Constraint

A constraint is a restriction placed on the Sudoku puzzle. All constraints are derived from the Rule.

Sometimes, constraint is used as an alias for House.

Constraint Types

Cell Constraints

Each cell must contain a digit between 1 and 9.

Unit Constraints

Each row must contain one of each digit.

Each column must contain one of each digit.

Each box must contain one of each digit.

Background Information

Sudoku is a Constraint Satisfaction Problem (CSP). The grid must be completed in such a way that all constraints are satisfied. The size of each house matches the number of available digits. In a standard Sudoku, there are 9 digits and each house therefore contains 9 cells. With 27 houses, there are 243 unit constraints. In addition, there are 81 cell constraints, giving a total of 324 constraints.

Each of these 324 constraints has 9 possible candidates.