CS 580 - Intro to AI

Jonathan Zinger
Flashcards by Jonathan Zinger, updated more than 1 year ago
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What Is AI? The study of human intelligence via computer models. –  The study of man-made computational intelligence. –  The study of the architecture of intelligent systems. –  The study of “user-friendly” systems.
Traditional AI Intelligence = ability to solve problems
Traditional AI Goal Directed Symbol Manipulation
General Problem Solver Examples Game playing •  Theorem proving •  Medical diagnosis •  Traveling from A to B •  Solving puzzles: – Sudoku, logic, crossword, …
General Problem Solving Process Given: – Symbol set, operators – Initial state, goal state •  Find: – Sequence of operations that transform the initial state into the goal state
General Problem Solving Strategy Develop knowledge-intensive techniques for efficiently constructing the required problem-solving sequences.
Game Playing Board is current state Legal Moves are symbols Start with Initial state Well understood goal state
Game Trees gametrees.png (image/png)
Game Playing Problem Space Chess: 10^120 board configs Table Lookup/exhaustive search not feasible
Knowledge Directed Search STatic Eval Functions Dynamic Look Ahead Tree Pruning Heuristics
Chess Status Very Strong Programs Higher Level Concepts Missing Brute Force Works - 6-8 levels look-ahead
AI Paradigm 1. Identify problem humans solve 2. analyze activities 3. build a conceptual model 4. build a computational model 5. Implement 6. Simulate(Execute) 7. Revise
Knowledge Representation Issues Knowledge: 1 Large/unwieldy 2 Difficult to Articulate 3 Inexact/Incorrect 4 Changes
Knowledge Representation Key Factors Must Be: 1 Effecient 2 Useful 3 Support reasoning/generalization 4 Facilitate Acquisition 5. Understandable by humans
KR Approaches Natural Language Formal Logic Production Rules Semantic Nets Frames, Scripts
AI Subdomains NL Understanding Scene Understanding Multi-Sensor Integration Robotics Expert Systems Machine Learning ...
Modern AI Perspective 1 Agent Oriented 2 Sensors/Effectors - environmental interaction 3 open ended problem solving
Agents Embodied Intelligence Sense-Think-Act Meta Cognition
Common Threads 1 Signal->Symbol 2Mapping 3 General Purpose vs Specialized systems 4 Knowledge Representation 5 Knowledge Acquisition 6 Learning
Turing Test Imitation Game Interrogator tries to figure out which is human, A or B, via conversation
Weak AI Study of algorithms which enable computers to do tasks which previously only humans could perform
Strong AI Pursuit of research leading to the development of facsimiles of the human mind
Major Areas of AI Optimization Induction Deduction Interaction
AI Area: Optimization Process of wandering through an environment, hunting for samples which have the highest quality.
AI Area: Stochastic Optimization Metaheuristics - genetic algorithm, simulated annealing, ant colony optimization
LISP Symbol Manipulation Fundamental Data Objects
LISP: Symbols 1 Manipulated as data 2 Fundamental distinction: Symbol names vs values X, `X, "X"
LISP: Symbolic Expressions Formed from atomic elements and/or s expressions Lists - (a (b c) d)
LISP: Underlying Representation 1-way linked list of CONS cells with CAR, CDR fields (a (b c) d)
LISP: list construction cons, list, append (setf L (cons a b)) (setf L (list 'a 'b))
LISP: list traversal car, cdr, caar, cadr (print (car L))
LISP - list manipulation Shared, non-desctructive
LISP functions assume CAR specifies a function and CDR its arguments
LISP Program collection of s expressions to be manipulated or evaluated
LISP control flow IF, COND, LOOP, DOLIST, DOTIMES, PROG, MAP
LISP REPL read evaluate print loop (loop (print (eval (read)))
AI Goal Develop Knowledge Intensive techniques for efficiently constructing the required problem-solving sequences "Heuristics"
Cognitive Approach Build a computational mode of human problem solving
CS/Engineering Approach Build a computationally efficient problem solver (not necessarily modeling human behavior/preference)
State Space Search Initial State: 1. Root Node(of tree/graph) 2. Intermediate Nodes(partial solutions) 3. Leaf Nodes(solutions)
State Space Search Goal State stopping condition. eg Tour of minimal cost in TSP
State Space Search Generator Depth First Breadth First
State Space Search TSP Initial State: 1 visited city N-1 unvisited Goal State: Tour of minimal cost Generator: Depth first by city
State Space Search TSP Problems Size of space: (N-1)! leaf nodes >(N-1)! interior nodes Measuring efficiency: nodes visited Heuristics: 1 Closed Neighbor next(greedy) 2 No-overlapping paths
State Space Search Boolean Sat Initial State: N boolean variables unassigned Goal State: Entire expression evals to true Generator: Depth first by variable
General Search Strategies Data Driven vs Goal Directed Uninformed vs Informed Admissible vs Inadmissible Heuristics
