Grafite Sudoku
The hidden single in sudoku
The hidden single is the most used sudoku technique of all. The idea: instead of asking which number goes in a cell, ask where a given number can go inside a row, a column or a box. If there is only one possible spot, the number goes there.
It is called "hidden" because the cell may still have many candidates: you would not see it by looking at the cell. You see it by looking at the number.
How to find it
- Pick a box and a number it does not contain yet, for example 7.
- For every 7 already in the grid, follow its row and its column: wherever they cross the box, 7 cannot go.
- Also rule out the cells already filled with other numbers.
- If only one free cell is left, you found the hidden single.
In the example, the 7s in rows 2 and 3 and in columns 2 and 3 rule out almost the whole top-left box: the 7 can only go in row 1, column 1.
When to use it
All the time: at the start of every puzzle and every time you write a new number, since each number you place takes spots away from others. In easy sudoku it is enough on its own to finish.
In the app: "The hidden single" lesson in the Sudoku Academy
In the Sudoku Academy of Grafite Sudoku this technique is an interactive lesson on the grid, with exercises from real puzzles.