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Depth-First Searches

Depth-First Searches. Introduction to AI. Depth-First Search. Depth first search explores one branch of a tree before it starts to explore another branch. It can be implemented by adding newly expanded nodes at the front of the queue.

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Depth-First Searches

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  1. Depth-First Searches Introduction to AI

  2. Depth-First Search • Depth first search explores one branch of a tree before it starts to explore another branch. It can be implemented by adding newly expanded nodes at the front of the queue. • Using the general search algorithm we can implement it as follows: Function DEPTH-FIRST-SEARCH(problem) returns a solution or failure Return GENERAL-SEARCH(problem,ENQUEUE-AT-FRONT) • The following slide illustrates a depth-first search of 26 nodes (states) with an initial state of node A and a goal state of node L. Press space to see the example node set.

  3. The example node set Initial state A A B C D E F Goal state G H I J K L L M N O P Q R S T U V W X Y Z Press space to see a DFS of the example node set

  4. The search then moves to the first node in the queue. Press space to continue Node B is expanded and removed from the queue. Revealed nodes are added to the FRONT of the queue. Press space. The process now continues until the goal state is achieved. Press space. Node A is removed from the queue. Each revealed node is added to the FRONT of the queue. Press space to continue. We begin with our initial state: the node labeled A. Press space to continue This node is then expanded to reveal further (unexpanded) nodes. Press space. A A B B C C D D E F G G H H I I J J K K L L L L L L L L Node L is located and the search returns a solution. Press space to end. Q Q R R S S T T U U Press space to begin the search Press space to continue the search Press space to continue the search Press space to continue the search Press space to continue the search Press space to continue the search Press space to continue the search Press space to continue the search Press space to continue the search Press space to continue the search Press space to continue the search Press space to continue the search Press space to continue the search Size of Queue: 0 Size of Queue: 0 Size of Queue: 1 Size of Queue: 5 Size of Queue: 6 Size of Queue: 5 Size of Queue: 3 Size of Queue: 4 Size of Queue: 4 Size of Queue: 4 Size of Queue: 3 Size of Queue: 5 Queue: A Queue: Empty Queue: R, C, D, E, F Queue: H, C, D, E, F Queue: Q, H, C, D, E, F Queue: G, H, C, D, E, F Queue: B, C, D, E, F Queue: C, D, E, F Queue: U, L, E, F Queue: I, J, D, E, F Queue: S, J, D, E, F Queue: T, D, E, F Queue: D, E, F Queue: K, L, E, F Queue: Empty Queue: J, D, E, F Queue: L, E, F Nodes expanded: 12 Nodes expanded: 13 Nodes expanded: 0 Nodes expanded: 7 Nodes expanded: 8 Nodes expanded: 1 Nodes expanded: 6 Nodes expanded: 3 Nodes expanded: 2 Nodes expanded: 5 Nodes expanded: 10 Nodes expanded: 4 Nodes expanded: 11 Nodes expanded: 14 Nodes expanded: 9 Current Action: Expanding Current Action: Backtracking Current Action: Backtracking Current Action: Backtracking Current Action: Expanding Current Action: Expanding Current Action: Backtracking Current Action: Backtracking Current Action: FINISHED SEARCH Current Action: Expanding Current Action: Expanding Current level: 3 Current level: 1 Current level: 2 Current level: 3 Current level: 2 Current level: 2 Current level: 1 Current level: 0 Current level: 2 Current level: 0 Current level: 2 Current level: 3 Current level: 2 Current level: 1 Current level: 1 Current level: 3 Current level: 1 Current level: 2 Current level: n/a Current level: 1 Current level: 0 Current level: 2 Current level: 2 Current level: 3 Current level: 1 Current level: 2 Current level: 1 Current level: 2 DEPTH-FIRST SEARCH PATTERN

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