in general

26 DCP 1172, Ch. 7 26 Types of logic

27 DCP 1172, Ch. 7 27 The Semantic Wall Physical Symbol System World +BLOCKA+ +BLOCKB+ +BLOCKC+ P 1 :(IS_ON +BLOCKA+ +BLOCKB+) P 2 :((IS_RED +BLOCKA+)

28 DCP 1172, Ch. 7 28 Truth Depends on Interpretation (e.g., Anti-spam or anti-virus mail filtering ) Filteringwith H 1 (msg) FilteringWith H 2 (msg) Mail Spool Discard MTA 1 (or MUA 1 ) Accept MTA 0 MTA = Mail Transfer Agent MUA = Mail User Agent MTA 2 (or MUA 1 )

29 DCP 1172, Ch. 7 29 Entailment Entailment is different than inference

30 DCP 1172, Ch. 7 30 Reasoning - Logic as a representation of the World

31 DCP 1172, Ch. 7 31 Models

32 DCP 1172, Ch. 7 32 Inference

33 DCP 1172, Ch. 7 33 Basic symbols Expressions only evaluate to either “true” or “false.” P“P is true” ¬P“P is false”negation P V Q“either P is true or Q is true or both”disjunction P ^ Q“both P and Q are true”conjunction P => Q“if P is true, the Q is true”implication P  Q“P and Q are either both true or both false” equivalence

34 DCP 1172, Ch. 7 34 Propositional logic: syntax

35 DCP 1172, Ch. 7 35 Propositional logic: semantics

36 DCP 1172, Ch. 7 36 Truth tables Truth value: whether a statement is true or false. Truth table: complete list of truth values for a statement given all possible values of the individual atomic expressions. Example: PQP V Q TTT TFT FTT FFF

37 DCP 1172, Ch. 7 37 Truth tables for basic connectives PQ¬P¬QP V QP ^ QP=>QP  Q TTFFTTTT TFFTTFFF FTTFTFTF FFTTFFTT

38 DCP 1172, Ch. 7 38 Propositional logic: basic manipulation rules ¬(¬A) = ADouble negation ¬(A ^ B) = (¬A) V (¬B)Negated “and” ¬(A V B) = (¬A) ^ (¬B)Negated “or” A ^ (B V C) = (A ^ B) V (A ^ C)Distributivity of ^ on V A => B = (¬A) V Bby definition ¬(A => B) = A ^ (¬B)using negated or A  B = (A => B) ^ (B => A)by definition ¬(A  B) = (A ^ (¬B))V(B ^ (¬A))using negated and & or …

39 DCP 1172, Ch. 7 39 Propositional inference: enumeration method

40 DCP 1172, Ch. 7 40 Enumeration: Solution

41 DCP 1172, Ch. 7 41 Propositional inference: normal forms “sum of products of simple variables or negated simple variables” “product of sums of simple variables or negated simple variables”

42 DCP 1172, Ch. 7 42 Deriving expressions from functions Given a boolean function in truth table form, find a propositional logic expression for it that uses only V, ^ and ¬. Idea: We can easily do it by disjoining the “T” rows of the truth table. Example: XOR function PQRESULT TTF TFTP ^ (¬Q) FTT(¬P) ^ Q FFF RESULT = (P ^ (¬Q)) V ((¬P) ^ Q)

43 DCP 1172, Ch. 7 43 A more formal approach To construct a logical expression in disjunctive normal form from a truth table: -Build a “minterm” for each row of the table, where: - For each variable whose value is T in that row, include the variable in the minterm - For each variable whose value is F in that row, include the negation of the variable in the minterm - Link variables in minterm by conjunctions -The expression consists of the disjunction of all minterms.

44 DCP 1172, Ch. 7 44 Example: adder with carry Takes 3 variables in: x, y and ci (carry-in); yields 2 results: sum (s) and carry- out (co). To get you used to other notations, here we assume T = 1, F = 0, V = OR, ^ = AND, ¬ = NOT. co is: s is:

45 DCP 1172, Ch. 7 45 Tautologies Logical expressions that are always true. Can be simplified out. Examples: T T V A A V (¬A) ¬(A ^ (¬A)) A  A ((P V Q)  P) V (¬P ^ Q) (P  Q) => (P => Q)

46 DCP 1172, Ch. 7 46 Validity and satisfiability Theorem

47 DCP 1172, Ch. 7 47 Proof methods

48 DCP 1172, Ch. 7 48 Inference Rules

49 DCP 1172, Ch. 7 49 Inference Rules

50 DCP 1172, Ch. 7 50 Wumpus world: example Facts: Percepts inject (TELL) facts into the KB [stench at 1,1 and 2,1]  S1,1 ; S2,1 Rules: if square has no stench then neither the square or adjacent square contain the wumpus R1: !S1,1  !W1,1  !W1,2  !W2,1 R2: !S2,1  !W1,1  !W2,1  !W2,2  !W3,1 … Inference: KB contains !S1,1 then using Modus Ponens we infer !W1,1  !W1,2  !W2,1 Using And-Elimination we get: !W1,1 !W1,2 !W2,1 …

51 DCP 1172, Ch. 7 51 Limitations of Propositional Logic 1. It is too weak, i.e., has very limited expressiveness: Each rule has to be represented for each situation: e.g., “don’t go forward if the wumpus is in front of you” takes 64 rules 2. It cannot keep track of changes: If one needs to track changes, e.g., where the agent has been before then we need a timed-version of each rule. To track 100 steps we’ll then need 6400 rules for the previous example. Its hard to write and maintain such a huge rule-base Inference becomes intractable

52 DCP 1172, Ch. 7 52 Summary

53 DCP 1172, Ch. 7 53 Next time First-order logic:[AIMA] Chapter 7

54 DCP 1172, Ch. 7 54 Knowledge Representation A knowledge representation is a formal scheme that dictates how an agent is going to represent its knowledge. Syntax: Rules that determine the possible strings in the language. Semantics: Rules that determine a mapping from sentences in the representation to situations in the world.

55 DCP 1172, Ch. 7 55 Reasoning The knowledge base can ’ t be a simple table. It has to be set up so that an agent can conclude facts about the world that are not already represented in the knowledge base. In other words, it has to reason about unseen worlds

56 DCP 1172, Ch. 7 56 Reasoning

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