Single Candidate Technique: The Foundation

Master the single candidate technique, the most fundamental and commonly used solving method in Sudoku puzzles.

What is the Single Candidate Technique?

The single candidate technique (also known as "naked single") is the most basic solving method in Sudoku. It occurs when a cell has only one possible number that can be placed in it. This happens when all other numbers 1-9 are already present in the same row, column, or 3×3 box.

How to Identify Single Candidates

To find single candidates, follow this systematic approach:

Example: Finding a Single Candidate

Let's look at a practical example:

In this example, let's look at the empty cell in row 1, column 3. The row contains: 5, 3, 7. The column contains: 6, 9, 8, 4, 7, 6, 2, 4. The box contains: 5, 3, 6, 9, 8. The missing number is 1, so 1 must go in this cell.

When to Use Single Candidates

Single candidates are most useful in these situations:

• Early in the Puzzle: When many numbers are already placed
• After Each Placement: Always check for new single candidates after placing a number
• When Stuck: When you can't find other obvious moves
• Systematic Solving: As part of a regular solving routine

Systematic Search Method

To efficiently find single candidates, use this systematic approach:

1. Row by Row: Check each row for empty cells and their possible numbers
2. Column by Column: Check each column for empty cells and their possible numbers
3. Box by Box: Check each 3×3 box for empty cells and their possible numbers
4. Repeat: Continue until no more single candidates are found

Common Mistakes to Avoid

Beginners often make these errors when using single candidates:

• Incomplete Checking: Not checking all three units (row, column, box)
• Missing Numbers: Overlooking numbers that are already present
• Rushing: Not taking time to verify the logic
• Forgetting Updates: Not checking for new single candidates after placements

Advanced Applications

Single candidates can be combined with other techniques:

• With Pencil Marks: Use pencil marks to make single candidates easier to spot
• With Hidden Singles: Single candidates may reveal hidden singles
• With Naked Pairs: Eliminations may create new single candidates
• With Advanced Techniques: Single candidates are the foundation for more complex methods

Practice Strategies

To improve your single candidate skills:

1. Start with Easy Puzzles: Practice on puzzles with many obvious placements
2. Be Systematic: Use a consistent method for checking cells
3. Practice Speed: Work on finding single candidates quickly
4. Use Pencil Marks: Mark candidates to make single candidates more visible
5. Regular Practice: Solve puzzles regularly to develop pattern recognition

Single Candidates vs. Hidden Singles

It's important to understand the difference:

Single candidates are the most basic technique and should be mastered before moving on to hidden singles and other advanced methods.

The single candidate technique is the cornerstone of Sudoku solving. By mastering this fundamental method, you'll build a solid foundation for learning more advanced techniques and become a more efficient solver.