Complete Multi-Select Guide: Efficient Techniques for Marking Candidates

When solving Sudoku, marking candidate numbers (also called notes or pencil marks) is a crucial technique. The multi-select feature provided by this website allows you to efficiently fill in candidate numbers, significantly improving your solving speed. This guide will detail how to use the multi-select feature and its applications in solving.

I. Basic Multi-Select Operations

1. Basic Selection Operations

The basic operations of the multi-select feature are intuitive:

• Click a cell: Select a single cell
• Drag mouse: Select multiple cells (all cells in the drag path)
• Drag again: Clear previous selection and select new cells

2. Maintaining Selection State

When you need to select multiple non-continuous cells, there are two methods to maintain previous selections:

Method 1: Use the multi-select button in the tool panel

• Click the multi-select icon () on the tool panel
• When the button is highlighted, clicking or dragging again won't clear previously selected cells
• You can click or drag multiple times to accumulate more cell selections
• Click the multi-select button again to exit this mode

Method 2: Use Ctrl shortcut key (PC)

• Hold the Ctrl key while clicking or dragging cells
• Achieves the same effect as the tool panel without switching panel state
• Release the Ctrl key to return to normal selection mode

II. Smart Multi-Select: Double-Click Feature

Besides manual cell selection, the system provides powerful smart multi-select functionality through double-clicking to quickly select related cells.

1. Double-Click Large Numbers (Confirmed Answers)

Default behavior: Double-click large number

• Selects all cells with the same large number
• Also selects cells whose candidates contain this number
• Example: Double-click "5" → Selects all cells with answer "5" + cells with candidates containing "5" (like "1,5,7")

Shift + Double-click large number: Exact match

• Hold Shift key and double-click large number
• Only selects cells with the same large number, excluding candidates
• Example: Shift + double-click "5" → Only selects cells with answer "5"

2. Double-Click Candidates (Small Numbers)

Default behavior: Subset matching

• Double-click a cell's candidates
• Selects all cells whose candidates are subsets of that cell's candidates
• Example: Double-click cell with candidates "1,3,5" → Selects "1,3,5", "1,3", "1,5", "3,5", "1", "3", "5", etc.

Shift + Double-click candidates: Exact match

• Hold Shift key and double-click candidates
• Only selects cells with exactly the same candidates
• Example: Shift + double-click "1,3,5" → Only selects cells with candidates exactly "1,3,5"

3. Append Selection and Intersection Selection

Ctrl + Double-click: Append selection

• Hold Ctrl key and double-click
• Newly selected cells are added to existing selection, not replaced
• Example: First double-click "5" to select cells → Ctrl + double-click "7" → Selects all cells containing "5" and "7"

Alt + Double-click: Track intersection (cells containing multiple numbers)

• First double-click a number (like "1"), selecting all cells containing "1"
• Hold Alt key and double-click another number (like "3")
• Only keeps cells containing both numbers
• Example: Double-click "1" → Alt + double-click "3" → Only selects cells containing both "1" and "3" (like "1,3", "1,2,3", "1,3,5", etc.)

III. Multi-Select Applications in Sudoku Solving

After mastering the basic operations, let's see how to apply multi-select to actual Sudoku solving techniques.

Technique 1: Naked Single

Solving scenario: A cell has only one candidate number left, the answer can be determined.

Using multi-select:

1. Double-click a cell with only one candidate (like only "7")
2. System selects all subsets of candidate "7" (including single "7")
3. Quickly find other cells that can be determined

Technique 2: Hidden Single

Solving scenario: In a row/column/box, a number can only appear in one position.

Using multi-select:

1. Double-click a large number (like "5")
2. Observe which cells in a row/column/box have candidates containing "5"
3. If only one cell, it must be "5"

Technique 3: Naked Pairs

Solving scenario: Find two cells in a row/column/box with exactly the same candidates (like both "2,7"), these two numbers can be excluded from other cells.

Using multi-select:

1. Double-click a cell with only two candidate numbers (like "2,7")
2. Hold Shift and double-click for exact match
3. If another cell in the same row/column/box is selected, found a pair
4. Can exclude "2" and "7" from other cells in that row/column/box

Technique 4: Naked Triples

Solving scenario: Similar to pairs, but three cells' candidates form a triple (like "1,3,5").

