The rules of sudoku

In brief

The grid has 81 cells, arranged in 9 rows, 9 columns and 9 boxes of 3 × 3 cells. Some cells are already filled in. You must fill in all the others with the numbers from 1 to 9, so that each row, each column and each box contains each number only once. The game is won when the grid is full with no repeated numbers.

The grid

Here is an Easy grid as the app presents it. The cells with a bluish background are the given numbers: they are part of the puzzle and cannot be changed. The thick lines separate the boxes.

5	empty	empty	empty	3	4	empty	6	9
4	2	6	8	empty	9	5	3	7
3	8	empty	6	5	7	empty	2	1
1	empty	5	empty	empty	3	2	empty	empty
8	3	7	empty	9	empty	empty	4	6
6	9	2	4	8	empty	7	5	empty
2	1	empty	empty	7	6	empty	8	empty
empty	5	empty	9	2	empty	6	empty	4
9	6	empty	empty	empty	empty	3	empty	empty

The rule

Each empty cell gets a number from 1 to 9.
A number cannot appear twice in the same row, the same column or the same box.
A complete row, column or box therefore contains each of the 9 numbers exactly once.

Each grid in the app has only one solution: you find it by reasoning, without guessing (see the level grids).

The end of the game

The app declares the grid solved as soon as all the cells are filled in with no repeats in any row, column or box. The timer stops, and the game is added to the statistics and to the Saves screen. There is no losing: you can take as long as you like, and you can pause the game by tapping the timer.

Playing in the app

Tap an empty cell, then a number on the keypad: the number is placed.
Notes switches to notes mode: the numbers you tap are written small in the cell, like pencil candidates. Placing a number clears the notes in its cell.
Erase empties the selected cell. Double-tapping a filled cell, within 400 milliseconds, also erases it.
There is no button to undo the last move.

Mistakes

The app never rejects a number and never compares it to the solution: it only checks whether the number is repeated in its row, column or box. Such a number is in conflict. When you place it, the cell shakes briefly.

With the "Show mistakes" setting, which is off by default, every cell in conflict stays colored red.
When the grid is full but not solved, an "Errors" banner appears over the grid and an Errors button shows up: it clears all the numbers in conflict at once.

The hint

The Hint button fills a randomly chosen empty cell with the correct number, not necessarily the selected cell. You get 5 per game; the number remaining is shown on the button. The revealed number is written in red and can no longer be changed.