Hidden Single in a Row or Column
Hidden Single (line)
In a row or column, a digit has exactly one cell left where it can go.
Rule
In a row (column), digit d has only one valid cell → that cell is d.
Pick a row or column and a digit
Look at digit 4 in row 2. Row 2 has no 4 yet and 6 empty cells.
The idea
The same idea as the hidden single in a box, but the unit is a row or a column. For each empty cell in the row, ask two questions: does this cell's column already have d? Does its box already have d? If so, the cell is blocked.
It is a little harder to see because the blocking rays come from two directions: along columns (across the row) and from boxes. It is especially useful when a row or column has only 3–4 empty cells left.
How to spot it
- Pick a row or column with few empty cells, list the missing digits, and try each one.
- For each missing digit, mark which cells are blocked by their column or their box.
Common mistakes
- Checking only the column and forgetting the box: a d in the same box blocks the cell too.
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Related techniques
References
The examples on this page are generated from random puzzles, not taken from these sources.