Uninformed Search Examples Depth First Breadth First Depth Limited Iterative Deepening Breadth limited bidirectional
Search: Informed Examples w/ Static eval function Hill Climbing Greedy Prioritized depth first Prioritized breadth first Best first
Search: Informed Examples w/ a future cost estimator A*
A* Total Est Cost = current cost + heuristic estimate Keep "best so far" cost prune if est.clost > bsf cost Admissible if never overestimates future costs
Search: Informed Examples Constraint Violoation Assumes constraints Admissible Efficiency Constraint Issues
Search: Informed Look Ahead Simulate Future Possibilities extract useful information make choices, via pruning
Special Search: Adversarial Games w/ Lookahead capability goal: 1. Sim Future Poss 2. Extract Useful Info 3. Make choices, prune: Minimax Algs Alpha/Beta pruning
Game Tree tree.png (image/png)
Minimax Assume symmetric static eval function Max values, good for me Min Values, bad for me Opponent always chooses optmial min Exhaustive search to fixed path
State Space Approach Generate & test with pruning
Solution State Search Move around in solution space rather than constructing solutions incrementally
Solution Space search TSP Solution Space: All permutations Move from one perm to another via 2 city swap Segment inversion
Solution Space Search Heuristics Hill Climbing/descent Greedy Ascent/Descent Gradient ascent/descent
Special Search Natural Computation Simulated Annealing Evolutionary Computation Ant Colony Optimization
Special Search: Natural Search: Simulated Annealing Energy levels <==> random jumps initial "temperature high" cool slowly
Special Search Natural Computation Evolutionary Computation Simulate Darwinian Evolution Survival of the fittest Parental fitness -> offspring Offspring inheritance
Special Search Natural Logic Ant Colony Optimization Distributed problem solving Initially parallel random search Pheremone trails
Datadrive vs Goal Directed Forward Chaining vs backward chaining
General Search Uninformed vs Informed Blind vs knowledge guided
General Search Admissible vs Inadmissible Provably correct vs generally correct
Uninformed Search Breadth First all nodes at each level expanded before next level
Uninformed Search Breadth First - Pros Complete
Uninformed Search Breadth-First Cons Time & Space All nodes in memory - at 8 levels deep, need 1TB (1k per node, 10^9 nodes)
Uninformed Search Uniform-cost search Breadth-First search which follows lowest cost node instead of all nodes at a level
Uninformed Search Depth-first Search Extends the deepest node on the current fringe of the tree
Uninformed Search Depth-first search Pros & Cons Low memory Reqmts - only one path need be in memory Can get stuck going down long or infinite path if it makes a wrong choice. Not complete or optimal.
Uninformed Search Depth- Limited Search Set a depth l which we treat as if it has no successors. Use depth-first search but stop at l. depth-first is special case with l=d.
Uninformed Search Dept-First Search Pros & Cons Solves infinite path problem incomplete if l<d (shallowest goal is beyond depth limit) non-optimal if we choose l>d. useful for applying heuristics - like state-space diameter(known max step count)
Uninformed Search Iterative Deepening Depth First Search find the best depth limit by gradually increasing the limit - 0,1,2 etc. Repeating entire search each time.
Uninformed Search Iterative Deepening depth-first search Preferred uninformed search for large search space and unknown solution depth
Uninformed Search Iterative Deepening Pros & Cons Small memory requirements like depth first search, complete and optimal under right conditions like breadth first.
Uninformed Search Bidirectional Search Two simultaneous searches, one forward from init state and one back from the goal, stopping when they meet.
Uninformed Search Bidirectional Search Pros & Cons Complete and Optimal if both searches are breadth first. Hard to know how to work back from goal in many cases.
Uninformed Search Strategies Comparison BF: Complete, Optimal, Memory issues UniformCost: Complete, Optimal, memory issues DF: Not complete, not optimal, small memory Depth-Limited: Not complete, not optimal, smaller memory Iterative Deepening: Optimal, Complete, good memory use Bidirectional: Complete, Optimal, if BF, hard to use
Informed Search Use Problem Specific Knowledge beyond the problem description
Heuristic function Estimated cost of the cheapest path from node n to a goal node
Greedy Best First Expand node that is closest to the goal, according to the heuristic function
Greedy Best First Pros & Cons Minimizing cost is prone to false starts. May choose wrong path just because first step appears to get you closer than another. Much like Depth First search - not complete or optimal. A good heuristic solves all.
