# Introduction to Artificial Intelligence Announcements by wpr1947

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```									           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.

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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

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A Simple Semantic Net

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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.

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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).
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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.

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Frames: A Simple Example

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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

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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.

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Representation

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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

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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

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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.

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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
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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.

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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

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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

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

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The Search Tree
• Cycles have been removed.
• Nodes represent states,
edges represent operators.
• There are two shortest paths

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What Other Representations are
Possible?

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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).

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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.

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Problem Reduction: Example

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