Hidden Single

Easy

Description

Select a 3x3 block and a digit. Check how many cells in that block can contain the selected digit. If there is only one such cell, you can fill that cell with the selected digit. The same logic is applicable for rows and columns.

A cell can contain a digit if there is no such digit in the same row, column and 3x3 block.

Important: all Easy puzzles at Sudoku Mood can be solved only by looking for Hidden Singles in blocks. No need to look for Hidden Singles in rows or columns.

In the first example below there is only one cell in block 5 where digit 8 is allowed. In the second example digit 2 in column 7 can be placed in only one cell.

Example 1.
Example Sudoku 1 Example Sudoku 1
Example 2.
Example Sudoku 2 Example Sudoku 2

Demo

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How to find Hidden Singles

Select a digit. Then look at every block and check how many cells in that block can contain the selected digit. If you haven't found a Hidden Single, move on and check another digit. Then another - until you check all digits. If you found a Hidden Single in some block and placed it, recheck all other blocks again - new Hidden Singles might have appeared after you placed a digit on the grid. Moreover, you need to recheck all other digits as well. For instance, it might be that you checked digit 1 and haven't found any Hidden Singles in any blocks. Then you checked digit 2 and found a Hidden Single in some block. Now it may be that in that block there is only one cell left where digit 1 can be placed. So you need to recheck digit 1 again. And all other digits too.

The same works for rows and columns.

There are various strategies for finding Hidden Singles. For example:

  • First you look for Hidden Singles in blocks only. After you checked all digits and all blocks, start looking for Hidden Singles in rows and/or columns.
  • Alternatively, once you selected a digit, you may decide to check not only blocks, but also rows and columns.
  • Or you can even choose a row, column or block first and then check how many cells in the selected group can contain each digit.

It's worth noticing, that looking for Hidden Singles in blocks is typically easier and less time consuming than looking for Hidden Singles in rows or columns.

How Hidden Single is different from Naked Single

Naked Single is another simple yet powerful technique for solving Sudoku.

However, the difference between Hidden Single and Naked Single techniques is a typical source of confusion for beginner Sudoku players. Try to remember:

  • Hidden Single is when there is only one possible cell in a row, column or block, where the digit can be placed.
  • Naked Single is when there is only one allowed digit in a cell.

Try it yourself

See also