SudokuPlay
Rules of Sudoku
To play Sudoku, follow these steps:
​
1. **Understand the grid**: Sudoku is played on a 9x9 grid, which is divided into nine 3x3 subgrids, called squares. The puzzle will start with some numbers already filled in.
​
2. **Goal**: The aim is to fill in the remaining empty cells with numbers from 1 to 9.
​
3. **Basic rules**:
- Each row must contain the numbers 1 to 9 without any repetition.
- Each column must also contain the numbers 1 to 9 without repeating.
- Each 3x3 square (subgrid) must have the numbers 1 to 9, without duplicates.
The common term used for rows, columns and squares is "unit".
​
4. **Look for clues**: Begin by looking at units (rows, columns, or squares) that already have many numbers filled in. Use these as clues to figure out where missing numbers should go.
​
5. **Process of elimination**: For each empty cell, consider which numbers are missing in the row, column, and square it belongs to. Eliminate numbers that would cause duplicates, and narrow down the possibilities.
​
6. **Pencil in possible numbers**: For trickier cells, you can enter possible numbers and refine your options as you fill in other parts of the grid. To edit a cell in SudokuPlay, tap it, then tap the edit box that comes up to display the cursor and keyboard. Add and delete possibles according to the rules.
​
7. **Use logic, not guessing**: Keep solving by using logical reasoning to place each number. Guessing can lead to mistakes, so it's best to rely on deductive reasoning. However, many puzzles at some point give no clues as to what numbers go where.
​
8. **Complete the puzzle**: Continue this process until every cell is filled, with each row, column, and square containing the numbers 1 to 9, following the game's rules.
That’s how you play Sudoku! The difficulty varies depending on how many numbers are provided at the start.
In the example above there are 81 cells. The first cell is cell 0. Row, column and square numbers go from zero to 8. So, a cell is referred to by its cell number or by its row and column numbers.
​
Considering only the large, black numbers (pretend the green numbers aren’t there!), there aren’t any cells that can be filled in immediately. So, find a unit that has almost all numbers filled in, pick an empty cell in the unit and scan the cell’s row, column and square to see what numbers can be placed there. In the example in column 5, row 3, only a 4 can be entered without repeating a number. The 4 is coloured green to show we have just entered it.
​
Next, look at square #1. We know the square needs a 3 and that 3’s occupy rows and columns that pass through the square. Follow these rows and columns into the square and you’ll see that a 3 can only go in cell 5 (that’s row 0, column 5). That 3 is coloured green in the example.
​
Now check out column 4. It needs a 1 and a 4. Square 4 already has a 4 that was placed during play, so the empty cell in column 4 that is also in square 4 must be a 1. The remaining empty cell must be a 4. Those are coloured green as well to show that we just added them.
​
In puzzles that don’t yield many clues, one approach is to fill in all of the numbers that are possible for each cell. Studying the possibles may show easy wins and may reveal patterns in the numbers that can be used to delete some of the possibles from some cells. Notice the small (green) numbers in the cells in row 8 (the last row). Cell 73 (row 8, column 1) can only contain a 4 and a 6. Same with cell 79 (row 8, column 7). When one of the cells is a 4 the other is a 6, and vice versa. But that means that 4 and 6 cannot go anywhere else in row 8. So, when filling in the possibles for the other blank cells in row 8 we can leave out 4 and 6, or we can remove them if we’ve already filled in the possibles. This leaves fewer possibles in these cells and often results in the solution (a win) for some cells.
​
The example above has 33 cells filled (given) in at the start of the game and is an easy puzzle. As the number of given cells decreases it becomes harder to solve the puzzle. As well, random chance just makes some puzzles very difficult to solve.