Naked single
A cell with only one digit left that fits. It sounds like the same thing as a hidden single and it is the mirror image: there you look at a digit and find its cell, here you look at a cell and find its digit.
How to spot one
- Pick an empty cell and list the digits that are still legal in it.
- A digit is illegal if it already appears in that cell's row, column or box.
- If only one digit survives, the cell has to be that digit.
A real example
This grid is not a drawing. It came out of our generator, and the position below is one a solver actually reaches: every digit the reasoning uses is on the board, so you can check it yourself without taking anything on trust. The cells it looks at are green; the cells it changes are amber.
Check it: in r1c2 the digits still legal are 5. Everything else already appears in that cell's row, column or box.
The same move on a different grid
One example can be a fluke of the grid it sits in, so here is another, dug at
medium from the seed second:naked-single:100 with
29 starting digits. The technique fires in a different place, for a
different digit, and the point is what it buys you afterwards.
What the solver does next, from exactly that position:
- hidden single — In row 3, 1 has only one cell left it can go in. r3c9 takes 1, the only digit it had left.
- hidden single — In row 4, 1 has only one cell left it can go in. r4c2 takes 1, the only digit it had left.
- hidden single — In row 1, 1 has only one cell left it can go in. r1c1 takes 1, the only digit it had left.
That is the pay-off a naked single is worth: not the one cell it names, but the run of cheap moves it unlocks behind it.
Where you meet it
Our grids are graded by the hardest technique they force, so a naked single is what makes a medium grid what it is. Pencil marks start to pay off. When you ask for a hint on one of them, the first thing the game says is the name of the technique and which row, column or box to use it in, and that step costs nothing — you still get to find the cell yourself.
The other techniques
Ready to use it? Play medium sudoku, take on the daily grid, or start from how to play.