Look-ahead (backtracking)
（重定向自Forward checking）
![In this example, x1=2 and the tentative assignment x2=1 is considered.](/Images/godic/202501/14/Forward-arc-0.svg5946.png")
![Forward checking only checks whether each of the unassigned variables x3 and x4 is consistent with the partial assignment, removing the value 2 from their domains.](/Images/godic/202501/14/Forward-arc-1.svg5946.png")
![Arc consistency look ahead also checks whether the values of x3 and x4 are consistent with each other (red lines) removing also the value 1 from their domains.](/Images/godic/202501/14/Forward-arc-2.svg5946.png")
In backtracking algorithms, look ahead is the generic term for a subprocedure that attempts to foresee the effects of choosing a branching variable to evaluate or one of its values. The two main aims of look-ahead are to choose a variable to evaluate next and the order of values to assign to it.