Sudoku is a number-grid puzzle that builds patience, pattern recognition, and logical thinking. The goal is to fill a 9 by 9 grid so that every row, column, and 3 by 3 box contains the digits 1 through 9 exactly once. Even though numbers are used, Sudoku is not about arithmetic.
It is about organizing possibilities and making careful deductions.
Key Facts
- A standard Sudoku grid has 9 rows, 9 columns, and nine 3 by 3 boxes.
- Each row must contain the digits 1 through 9 exactly once.
- Each column must contain the digits 1 through 9 exactly once.
- Each 3 by 3 box must contain the digits 1 through 9 exactly once.
- Candidate set for a cell = {1,2,3,4,5,6,7,8,9} minus digits already in its row, column, and box.
- A naked single occurs when a cell has exactly one possible candidate left.
Vocabulary
- Cell
- A cell is one small square in the Sudoku grid where a single digit from 1 to 9 can be placed.
- Row
- A row is a horizontal line of 9 cells that must contain each digit from 1 to 9 exactly once.
- Column
- A column is a vertical line of 9 cells that must contain each digit from 1 to 9 exactly once.
- Box
- A box is a 3 by 3 region of the grid that must contain each digit from 1 to 9 exactly once.
- Candidate
- A candidate is a digit that could legally fit in an empty cell based on the current row, column, and box.
Common Mistakes to Avoid
- Guessing too early is a mistake because one wrong digit can spread through the puzzle and make later logic impossible.
- Checking only the row is a mistake because every placement must also fit the column and the 3 by 3 box.
- Forgetting to update pencil marks is a mistake because old candidates can hide the next logical move.
- Placing a repeated digit in a box is a mistake because each 3 by 3 box must contain the digits 1 through 9 only once.
Practice Questions
- 1 In a Sudoku row, the digits shown are 1, 2, 3, 4, 6, 7, 8, and 9. What digit must fill the empty cell?
- 2 An empty cell is in a row containing 2, 5, and 9, a column containing 1, 3, and 8, and a 3 by 3 box containing 4 and 7. What is the only possible digit for the cell?
- 3 A cell has candidates 2 and 6. Later, a 6 is placed in the same column. Explain why the cell must now be 2.