Sudoku Solver

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Swordfish (Row)

A 3x3 pattern where a candidate appears in only three rows, restricted to the same three columns.

Swordfish (Row) is the horizontal cousin of the Swordfish (Column) and the expanded version of the X-Wing (Row). It uses a 3x3 pattern involving three rows and three columns.

It proves that "since the numbers for these 3 rows MUST be in these 3 columns, no other cells in those columns can have that number."

Interactive Example

1 2 3 4 5 6 7 8 9
7
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
8
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
6
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
9
1 2 3 4 5 6 7 8 9
7
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
2
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
6
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1
1 2 3 4 5 6 7 8 9
4
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
6
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
5
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
4
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
6
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
8
1 2 3 4 5 6 7 8 9
7
1 2 3 4 5 6 7 8 9
7
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
4
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
3
1 2 3 4 5 6 7 8 9
1
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
2
1 2 3 4 5 6 7 8 9
6
1 2 3 4 5 6 7 8 9
1
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
4
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
7
1 2 3 4 5 6 7 8 9
9
1 2 3 4 5 6 7 8 9

Click "Apply Logic" to see the strategy in action.

Real Example Walkthrough

In the example puzzle above, the strategy targets the number 5:

1. The Base Sets (Rows) We look at three specific rows: - Row 2 - Row 6 - Row 7

In these three rows, the candidate 5 appears only in three columns: - Column 4 - Column 5 - Column 8

2. The Pattern Let's map out where 5 can go in our base rows: - Row 2: Locked to Cols 4, 5, 8 - Row 6: Locked to Cols 4, 5 - Row 7: Locked to Cols 5, 8

Notice that across all three rows, the candidates are restricted entirely to Columns 4, 5, and 8.

3. The Logic - We need to place three 5s—one for Row 2, one for Row 6, and one for Row 7. - We have exactly three columns available for them (Column 4, 5, 8). - Therefore, these three columns must contain the 5s for our three rows. - Consequently, no other cell in Column 4, Column 5, or Column 8 can contain a 5.

4. The Elimination We can eliminate 5 from any cell in the cover columns (4, 5, 8) that is NOT part of our Swordfish pattern. - Eliminate 5 from R8C5, R9C4, R9C5.

How to Spot a Swordfish (Row)

  1. Pick a Candidate: Focus on one number (e.g., 5).
  2. Count Candidates in Rows: Look for rows where the candidate appears 2 or 3 times.
  3. Find the Match: Look for three such rows where the candidates all fall into the same three columns.
    • It's a "Row-based" Swordfish because you start by finding the pattern in the rows.
    • You eliminate vertically from the columns.

Visual Guide

C1 C2 C3 R1 X X . <- Row 1 (Base Set) R2 X . X <- Row 2 (Base Set) R3 . X X <- Row 3 (Base Set) | | | v v v Eliminate from Cols

  • Be sure to eliminate only from the columns, and not from the rows you used to find the pattern.

Common Mistakes

  • Wrong Direction: If you find the pattern in Rows, you MUST eliminate from Columns. If you eliminate from Rows, you break the puzzle!
  • Missing a Column: Make sure all candidates in your 3 rows generally fit into exactly 3 columns. If they spill over into a 4th column, it is not a standard Swordfish.

Tips for Beginners

  • Pencil Marks: This strategy is nearly impossible to spot without full candidate notation.
  • X-Wing First: Always look for X-Wings first. They are more common. Swordfish is essentially an "X-Wing plus one".

Comparison Table

Strategy Pattern Size Base Sets Cover Sets Elimination
X-Wing (Row) 2x2 2 Rows 2 Cols Eliminates from Cols
Swordfish (Row) 3x3 3 Rows 3 Cols Eliminates from Cols
Jellyfish (Row) 4x4 4 Rows 4 Cols Eliminates from Cols

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