Announcements

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Want to cross a river using one canoe. Canoe can hold up to two people. ... will show number of cannibals, missionaries and canoes on each side of the river. ...
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Title: Announcements 1 Announcements
  • Homework 1 due today write up on The Thinking Machine
  • Department Picnic Thursday, September 13 120 to 230
  • Lab 0 due Thursday, September 13
  • Writing Assignments Posted
  • Caves of Steel due 10/4
  • Current Events Presentation
  • 2 Representations Semantic Nets, Frames, and Trees
  • Lecture 3
  • 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.
  • 4 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
  • 5 A Simple Semantic Net 6 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.
  • 7 Your Semantic Web 8 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 hes male then he probably wont give birth).
  • 9 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.
  • 10 Frames A Simple Example 11 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
  • 12 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.
  • 13 Your Frame-Based Representation 14 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
  • 15 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
  • 16 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.
  • 17 Search Trees An Example
  • A is the root node.
  • L is the goal node.
  • H, I, J, K, M, N and O are leaf nodes.
  • There is only one complete path
  • A, C, F, L
  • 18 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.
  • 19 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
  • C3,M3,B1 0,0,0
  • 20 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
  • 21 Operators
  • Now we have to choose suitable operators that can be applied
  • 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.
  • 22 The Search Tree
  • Cycles have been removed.
  • Nodes represent states, edges represent operators.
  • There are two shortest paths that lead to the solution.
  • 23 What Other Representations are Possible? 24 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).
  • 25 Problem Reduction
  • Breaking a problem down into smaller sub-problems (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.
  • 26 Problem Reduction Example
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