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The topic of search has been at the heart of AI since GPS. Some fundamental search algorithms were generate and test, branch and bound, the A* algorithm, iterative deepening, and tree search guided by the global problem structure, or by information elicited during search, or by intelligent backtracking.

The contribution of constraint programming is to allow the end user to control the search, combining generic techniques and problem-specific heuristics. A nice illustration of this is the n-queens problems: how to place n queens on an nXn chess board, so that no queens can take each other. For n=8, depth-first generate-and-test with backtracking is quite sufficient, finding a solution in a few seconds. However, when n=16, it is necessary to interleave constraint propagation with the search so as to obtain a solution quickly. However when n=32 it is necessary to add more intelligence to the search algorithm. A very general technique is to choose as the next variable to label the one with the smallest domain - i.e. the smallest number of possible values. This is called first-fail. It is particularly effective in conjunction with constraint propagation, which reduces the size of the domains of the unlabelled variables (as described for arc-consistency above), For n queens, the first-fail technique works very well, yielding a solution within a second. Unfortunately even first-fail doesn't scale up beyond n=70. However there is a problem-specific heuristic which starts by placing queens in the middle of the board, and then moving outwards. With the combination of depth-first search, interleaved with constraint propagation, using the first-fail ordering for choosing which queen to place next, and placing queens in the centre of the board first, the 70-queens problem is solved within a second, and the algorithm scales up easily to 200 queensgif.

Mark Wallace
Wed Sep 3 18:36:40 BST 1997