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Representation

- A farmer wants to move himself, a hunrgry silver fox, a hungry, fat goose, and a sack of tasty grain across a river. Unfortunately, his boat is so tiny he can move only one of his possessions across on any trip. What is he to do?

Fox, Farmer, Goose, Grain

- 5 minute break for solving the problem
- Keep track of how you solve the problem (think about how you think to solve the problem)

Representation

- Look at the representation in figure 2.1.
- Graph like representation
- Nodes: A safe distribution of entities on both banks of the river
- Edges: connect nodes separated by a single boat trip
- Solution: draw a line from start state to end state

Analysis

- How many nodes?
- How many safe nodes?
- How many links?
- How many legal links?

Representation is key

- Node/Link representation is good
- Once represented using nodes and links, problem is simple to solve

Representation

- A lexical part: What symbols are allowed in your vocabulary/lexicon
- A structural part: constraints on how symbols can be arranged
- A procedural part: access procedures to create, modify descriptions
- A semantic part: associates meaning with description

Semantic Nets

- Lexical: Nodes, links, and app-specific link labels
- Structural: Each link connects a tail node to a head node
- Semantic: Nodes and links denote application specific entities
- Procedural: node, link constructors, readers/visitors to traverse the net

Good Representations

- Example of a good representation in object identification
- Identify an object as belonging to a class
- Basic Method: Describe and Match
- Describe Object (using a “good” representation)
- Match to library of object descriptions
- Return closest match

Object Identification

- Feature based object identification
- Feature extractor, extracts easily identifiable object features (area, perimeter, hole count, hole area, …)
- Values of these features locate the object in an n-dimensional feature space (an object occupies a feature point in feature space)
- Compare the distance of unknown object’s feature point to library of known (idealized) object’s feature points. The most likely identity is determined by smallest distance.

Administrivia

- Read Chapters 1 and 2
- Assignment 1 considers Missionaries and Cannibals is available, due Feb 4.
- Back of book contains hints
- Need help, talk to me

3 Missionaries 3 Cannibals

- Three cannibals and three missionaries are standing on the west bank of a river. A boat is available that will hold either one or two people. If the missionaries are ever outnumbered -- on either bank or in the boat -- the cannibals will eat them. Your job is to design a program (come up with a representation and implement a search algorithm) that will get everyone across the river to the east bank safely.

Burglar and combination lock

- Generate a combination
- Test if it opens the safe
- Consider 6 digit combination
- 10^6 combinations
- 10^6/2 combinations on average to find correct combination, 3/min 16, 24 hr weeks

Generate and Test

- Powerful problem solving method
- Until a satisfactory solution is found or no more solutions can be generated
- Generate the candidate solution
- Test the candidate solution
- If an acceptable solution is found, announce it; otherwise announce failure

Dendral

- Good generator – complete, nonredundant, only generates structures arising from known formula
- Tester – Spectrogram comparator

Terminology

- State (of a system): Description of a system that is sufficient to determine the future
- State space: Each node denotes a state, and each link denotes a possible one-step transition from one state to another

State Space

- A state space is a representation
- That is a semantic net
- In which
- Nodes denote states
- Links denote transitions between states
- Initial state, current state, goal state

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