Hidden Single

The other basic technique. Together with naked singles, it solves every Easy and Medium puzzle on puzuru.

What is a hidden single?

A hidden single is a number that has only one possible cell left inside a row, a column or a 3x3 box. The cell itself may still have several candidates. That is why it is called hidden: if you only look at the cell, you won't see it. If you look at the number, it is obvious.

the only cell where the number fits the box we are looking at

Example from a real puzuru puzzle. Rows and columns are numbered 1 to 9 from the top left.

The example

Look at box 1 (highlighted). Ask: where can a 2 go in this box? Every other empty cell in the box shares a row or a column with a 2 that is already on the grid, so a 2 can't go there.

Only one cell is left: row 2, column 2 (marked ?). So that cell must be 2, even though, judged on its own, it could still hold other numbers.

How to find hidden singles fast

Pick one number, say 1, and sweep it across all nine boxes. In each box, cross off cells that see a 1 in the same row or column. If one cell is left, place the 1.

Then do the same with 2, 3 and so on up to 9. Going through the numbers in order makes sure you don't skip any.

Start with numbers that already appear many times on the grid. They block the most cells, so they give the most hidden singles.

Hidden single vs naked single

A naked single is the opposite view: a cell where only one number is possible after removing everything in its row, column and box.

Both are basic techniques. Together they solve every Easy and Medium puzzle on puzuru, and they are always the first thing to try in harder puzzles too.

Practice on Medium puzzles →

← Naked Single　Next: Naked Pair →

FAQ

Is a hidden single the same as a naked single?

No. A naked single is a cell with only one possible number. A hidden single is a number with only one possible cell in a row, column or box.

Do I need notes to find hidden singles?

No. You can find them by scanning one number at a time. Notes help in harder puzzles, but hidden singles work without them.

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