Should You Turn On Auto Candidates in Sudoku?
Auto-candidates is a digital Sudoku feature that instantly fills every empty cell with all digits that do not yet conflict with given numbers in the same row, column, or 3x3 box. This automation replaces the manual process of [pencil marking](/blog/what-is-pencil-marking-in-sudoku) candidates. While it can dramatically speed up solving for many players, it also conceals the fundamental scanning and elimination steps that are the bedrock of Sudoku logic. Understanding what this tool does, its benefits, and its drawbacks is key to using it effectively without hindering your development as a solver.
What Exactly Does the Auto-Candidate Feature Compute?
The auto-candidate algorithm performs a simple but exhaustive scan of the entire grid. For each empty cell, it checks the row, column, and 3x3 box to see which digits from 1 to 9 are already placed. Any digit that appears in these three 'units' is ruled out as a candidate for that cell. The feature then fills the cell with the remaining digits that are not yet present, creating a complete set of pencil marks.
This computation happens instantly and updates in real-time. Whenever you place a new digit, the algorithm immediately recalculates all affected candidates, removing the placed digit from the candidate lists in the same row, column, and box. This dynamic update maintains a perfectly accurate candidate picture at all times, which is its primary mechanical advantage.
Why Does It Make Easy and Medium Puzzles Faster?
Auto-candidates accelerate solving by eliminating the tedious manual work of candidate notation. In puzzles rated Easy or Medium, a significant portion of the solve path often involves placing Naked Singles—cells where only one candidate remains. With auto-candidates enabled, these singles become visually obvious because the cell will display only one tiny digit.
The solver no longer needs to slowly scan each row, column, and box to mentally eliminate possibilities. Instead, they can quickly scan the grid for cells with a single candidate and place the digit. This turns a process of active deduction into one of rapid pattern recognition. For puzzles whose entire solution can be found through consecutive Naked Singles, the speed increase is most dramatic, as the tool perfectly reveals the next logical move at every step without any cognitive effort from the player.
How Does It Hide the Reasoning Beginners Need to Practice?
The core skill in Sudoku is not placing digits but learning how to find them through systematic elimination. Auto-candidates performs this elimination for you, which can create a dependency that stunts skill development. Beginners who rely on the feature may never internalize the essential scanning technique of looking across a row, down a column, and within a box to see what's missing.
This automatic process skips the fundamental 'why' behind each placement. A player might see a cell with only the candidate '5' and place it, but they haven't practiced identifying that the '5' was eliminated from all other cells in the unit by other given digits. Practicing manual pencil marking forces you to engage with this logic, strengthening your mental model of the puzzle's constraints and preparing you for more advanced techniques that build on these basics.
How Do Auto-Candidates Interact with Manual Corner Marks?
Most Sudoku apps allow you to use both systems simultaneously. You can have auto-candidates populate the small 'center marks' in each cell while you manually add your own 'corner marks' for personal notation or advanced strategy work. This hybrid approach is common among intermediate solvers.
The key interaction is that auto-candidates are the authoritative source for basic candidate elimination. Your manual corner marks should not contradict them. For instance, you might use corner marks to note a potential Naked Pair you've spotted, while the auto-candidates show all remaining possibilities. It's crucial to remember that the auto-candidate list is dynamic; if you place a digit based on your corner mark analysis, the auto-candidates will immediately update, possibly invalidating your other manual notes. This can lead to note overload and confusion if you're not careful.
What's a Simple Rule for When to Switch It On or Off?
A practical, skill-based rule is this: Use auto-candidates when your goal is relaxed, speedy solving of easier puzzles. Turn it off when your goal is to learn, practice, or tackle harder puzzles that require deep logical deduction.
For learning and practice, especially for beginners, keeping the feature off is advisable. It forces you to build the essential scanning muscle. As you progress to Medium puzzles, you might turn it on to save time, but periodically switch it off to ensure your core skills remain sharp. For Hard and Expert puzzles, many advanced techniques like X-Wings or Swordfish require you to think in terms of candidate positions and patterns. Auto-candidates can create a cluttered visual field that makes these patterns harder, not easier, to spot. In these cases, disciplined manual notation is often the more powerful tool.
- Beginners: Keep it OFF to learn fundamentals.
- Intermediate players: Use it ON for speed on Easy/Medium, OFF for practice.
- Advanced players: Use it OFF for complex puzzles where pattern spotting is key.
Key Facts
- ▪Auto-candidates automatically fill each empty cell with every digit (1-9) not already present in that cell's row, column, or 3x3 box.
- ▪The feature updates in real-time, removing candidates as you place digits to always reflect the current puzzle state.
- ▪It significantly speeds up Easy and Medium puzzles by making Naked Singles (cells with one candidate) visually obvious.
- ▪Relying on auto-candidates can prevent beginners from learning the essential scanning and elimination skills required for Sudoku.
- ▪Most apps allow manual corner marks and auto center marks to be used simultaneously, but auto-candidates are the authoritative source for basic elimination.
- ▪A good rule is to use auto-candidates for relaxed, fast solving of easier puzzles, but turn it off when practicing fundamentals or solving harder puzzles.
- ▪In puzzles solved entirely by Naked Singles, auto-candidates reveal the complete solve path without any deduction from the player.
- ▪For advanced techniques like X-Wing or Swordfish, a cluttered auto-candidate display can sometimes hinder pattern recognition.
Frequently Asked Questions
Do auto-candidates make the puzzle easier to solve?
They make the mechanical process faster by showing possibilities, but they don't solve the logic for you. For a true beginner, they can make the puzzle harder to learn from.
Can auto-candidates make a mistake?
No. They algorithmically show all valid candidates based on current placements. Any error would stem from an incorrect digit placement by the player.
Should I use auto-candidates as a beginner?
It's not recommended initially. Learning manual pencil marking builds critical scanning skills that auto-candidates bypass.
Do professional Sudoku solvers use auto-candidates?
In official competitions, players use pencil and paper. They rely on their own notation skills, not automation, which demonstrates the value of practicing without it.
How do I avoid clutter with auto-candidates on?
Focus on finding cells with only one candidate (Naked Singles). Ignore the multi-candidate cells until placements create new singles.
Can I turn auto-candidates on mid-puzzle?
Yes, in most apps. Turning it on will instantly populate all empty cells with the correct candidates for the current puzzle state.