Informed Search A* Combines g(n), the cost to reach thenode, and h(n) the cost from the node to the goal. Estimates the cheapest solution through n.
A* Heuristic Complete and Optimal only if h(n) is admissible - that is, it never overestimates the cost to reach the goal
Informed Search Hill Climbing Continually moves in the direction of increasing value. Stops when no neighbor has higher value.
Hill Climbing Issues Get stuck because: Local Maxima Ridges Plateaux
Gradient Descent Minimize cost by seeking minima
Simulated Annealing Start at high temperature(make a lot of random moves) and then decrease temperature until reaching steady state at solution.
Solution State Search genetic algoritm stochastic hill climbing search in which a large population of states is maintained. new states generated by mutation and crossover.
Natural Computation Evolutionary Computation Simulate Darwin Survival of fittest Parent fitness <==> offspring Offspring inheritance/variation(crossover and mutation)
Evolution Computation Pseudocode Create initial population of solutions until stopping condition: select parents based on fitness produce offspring select individuals to die end do return result
Minimax algorithm Complete depth-first search of a game tree in order to backup values from each leaf node, labeling parent nodes with the best possible score on that path
Alpha Beta Pruning Prune nodes which result in a choice the player would never make. For MAX, prune away all but the branch with the highest low score(which MIN will optimally choose). Highly order dependent.
KR Types of Knowledge Declarative(Facts) Procedural(HowTo)
Properties of Knowledge Temporal Extent Context Ambiguities, Conflicts
Knowledge Representation Issues Ease of Acquisition Ease of use in problem solving Ease of Maintenance Handling Noise, inconsistencies Generalization
KR Extremes Natural Language Formal Logics
KR - Practical Systems Rule-based Network Representations Logic Programming
Propositional Logic Constants: T,F Operators: AND,OR,NOT,=> Symbol Manipulation: DeMorgans Law Truth Tables!
Truth Table Specifies truth value for each possible assignment of true values to its components.
Modus Ponens A==>B, a / B If A=>B and a is given, then B can be inferred.
And-Elimination From a conjunction, any conjunct can be inferred. A^B/A. if (A^B) is given, A can be inferred.
Biconditional Inference Rules A<=>B/(A=>B) ^ (B=>A) also the reverse
deMorgan's Rule !(A^B) == (!A || !B) !(A||B) == (!A ^ !B)
Satisifiability A sentence is satisfiable if it is true in some model
Propositional Logic Validity A sentence is valid if it is true in ALL models
Propositional Logic Logical Equivalence A and B are logically equivalent if they are true in the same set of models
Propositional Logic Monotonicity Set of entailed sentences can only increase as information is added to the knowledge base IF KB entails A, then KB ^ B entails A too. "The conclusion of rules must follow regardless of what else is in the knowledge base".
Propositional Logic Resolution Applies only to disjunctions of literals. Uses inference rules to prove a series of statements.
Propositional Logic Resolution Takes two clauses and produces a new clause containing all the literals of the two original clauses except the complimentary literals. (L1 || L2) ^ (!L2 || L3) / (L1 || L3)
Conjunctive Normal Form every sentence of propositional logic is logically equivalent to a conjunction of disjunctions of literals.
CNF Sample Conversion B<=>(P12 | P21) 1. Eliminiate <=> B=>(P12 | P21) ^ (P12 | P21) => B 2. Eliminate => Replace a=>b with !a | B (!B | P12 | P21) ^ (!(P12 | P21) | B) 3. Move NOT inwards so it applies only to literals (!B | P12 | P21) ^ ((!P12 ^ !P21) | B) 4. Distribute OR over AND (!B | P12 | P21) ^ (!P12 | B) ^ (!P21 | B)
Forward Chaining Deductive Closure Facts: A, A=>B ^ C B => R C=> Q Query: Q? Yes
Backward Chaining Facts: A, A=>B &C, B=>R,C=>Q Query: Q? Q?: Q<=C C?: B&C <=A A?: A = yes.
KR Using Prop Logic Limitations Expressiveness - No Variables
first order logic Propositional Calculus plus ForAll and ThereExists Variables, relations, functions.
FOL Unification Agree to allow one variable to act as another. Is Q(Mary) true? If Q(Z) is true, and we unify Z and Mary, then Q(Mary) is true.
KR using Pred Logic FOL Example Simplification Facts: FA m A(m), FAx A(x)=>B(x) ^ C(x) FAy B(y)=>R(y) FAz C(z)=>Q(z)
FOL PROS & Cons Much more expressive Computation Issues
Prolog HORN Clauses Only one variable on right side(no conjunctions) Negation as Failure: If we can't find the answer query is false. Closed World.
FOL Quantifiers Universal Quantification "For All..."
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