All Sudoku Variants
9×9 · Constraint

Friendly Cells Sudoku

Each specially marked cell must hold a digit equal to its own row number, its column number, or its box number.

1 2 5 3

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.

238

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: