Alphabeta

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Alphabeta

  1. 1. An Alpha-Beta Primer Stephen D. BayMinimax search with alpha-beta pruning involves depth rst search of a game tree, keepingtrack of Alpha: highest value seen so far on a maximizing level Beta: lowest value seen so far on a minimizing levelNote that when we look at alpha and beta values we look only at nodes along the path fromour current node to the root.Pruning is done according to the following two rules: When on a maximizing level, do not expand any more sibling nodes once a node has been seen whose evaluation is lower than or equal to Alpha. When on a minimizing level, do not expand any more sibling nodes once a node has been seen whose evaluation is greater than or equal to Beta.For this primer, we will consider the following search tree: A MAX B C MIN D E F G MAX H I J K L M N O MIN 2 -3 8 5 -2 1 5 4This is the full search tree to depth 3. The numbers beneath the leaves are values returnedby the evaluation function. 1
  2. 2. Step 1 A MAX MIN MAX MINalpha = unde ned, beta = unde ned, Expand AStep 2 A MAX B C MIN MAX MINalpha = unde ned, beta = unde ned, Expand B 2
  3. 3. Step 3 A MAX B C MIN D E MAX MINalpha = unde ned, beta = unde ned, Expand DStep 4 A MAX B C MIN D E MAX H I MIN 2alpha = unde ned, beta = unde ned, Evaluate H and propagate values upwards. 3
  4. 4. Step 5 A MAX B C MIN >= 2 D E MAX H I MIN -3alpha = 2, beta = unde ned, Evaluate I and propagate values upwards.Step 6 A MAX <= 2 B C MIN 2 D E MAX H I MINalpha = unde ned, beta = 2, Expand E. Note that alpha is unde ned since we are at E andwe only consider alpha-beta values from E to the root A i.e. values at B and A. 4
  5. 5. Step 7 A MAX = 2 B C MIN D E MAX H I J K MIN 8alpha = unde ned, beta = 2, Evaluate J and propagate values upwards.Step 8 A MAX = 2 B C MIN = 8 D E MAX H I J K MINalpha = 8, beta = 2, Prune K as evalj 8 2. Once K has been pruned we canpropagate values up to A. 5
  6. 6. Step 9 = 2 A MAX 2 B C MIN D E MAX H I J K MINalpha = 2, beta = unde ned, Expand CStep 10 = 2 A MAX B C MIN D E F G MAX H I J K MINalpha = 2, beta = unde ned, Expand F 6
  7. 7. Step 11 = 2 A MAX B C MIN D E F G MAX H I J K L M MIN -2alpha = 2, beta = unde ned, Evaluate L and propagate values upward.Step 12 = 2 A MAX B C MIN = -2 D E F G MAX H I J K L M MIN 1alpha = 2, beta = unde ned, Evaluate M and propagate values upward. 7
  8. 8. Step 13 = 2 A MAX = 1 B C MIN 1 D E F G MAX H I J K L M MINalpha = 2, beta = 1, Prune G as evalF 1 2. Once G has been pruned we canpropagate values up to A.Step 14 2 A MAX 1 B C MIN D E F G MAX H I J K L M MINThe computer choses the move that leads to B. 8

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