Chapter 6, Sec. 1 - 4. Adversarial Search. Outline. Optimal decisions α-β pruning Imperfect, real-time decisions. Games vs. search problems. Competitive multiagent environment in which the agents’ goals are in conflict: adversarial search problem - games .
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to the final choice
of a state without doing a complete search.
gives a numeric value for the terminal states. -- +x, -x, 0
if n is a MAX node
if n is a MIN node
which can be assigned utilities according to the rules of the game.
Consider a node n somewhere in the tree, s.t. Player has a choice of moving to that node. If Player has a better choice m either at the parent node of n or at any choice point further up, then n will never be reached in actual play.
-- highly dependent on the order of successors examined.
= estimated desirability(utility) of position
even if it can see ahead all the way to the end of the game.
the states in each category have the same values for all the features
as a single value which reflects the proportion of states with each outcome
– the weighted average or expected value.
– too much to estimate all the probabilities of winning for too may categories
Eval(s)=w1 f1 (s)+w2 f2 (s)+ …. +wn fn (s)
Eval(s) = w1 f1(s) + w2 f2(s) + … + wn fn(s)
f1(s) = (number of white queens) – (number of black queens), etc.
won’t exceed what the rules of the game allow.
bm = 106, b=35 m=4
Chinook ended 40-year-reign of human world champion Marion Tinsley in 1994. Used a precomputed endgame database defining perfect play for all positions involving 8 or fewer pieces on the board, a total of 444 billion positions.
Deep Blue defeated human world champion Garry Kasparov in a six-game match in 1997. Deep Blue searches 200 million positions per second, uses very sophisticated evaluation, and undisclosed methods for extending some lines of search up to 40 ply.
human champions refuse to compete against computers, who are too good.
human champions refuse to compete against computers, who are too bad. In go, b > 300, so most programs use pattern knowledge bases to suggest plausible moves.