ifs.heuristics
Interface VariableSelection

All Known Implementing Classes:

DbtVariableSelection, GeneralVariableSelection, LectureSelection

public interface VariableSelection

Variable selection criterion.

The IFS algorithm requires a function that selects a variable to be (re)assigned during the current iteration step. This problem is equivalent to a variable selection criterion in constraint programming. There are several guidelines for selecting a variable. In local search, the variable participating in the largest number of violations is usually selected first. In backtracking-based algorithms, the first-fail principle is often used, i.e., a variable whose instantiation is most complicated is selected first. This could be the variable involved in the largest set of constraints or the variable with the smallest domain, etc.

We can split the variable selection criterion into two cases. If some variables remain unassigned, the “worst” variable among them is selected, i.e., first-fail principle is applied. This may, for example, be the variable with the smallest domain or with the highest number of hard and/or soft constraints.

The second case occurs when all variables are assigned. Because the algorithm does not need to stop when a complete feasible solution is found, the variable selection criterion for such case has to be considered as well. Here all variables are assigned but the solution is not good enough, e.g., in the sense of violated soft constraints. We choose a variable whose change of a value can introduce the best improvement of the solution. It may, for example, be a variable whose value violates the highest number of soft constraints.

It is possible for the solution to become incomplete again after such an iteration because a value which is not consistent with all hard constraints can be selected in the value selection criterion. This can also be taken into account in the variable selection heuristics.

Version:

1.0

Author:

Tomáš Müller

See Also:

Solver

Method Summary

void	init(Solver solver) Initialization
Variable	selectVariable(Solution solution) Variable selection

Method Detail

init

void init(Solver solver)

Initialization

selectVariable

Variable selectVariable(Solution solution)

Variable selection

Parameters:

solution - current solution