How to start a Sudoku: make your first move with logic
Choose a row, column, or 3×3 box that is nearly full. Find the missing digits first. Then check each empty cell’s crossing row or column before you place anything.
Definition
Start a Sudoku by finding what is missing, then checking the crossing row, column, and 3×3 box.
One visible proof
This repeated check is called scanning: find what is missing, then rule out the places where it cannot go.
- 1. Find what is missingRow 1 is missing 2 and 7.
- 2. Check the crossing columnColumn 3 already contains 2, so the first empty cell cannot contain 2.
- 3. Place only what is provedThe first empty cell must contain 7. The other empty cell then contains 2.
5364891
The row needs 2 and 7. The crossing column rules out 2, so the outlined cell must contain 7.
Editorial method
A reference verified by Doku.su
Doku.su checks every placement or elimination in this example with its deterministic Sudoku engine. Illustrative candidates are labelled when they do not represent a complete puzzle state.
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