Sudoku rules
A sudoku puzzle consists of a 9x9 grid, with a total of 81 cells.
A cell can be empty, or filled with a "digit". This digit can be
any digit between 1 and 9.
Rows, Columns and Boxes
The grid can be divided into 9 "rows", each containing 9 cells.
These rows are numbered 1 through 9 from top to bottom.
The grid can also be divided into 9 "columns", each containing
9 cells. These columns are numbered 1 through 9 from left to right.
A third method to divide the grid is in 3x3 boxes, each containing 9 cells.
The Rule
The rules of Sudoku are that you should fill a number in to every cell in
the grid, using the numbers 1 to 9. The restriction is that you can only use
each number once in each row, each column, and in each of the 3x3 boxes.
Coordinates
Rows and columns are used as the coordinates of the cells in the grid. A
cell at row 4, column 5 will be addressed as R4C5.
The Givens
A number of digits are already in place. These are the "givens".
These givens are placed in such a way that there is only a single solution
to the grid. You are not allowed to remove or change them, because they are
part of the puzzle.
Currently, there is a search going on in the Sudoku community for the minimum
number of givens that a Sudoku puzzle can have. At the moment 17 is the minimum
so far.
The number of givens does not reflect the difficulty of the puzzle.The placement
of the givens and the information that you can extract from them is more important
than the sheer quantity of givens.
The Japanese publisher Nikoli limits its Sudokus to a maximum of 32 givens.
One can conclude, however, that a Sudoku with more than 40 givens is probably
not a very difficult one.
