## Intelligent Backtracking

# What is Intelligent Backtracking?

Performance of bruteforce backtracking can be improved by using a number of techniques such as variable ordering, value ordering, back jumping, and forward checking.

The order in which variables are instantiated can have a large effect on the size of the search tree. The idea of variable ordering is to order the variables form most constrained to least constrained. For example, if a variable has only a single value remaining that is consistent with the previously instantiated variable, it should be assigned that value immediately. In general, the variables should be instantiated in increasing order of the size of their remaining domains. This can either be done statically at the beginning of the search or dynamically, reordering the remaining variables each time a variable is assigned a new value.

The order in which the value of a given variable are chosen determines the order in which the tree is searched. Since it does not effect the size of the tree, it makes no difference if all solutions are to be found. If only a single solution is required, however, value ordering can decrease the time required to find a solution. In general, one should order the values from least constraining to most constraining in order to minimize the time required to find a first solution.

An important idea, originally called back jumping, is that when an impasse is reached, instead of simply undoing the last decision made, the decision that actually caused the failure should be modified. For example, consider the three-variable problem where the variables are instantiated in the order x,y,z. Assume that values have been chosen for both x and y, but that all possible values for z conflict with the value chosen for x. In chronological backtracking, the value chosen for y would be changed, and then all the possible values for z would be tested again, to no avail. A better strategy in this case is to go back to the source of the failure, and change the value of x before trying different values for y.

When a variable is assigned a value, the idea of forward checking is to check each remaining uninstantiated variable to make sure that there is at least one assignment for each of them that is consistent with the previous assignments. If not, the original variable is assigned its next value.

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