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					Depth-First Searches

 G5AIAI – 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.
• 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.
               The example node set
        Initial state
                                  A


    B          C                  D                  E             F
                                       Goal state

G H        I       J          K       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
Node B is expanded and removed node
     process now continues until the
The search then moves to the first from            This node removed from state: the node
                                                   Node A is is then expanded queue. Each
                                                   We begin with our initialtheto reveal
the queue.is achieved. Press continue
goal state Revealed nodes space.
in the queue. Press space toare added to       A   revealed node is added to the FRONT of
                                                   labeled A. Press space nodes. Press space.
                                                   further (unexpanded) to continue
the FRONT of the queue. Press space.               the queue. Press space to continue.


      B                  C                     D                 E                    F


                                                        Node L is located and the search
  G H                I        J            K       L    returns a solution. Press space to
                                                        end.


  Q      R          S        T             U

                                                 continue search
                                  Press space to begin thethe search

               3
               4
               6
               5
               1
Size of Queue: 0             Queue: A J, D, E, FF F
                                    T, D, D, E, FF
                                    L,
                                    U,
                                    K,
                                    D,
                                    J,
                                    S,
                                    I, C,
                                    C, H, C,
                                    R, L, D,
                                    H, D, E, D,
                                    Q, E, D, E,
                                    G, C, F E,
                                    B, J, E, F
                                    Empty F E,
Nodes expanded: 0
                14
                13
                12
                11
                10
                9
                8
                7
                6
                5
                4
                3
                2
                1                    Action: Expanding
                                             Backtracking
                             CurrentFINISHED SEARCH                                   3
                                                                                      2
                                                                                      1
                                                                                      0
                                                                       Current level: n/a

                             DEPTH-FIRST SEARCH PATTERN

				
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posted:9/10/2011
language:English
pages:4