Properties
IDDFS combines depth-first search's space-efficiency and breadth-first search's completeness (when the branching factor is finite). It is optimal when the path cost is a non-decreasing function of the depth of the node.
The space complexity of IDDFS is, where is the branching factor and is the depth of shallowest goal. Since iterative deepening visits states multiple times, it may seem wasteful, but it turns out to be not so costly, since in a tree most of the nodes are in the bottom level, so it does not matter much if the upper levels are visited multiple times.
The main advantage of IDDFS in game tree searching is that the earlier searches tend to improve the commonly used heuristics, such as the killer heuristic and alpha-beta pruning, so that a more accurate estimate of the score of various nodes at the final depth search can occur, and the search completes more quickly since it is done in a better order. For example, alpha-beta pruning is most efficient if it searches the best moves first.
A second advantage is the responsiveness of the algorithm. Because early iterations use small values for, they execute extremely quickly. This allows the algorithm to supply early indications of the result almost immediately, followed by refinements as increases. When used in an interactive setting, such as in a chess-playing program, this facility allows the program to play at any time with the current best move found in the search it has completed so far. This is not possible with a traditional depth-first search.
The time complexity of IDDFS in well-balanced trees works out to be the same as Depth-first search: .
In an iterative deepening search, the nodes on the bottom level are expanded once, those on the next to bottom level are expanded twice, and so on, up to the root of the search tree, which is expanded times. So the total number of expansions in an iterative deepening search is
For and the number is
- 6 + 50 + 400 + 3,000 + 20,000 + 100,000 = 123,456
All together, an iterative deepening search from depth 1 to depth expands only about 11% more nodes than a single breadth-first or depth-limited search to depth, when . The higher the branching factor, the lower the overhead of repeatedly expanded states, but even when the branching factor is 2, iterative deepening search only takes about twice as long as a complete breadth-first search. This means that the time complexity of iterative deepening is still, and the space complexity is like a regular depth-first search. In general, iterative deepening is the preferred search method when there is a large search space and the depth of the solution is not known.
Read more about this topic: Iterative Deepening Depth-first Search
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