Announcements Homework 1 due today write up on

Announcements • • Homework 1 due today – write up on The Thinking Machine Department Picnic: Thursday, September 13 – 1: 20 to 2: 30 Lab 0 due Thursday, September 13 Writing Assignments Posted • Caves of Steel due 10/4 • Current Events Presentation CS 484 – Artificial Intelligence 1

Representations: Semantic Nets, Frames, and Trees Lecture 3

The Need for a Good Representation • • A computer needs a representation of a problem in order to solve it. A representation must be: • Efficient – not wasteful in time or resources. • Useful – allows the computer to solve the problem. • Meaningful – really relates to the problem. CS 484 – Artificial Intelligence 3

Semantic Nets • • • A graph with nodes, connected by edges. The nodes represent objects or properties. The edges represent relationships between the objects. • Label to indicate nature of relationship CS 484 – Artificial Intelligence 4

A Simple Semantic Net CS 484 – Artificial Intelligence 5

Create a Semantic Net A Ford is a type of car. Bob owns two cars. Bob parks his car at home. His house is in California, which is a state. Sacramento is the state capital of California. Cars drive on the freeway, such as Route 101 and Highway 81. CS 484 – Artificial Intelligence 6

Your Semantic Web CS 484 – Artificial Intelligence 7

Inheritance • • Inheritance is the process by which a subclass inherits properties from a superclass. Example: • Mammals give birth to live young. • Fido is a mammal. • Therefore Fido gives birth to live young. • In some cases, as in the example above, inherited values may need to be overridden. (Fido may be a mammal, but if he’s male then he probably won’t give birth). CS 484 – Artificial Intelligence 8

Frames • • • A frame system consists of a number of frames, connected by edges, like a semantic net. Class frames describe classes. Instance frames describe instances. Each frame has a number of slots. Each slot can be assigned a slot value. CS 484 – Artificial Intelligence 9

Frames: A Simple Example CS 484 – Artificial Intelligence 10

Other relationships • Aggregation – one object being part of another object • Fido has a tail • Association – explains how objects are related to each other • "chases relationship" – how Fido and Fang are related CS 484 – Artificial Intelligence 11

Create a frame-based representation A Ford is a type of car. Bob owns two cars. Bob parks his car at home. His house is in California, which is a state. Sacramento is the state capital of California. Cars drive on the freeway, such as Route 101 and Highway 81. CS 484 – Artificial Intelligence 12

Your Frame-Based Representation CS 484 – Artificial Intelligence 13

Why Are Frames Useful? • • Used as a data structure by Expert Systems All information about an object stored in one place • As opposed to rule-based systems • In real world systems frames have a large number of slots • Searching for all relevant information would take a long time CS 484 – Artificial Intelligence 14

Search Space • A set of possible choices in a given problem • One or more are the solution to the problem • • • Identify one or more goals Identify one or more paths to those goals Problem • set of states • states connected by paths that represent actions CS 484 – Artificial Intelligence 15

Search Trees • • • Semantic trees – a type of semantic net. Used to represent search spaces. Root node has no predecessor. Leaf nodes have no successors. Goal nodes (of which there may be more than one) represent solutions to a problem. CS 484 – Artificial Intelligence 16

Search Trees: An Example • • A is the root node. L is the goal node. l. H, I, J, K, M, N and O are leaf nodes. l. There is only one complete path: l. A, C, F, L CS 484 – Artificial Intelligence 17

Example: Missionaries and Cannibals • • • Three missionaries and three cannibals Want to cross a river using one canoe. Canoe can hold up to two people. Can never be more cannibals than missionaries on either side of the river. Aim: To get all safely across the river without any missionaries being eaten. CS 484 – Artificial Intelligence 18

A Representation • • • The first step in solving the problem is to choose a suitable representation. We will show number of cannibals, missionaries and canoes on each side of the river. Start state is therefore: • C=3, M=3, B=1 0, 0, 0 CS 484 – Artificial Intelligence 19

A Simpler Representation • • • In fact, since the system is closed, we only need to represent one side of the river, as we can deduce the other side. We will represent the finishing side of the river, and omit the starting side. So start state is: • 0, 0, 0 CS 484 – Artificial Intelligence 20

Operators • Now we have to choose suitable operators that can be applied: 1. 2. 3. 4. 5. Move one cannibal across the river. Move two cannibals across the river. Move one missionary across the river. Move two missionaries across the river. Move one missionary and one cannibal. CS 484 – Artificial Intelligence 21

The Search Tree • • • Cycles have been removed. Nodes represent states, edges represent operators. There are two shortest paths that lead to the solution. CS 484 – Artificial Intelligence 22

What Other Representations are Possible? CS 484 – Artificial Intelligence 23

Combinatorial Explosion • • • Problems that involve assigning values to a set of variables can grow exponentially with the number of variables. Some such problems can be extremely hard to solve (NP-Complete, NP-Hard). Reduce state space • select good representation help • using heuristics (see chapter 4). CS 484 – Artificial Intelligence 24

Problem Reduction • • • Breaking a problem down into smaller subproblems (or sub-goals). Can be represented using goal trees (or and-or trees). Nodes in the tree represent sub-problems. The root node represents the overall problem. Some nodes are and nodes, meaning all their children must be solved. CS 484 – Artificial Intelligence 25

Problem Reduction: Example CS 484 – Artificial Intelligence 26