Lesson 10 - Module 4 Constraint satisfaction problems...

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Module 4 Constraint satisfaction problems Version 1 CSE IIT, Kharagpur
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Lesson 10 Constraint satisfaction problems - II Version 1 CSE IIT, Kharagpur
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4.5 Variable and Value Ordering A search algorithm for constraint satisfaction requires the order in which variables are to be considered to be specified as well as the order in which the values are assigned to the variable on backtracking. Choosing the right order of variables (and values) can noticeably improve the efficiency of constraint satisfaction. 4.5.1 Variable Ordering Experiments and analysis of several researchers have shown that the ordering in which variables are chosen for instantiation can have substantial impact on the complexity of backtrack search. The ordering may be either a static ordering , in which the order of the variables is specified before the search begins, and it is not changed thereafter, or a dynamic ordering , in which the choice of next variable to be considered at any point depends on the current state of the search. Dynamic ordering is not feasible for all search algorithms, e.g., with simple backtracking there is no extra information available during the search that could be used to make a different choice of ordering from the initial ordering. However, with forward checking, the current state includes the domains of the variables as they have been pruned by the current set of instantiations, and so it is possible to base the choice of next variable on this information. Several heuristics have been developed and analyzed for selecting variable ordering. The most common one is based on the " first-fail " principle, which can be explained as " To succeed, try first where you are most likely to fail ." In this method, the variable with the fewest possible remaining alternatives is selected for instantiation. Thus the order of variable instantiations is, in general, different in different branches of the tree, and is determined dynamically. This method is based on assumption that any value is equally likely to participate in a solution, so that the more values there are, the more likely it is that one of them will be a successful one. The first-fail principle may seem slightly misleading, after all, we do not want to fail. The reason is that if the current partial solution does not lead to a complete solution, then the sooner we discover this the better. Hence encouraging early failure, if failure is inevitable, is beneficial in the long term. On the other end, if the current partial solution can be extended to a complete solution, then every remaining variable must be instantiated and the one with smallest domain is likely to be the most difficult to find a value for (instantiating other variables first may further reduce its domain and lead to a failure). Hence the principle could equally well be stated as: " Deal with hard cases first: they can only get more difficult if you put them off. " Version 1 CSE IIT, Kharagpur
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This note was uploaded on 09/20/2010 for the course MCA DEPART 501 taught by Professor Hemant during the Fall '10 term at Institute of Computer Technology College.

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Lesson 10 - Module 4 Constraint satisfaction problems...

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