Hidden Single
The hidden single is the move that solves most of every Sudoku. It is a digit with only one place left in a row, column or box, and the way to find it is scanning.
Ninesmith Sudoku ·
In short
- Pick a digit and a box, row or column that doesn't have it yet.
- Rule out every empty cell that already sees that digit in its row, column or box.
- If one cell is left, the digit goes there, whatever else that cell could hold.
An example
The cell at row 3, column 5 could still take 4, 8 or 9 by its own row, column and box. That is why the single is hidden: looking at the cell alone shows three options. Looking at the digit shows only one place.
Rows count from the top, columns from the left, and boxes in reading order, so box 2 is the top middle. You can play this puzzle on the board on the home page.
How to scan for it
- Start with the digit the board has most of. It shuts the most cells.
- Go through every box that doesn't have it yet, then every row and column.
- When you place one, look at that digit again: the new copy shuts more cells.
- Move on to the next digit, and come back round when you reach 9.
With notes written down, a hidden single is a digit that appears in the notes of only one cell in a house.
Hidden single or naked single?
A naked single starts from a cell: it sees eight different digits, so it can only take the ninth. A hidden single starts from a digit: it has one place left in a house. Scanning finds hidden singles; looking at crowded cells finds naked singles. Most puzzles need both.
Common mistakes
- Scanning boxes only. A digit with two places in a box can have one place in a row or column.
- Forgetting the cell's own box. A copy of the digit anywhere in the box rules the whole box out.
- Placing what fits. The digit must have no other place, not merely fit here.
Questions
What is a hidden single in Sudoku?
A digit that has only one possible cell left in a row, column or box. It must go there, even if that cell could take other digits.
Why is it called hidden?
Because the cell itself still shows several candidates. The single only appears when you look at where the digit can go.
Is a hidden single the same as scanning?
Scanning is how you find hidden singles: one digit at a time, through every box, row and column.
Next: Naked Pair · all techniques
Practise singles with the Smith
Ninesmith Sudoku is a calm Sudoku app for iPhone. The Apprenticeship starts from the one rule, the Pattern Trainer drills singles in sessions of 30 to 90 seconds, and when you're stuck the Smith shows where to look before anything goes in.
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