An example Friendly Cells puzzle: each shaded cell equals its own row, column or box number - 1, 2, 5, 3.
Friendly Cells Sudoku marks a handful of cells as "friendly". A friendly cell must hold a digit equal to one of its own coordinates - its row number, its column number, or its box number. Everything else is ordinary Sudoku. The marked cells are usually drawn with a heart; here they are shaded.
The rule: a cell that names its own place
Number the rows 1-9 top to bottom, the columns 1-9 left to right, and the boxes 1-9 left-to-right then top-to-bottom. A friendly cell must contain at least one of those three numbers. Below, the shaded cell sits in row 2, column 8, box 3 - so it can only be a 2, a 3 or an 8.
A friendly cell in row 2, column 8, box 3 is limited to {2, 3, 8} - its row, column and box numbers.
Why it helps
A friendly cell arrives with at most three candidates instead of nine, and often fewer once two of its coordinates coincide - a cell on the main diagonal, where row equals column, is down to two options before you place anything. Cells near a corner are the most restrictive, since their small row, column and box numbers overlap.
Solving strategy
- Pre-fill the candidates. Pencil in only the row, column and box numbers for each friendly cell - the rest are impossible there.
- Hunt the overlaps. When two of the three coordinates match, the cell has just two candidates; when all three match, it is solved outright.
- Combine with the lines. A friendly cell's short candidate list often collides with an ordinary row, column or box elimination to fix it exactly.
- Feed the grid. Each friendly digit placed behaves like a given for its row, column and box.
Common mistakes
- Requiring all three. The cell needs to match one of row, column or box - not all of them.
- Mis-numbering the boxes. Boxes run 1-9 left-to-right, top-to-bottom; get the box index right before trusting it.
- Marking unmarked cells. Only the specially marked cells are friendly; ordinary cells take any digit.
- Dropping ordinary Sudoku. Rows, columns and boxes still each need 1-9 once.
Can I play it here?
Not yet - Friendly Cells Sudoku isn't one of the puzzle types SudokuStreak generates for now. But you can learn its rules above, and there's plenty here to sink your teeth into meanwhile:
- Browse all the Sudoku variants we do offer.
- Or print free puzzles from the printable PDF page.
Where to play it
Friendly Cells Sudoku belongs to the constraint family and runs on 9×9. It is not one SudokuStreak generates yet - but the deduction it trains is the deduction every Sudoku runs on.
Difficulty and type are two separate dials here: the type sets the extra rule, the level sets how much of the grid you start with. Expert Sudoku leaves 25 of the 81 cells filled, where the beginner tools dry up almost entirely; Evil Sudoku leaves 23, with no easy moves at all. Take it at the gentler end while the pattern is still new, and move up once it stops being a fight.