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