Using multi-select:

1. Double-click a cell with three candidate numbers (like "1,3,5")
2. System automatically selects all subset cells ("1,3", "1,5", "3,5", etc.)
3. If exactly three cells are selected and they're in the same row/column/box, found a triple
4. Can exclude these three numbers from other cells

Technique 5: X-Wing

Solving scenario: A number appears in only two columns at the same positions in two rows, forming four rectangle vertices.

Using multi-select:

1. Double-click a number (like "6"), view all candidates containing "6"
2. Observe if there are two rows, each with only two positions containing "6"
3. If these four positions form a rectangle, can exclude other "6"s from corresponding columns

Technique 6: Combined Condition Filtering

Solving scenario: Find cells containing specific multiple numbers.

Using multi-select:

1. Double-click number "1", selects all cells containing "1"
2. Hold Alt key, double-click number "3"
3. Now only selects cells containing both "1" and "3" (like "1,3", "1,2,3", etc.)
4. Can continue Alt + double-click other numbers to further narrow the range

Technique 7: Batch Update Candidates

Solving scenario: After determining a cell's answer, need to remove that candidate from other cells in the same row/column/box.

Using multi-select:

1. Fill in the determined answer (like "9")
2. Double-click this answer "9"
3. System highlights all cells containing "9" (candidates or answers)
4. Manually remove "9" from candidates in the same row/column/box

IV. Usage Recommendations and Best Practices

💡 Recommendation 1: Number-First Approach

When solving, recommend number-first rather than cell-first:

• Recommended: Double-click number "1", process all cells related to "1", then process "2"...
• Not recommended: Analyze all possible candidates for each cell individually

Reason: Number-first makes it easier to discover patterns like pairs, triples, X-Wing.

💡 Recommendation 2: Use Exact Matching Wisely

When looking for pairs, triples:

• First use default double-click (subset matching) to observe the overall situation
• Then use Shift + double-click (exact match) to confirm cells with exactly the same candidates
• This avoids missing anything

💡 Recommendation 3: Flexible Use of Key Combinations

When filtering with multiple conditions:

• Use Alt + double-click to find intersection (containing all numbers)
• Use Ctrl + double-click to find union (containing any number)
• Choose flexibly based on solving needs

💡 Recommendation 4: Use with Color Marking

If the website supports color marking:

• After selecting a group of related cells, use color marking
• Helps memory and analysis of complex situations
• Especially suitable for marking special structures like pairs and triples

V. Frequently Asked Questions

Q: Why are more cells selected than expected when double-clicking candidates?

A: The default double-click behavior is "subset matching", selecting all cells whose candidates are subsets of that cell's candidates. If you only want to select cells with exactly the same candidates, use Shift + double-click.

Q: What's the difference between Alt + double-click and Ctrl + double-click?

A:

• Ctrl + double-click: Append selection (union), more and more cells selected
• Alt + double-click: Intersection filtering, fewer and fewer cells selected (must satisfy multiple conditions)

Q: How to use key combinations on mobile devices?

A: Mobile devices cannot use Shift/Ctrl/Alt key combinations. It's recommended to use basic click and drag functions with the multi-select button on the tool panel. Advanced double-click features are mainly for PC users.

Q: How to cancel current selection?

A: Click or drag any cell (without pressing key combinations) to clear previous selection and select new ones. Or click an already selected cell to deselect it.

Summary

The multi-select feature is a powerful tool for improving Sudoku solving efficiency. Through this article, you learned:

• ✅ Basic selection operations (click, drag, Ctrl to maintain selection)
• ✅ Smart double-click feature (large number/candidate double-click, subset/exact matching)
• ✅ Use of key combinations (Shift, Ctrl, Alt)
• ✅ Applications in naked single, hidden single, pairs and other techniques
• ✅ Advanced uses like intersection filtering and union selection

Beginners are advised to first familiarize themselves with basic click, drag and double-click operations, then try key combinations when proficient. With practice, the multi-select feature will become your capable assistant in Sudoku solving!