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## Depth First Search

Depth First Search (DFS) searches deeper into the problem space. Breadth-first search always generates successor of the deepest unexpanded node. It uses last-in first-out stack for keeping the unexpanded nodes. More commonly, depth-first search is implemented recursively, with the recursion stack taking the place of an explicit node stack.

### Algorithm: Depth First Search

1.If the initial state is a goal state, quit and return success.
2.Otherwise, loop until success or failure is signaled.
a) Generate a state, say E, and let it be the successor of the initial state. If there is no successor, signal failure.
b) Call Depth-First Search with E as the initial state.
c) If success is returned, signal success. Otherwise continue in this loop.

### Advantages of Depth-First Search

• The advantage of depth-first Search is that memory requirement is only linear with respect to the search graph. This is in contrast with breadth-first search which requires more space. The reason is that the algorithm only needs to store a stack of nodes on the path from the root to the current node.

• The time complexity of a depth-first Search to depth d is O(b^d) since it generates the same set of nodes as breadth-first search, but simply in a different order. Thus practically depth-first search is time-limited rather than space-limited.

• If depth-first search finds solution without exploring much in a path then the time and space it takes will be very less.

### Disadvantages of Depth-First Search

• The disadvantage of Depth-First Search is that there is a possibility that it may go down the left-most path forever. Even a finite graph can generate an infinite tree. One solution to this problem is to impose a cutoff depth on the search. Although the ideal cutoff is the solution depth d and this value is rarely known in advance of actually solving the problem. If the chosen cutoff depth is less than d, the algorithm will fail to find a solution, whereas if the cutoff depth is greater than d, a large price is paid in execution time, and the first solution found may not be an optimal one.

• Depth-First Search is not guaranteed to find the solution.

• And there is no guarantee to find a minimal solution, if more than one solution exists.

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# Uniform-Cost Search Algorithm

If all the edges in the search graph do not have the same cost then breadth-first search generalizes to uniform-cost search. Instead of expanding nodes in order of their depth from the root, uniform-cost search expands nodes in order of their cost from the root. At each step, the next step n to be expanded is one whose cost g(n) is lowest where g(n) is the sum of the edge costs from the root to node n. The nodes are stored in a priority queue. This algorithm is also known as Dijkstra’s single-source shortest algorithm.

Whenever a node is chosen for expansion by uniform cost search, a lowest-cost path to that node has been found. The worst case time complexity of uniform-cost search is O(bc/m), where c is the cost of an optimal solution and m is the minimum edge cost. Unfortunately, it also suggests the same memory limitation as breadth-first search.

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## Minimax Search

### Minimax Search Algorithm

The standard algorithm for two-player perfect-information games such as chess, checkers or othello is minimax search with heuristic static evaluation. The minimax search algorithm searches forward to a fixed depth in the game tree, limited by the amount of time available per move. At this search horizon, a heuristic function is applied to the frontier nodes. In this case, a heuristic evaluation is a function that takes a board position and returns a number that indicates how favourable that position is for one player relative to the other. For example, a very simple heuristic evaluator for chess would count the total number of pieces on the board for one player, appropriately weighted by their relative strength, and subtract the weighted sum of the opponent’s places. Thus, large positive values would correspond to strange positions for one player called MAX, whereas large negative values would represent advantageous situation for the opponent called MIN.

Given the heuristic evaluations of the frontier nodes, minimax search algorithm recursively computes the values for the interior nodes in the tree according to the maximum rule. The value of a node where it is MAX’s turn to move is the maximum of the values of its children, while the value of the node where MIN is to move is the minimum of the values of its children. Thus at alternative levels of the tree, the maximum values of the children are backed up. This continues until the values of the immediate children of the current position are computed at which point one move to the child with the maximum or minimum value is made depending on whose turn it is to move.

# Iterative Deepening A* (IDA*) Search

Just as iterative deepening solved the space problem of breadth-first search, iterative deepening A* (IDA*) eliminates the memory constraints of A* search algorithm without sacrificing solution optimality. Each iteration of the algorithm is a depth-first search that keeps track of the cost, f(n) = g(n) + h(n), of each node generated. As soon as a node is generated whose cost exceeds a threshold for that iteration, its path is cut off, and the search backtracks before continuing. The cost threshold is initialized to the heuristic estimate of the initial state, and in each successive iteration is increased to the total cost of the lowest-cost node that was pruned during the previous iteration. The algorithm terminates when a goal state is reached whose total cost does not exceed the current threshold.

Since Iterative Deepening A* performs a series of depth-first searches, its memory requirement is linear with respect to the maximum search depth. In addition, if the heuristic function is admissible, IDA* finds an optimal solution. Finally, by an argument similar to that presented for DFID, IDA* expands the same number of nodes, asymptotically, as A* on a tree, provided that the number of nodes, asymptotically, as A* on a tree, provided that the number of nodes grows exponentially with solution cost. These costs, together with the optimality of A*, imply that IDA* is asymptotically optimal in time and space over all heuristic search algorithms that find optimal solutions on a tree. Additional benefits of IDA* are that it is much easier to implement, and often runs faster than A*, since it does not incur the overhead of managing the open and closed lists.

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## Pure Heuristic Search

Pure Heuristic Search

The simplest of heuristic search algorithms, the pure heuristic search, expands nodes in order of their heuristic values h(n). It maintains a closed list of those nodes that have already been expanded, and a open list of those nodes that have been generated but not yet been expanded. The algorithm begins with just the initial state on the open list. At each cycle, a node on the open list with the minimum h(n) value is expanded, generating all of its children and is placed on the closed list. The heuristic function is applied to the children, and they are placed on the open list in order of their heuristic values. The algorithm continues until a goal state is chosen for expansion.

In a graph with cycles, multiple paths will be found to the same node, and the first path found may not be the shortest. When a shorter path is found to an open node, the shorter path is saved and the longer one is discarded. When a shorter path to a closed node is found, the node is inaved to open and the shorter path is associated with it. The main drawback of pure heuristic search is that since it ignores the cost of the path so far to node n, it does not find optimal solutions.

Breadth-first search, uniform-cost search, and pure heuristic search are all special cases of a more general algorithm called best-first search. In each cycle of a best-first search, the node that is best according to some cost function is chosen for expansion. These best-first search algorithms differ only in their cost functions the depth of node n for breadth-first search, g(n) for uniform-cost search h(n) for pure heuristic search.

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