Sudoku conflicts: find repeated digits without blaming the wrong cell
A Sudoku conflict is a repeated placed digit in a row, column or 3 × 3 box. At least one entry in that repeated group must change for the position to be valid. The conflict alone does not identify which entry was copied or played incorrectly.
The worked example
Read coordinates from the top left: r = row, c = column. Small digits are candidates. Outlines identify the cells discussed in the caption.
Open this example in the solverView the exact 81-cell input
Read left to right across each row, then top to bottom. A 0 means a blank.
223456789456789123789123456234567891567891234891234567345678912678912345912345678Trace one changed digit through three units
This teaching board was made by changing r1c1 from 1 to 2 in a completed grid. We know that history because we constructed the example. When checking a board someone else supplies, the repeated digits alone do not reveal that history.
There are three affected cells, not six separate wrong entries. A cell can participate in more than one conflict, and the same pair can conflict through both a row and a box. A good checker marks all involved cells without claiming that all are incorrect.
| Unit | Repeated digit | Positions |
|---|---|---|
| Row 1 | 2 | r1c1 and r1c2 |
| Column 1 | 2 | r1c1 and r4c1 |
| Top-left box | 2 | r1c1 and r1c2 |
Repair the position using the source
Start with the distinction between givens and your own entries. Givens define the puzzle; your moves are deductions made from them. If a move clashes with a correctly copied given, undo the move. If two givens clash, compare both with the original puzzle before changing either.
Step 1. Locate the repeated digit and name its unit, such as ‘two 2s in column 1’.
Step 2. Compare the affected cells with the original printed or saved puzzle.
Step 3. Correct a verified transcription error, or undo the unsupported player entry.
Step 4. Recheck the entire board. Removing one duplicate does not establish that the remaining puzzle has a solution.
No duplicate does not mean no mistake
A wrong digit can satisfy the three immediate constraints and still block every completion. A conflict checker will not detect that situation by looking for repeats. A solution search is needed to classify the complete position, or a logical contradiction may expose it during play.
Conversely, a repeated digit is already sufficient to reject a position under standard Sudoku rules. There is no need to search all possible completions first. The useful order is to check format, find immediate conflicts, then check whether a conflict-free position has zero, one or multiple solutions.
Repeated pencil marks are normal
Several empty cells in the same row can contain a small candidate 2. These are possibilities, not simultaneous placements, so they do not violate the rules. A conflict applies to large, placed digits.
Before pasting a puzzle into the solver, include only placed digits. Encode each blank as 0 and read rows from left to right, top to bottom. Do not paste a list of pencil marks as if each note were a cell value.
Check your understanding
If you erase the 2 at r1c2 in this example, have you repaired the board?
Show the reasoning
No. It removes that row and box duplicate, but r1c1 and r4c1 still contain 2 in column 1. The known construction tells us the actual change was at r1c1. On an unfamiliar puzzle, verify the original givens instead of selecting a highlighted cell at random.
About this example
This example was constructed for Sudoku Slate from a completed grid. Many givens are retained to make the specific deduction easy to inspect. Diagnostic examples are deliberately conflicting or ambiguous where stated; these are teaching positions, not difficulty-rated game puzzles.
Rules and terminology
- Nikoli: Sudoku rules
The standard row, column and 3 × 3 box rules used in these examples. The board and worked explanation on this page were created separately.