Swordfish

Level 9 · Purple Belt

Strategy:

Requires:

Learn after:

Finned Swordfish Jellyfish

Overview

Swordfish extends X-Wing to three rows and three columns. When a candidate appears 2-3 times in each of three rows, and all occurrences fall within the same three columns, eliminations can be made.

How It Works

Find three rows where a candidate appears in at most 3 cells each, all within the same 3 columns.
The candidate must appear at least once in each row and column of the pattern.
Eliminate the candidate from other cells in those three columns.

Example

Starting position

Look for Swordfish.

Sudoku grid showing the initial puzzle state for Look for Swordfish.

1

Scan base rows 1, 4 and 9 for 5

Base rows: 1, 4 and 9. Check where digit 5 can appear in those rows.

Sudoku grid illustrating step 1: Scan base rows 1, 4 and 9 for 5 — Base rows: 1, 4 and 9. Check where digit 5 can appear in those rows.

2

r1c1 is already filled

r1c1 is already filled with 7.

Sudoku grid illustrating step 2: r1c1 is already filled — r1c1 is already filled with 7.

3

r1c3 is already filled

r1c3 is already filled with 6.

Sudoku grid illustrating step 3: r1c3 is already filled — r1c3 is already filled with 6.

4

r1c4 is already filled

r1c4 is already filled with 4.

Sudoku grid illustrating step 4: r1c4 is already filled — r1c4 is already filled with 4.

5

r1c6 is already filled

r1c6 is already filled with 3.

Sudoku grid illustrating step 5: r1c6 is already filled — r1c6 is already filled with 3.

6

5 is blocked from r1c7 by the column

Digit 5 cannot go in r1c7 because r6c7 is already in this column. Remove the entry or choose a different digit.

Sudoku grid illustrating step 6: 5 is blocked from r1c7 by the column — Digit 5 cannot go in r1c7 because r6c7 is already in this column. Remove the entry or choose a different digit.

7

r1c9 is already filled

r1c9 is already filled with 9.

Sudoku grid illustrating step 7: r1c9 is already filled — r1c9 is already filled with 9.

8

r4c1 is already filled

r4c1 is already filled with 4.

Sudoku grid illustrating step 8: r4c1 is already filled — r4c1 is already filled with 4.

9

r4c3 is already filled

r4c3 is already filled with 1.

Sudoku grid illustrating step 9: r4c3 is already filled — r4c3 is already filled with 1.

10

r4c4 is already filled

r4c4 is already filled with 7.

Sudoku grid illustrating step 10: r4c4 is already filled — r4c4 is already filled with 7.

11

r4c6 is already filled

r4c6 is already filled with 2.

Sudoku grid illustrating step 11: r4c6 is already filled — r4c6 is already filled with 2.

12

5 is blocked from r4c7 by the column

Digit 5 cannot go in r4c7 because r6c7 is already in this column. Remove the entry or choose a different digit.

Sudoku grid illustrating step 12: 5 is blocked from r4c7 by the column — Digit 5 cannot go in r4c7 because r6c7 is already in this column. Remove the entry or choose a different digit.

13

r4c9 is already filled

r4c9 is already filled with 6.

Sudoku grid illustrating step 13: r4c9 is already filled — r4c9 is already filled with 6.

14

r9c1 is already filled

r9c1 is already filled with 9.

Sudoku grid illustrating step 14: r9c1 is already filled — r9c1 is already filled with 9.

15

5 is blocked from r9c3 by the column

Digit 5 cannot go in r9c3 because r2c3 is already in this column. Remove the entry or choose a different digit.

Sudoku grid illustrating step 15: 5 is blocked from r9c3 by the column — Digit 5 cannot go in r9c3 because r2c3 is already in this column. Remove the entry or choose a different digit.

16

r9c4 is already filled

r9c4 is already filled with 6.

Sudoku grid illustrating step 16: r9c4 is already filled — r9c4 is already filled with 6.

17

r9c6 is already filled

r9c6 is already filled with 7.

Sudoku grid illustrating step 17: r9c6 is already filled — r9c6 is already filled with 7.

18

5 is blocked from r9c7 by the column

Digit 5 cannot go in r9c7 because r6c7 is already in this column. Remove the entry or choose a different digit.

Sudoku grid illustrating step 18: 5 is blocked from r9c7 by the column — Digit 5 cannot go in r9c7 because r6c7 is already in this column. Remove the entry or choose a different digit.

19

r9c9 is already filled

r9c9 is already filled with 1.

Sudoku grid illustrating step 19: r9c9 is already filled — r9c9 is already filled with 1.

20

5 is confined to columns 2, 5 and 8

In each of these 3 rows, digit 5 is confined to the same 3 cover columns: 2, 5 and 8.

Sudoku grid illustrating step 20: 5 is confined to columns 2, 5 and 8 — In each of these 3 rows, digit 5 is confined to the same 3 cover columns: 2, 5 and 8.

21

The pattern forms a Swordfish

That forms a Swordfish: the cover columns must hold the digit for the base rows, so other rows cannot keep it there.

Sudoku grid illustrating step 21: The pattern forms a Swordfish — That forms a Swordfish: the cover columns must hold the digit for the base rows, so other rows cannot keep it there.

22

Extra 5 candidates in columns 2, 5 and 8

These are the extra 5 candidates in columns 2, 5 and 8 that must be removed: r5c2, r7c2, r5c5, r8c5, r3c8 and r7c8.

Sudoku grid illustrating step 22: Extra 5 candidates in columns 2, 5 and 8 — These are the extra 5 candidates in columns 2, 5 and 8 that must be removed: r5c2, r7c2, r5c5, r8c5, r3c8 and r7c8.

23

Remove 5 from columns 2, 5 and 8

Remove 5 from the extra cells in columns 2, 5 and 8: r5c2, r7c2, r5c5, r8c5, r3c8 and r7c8.

Sudoku grid illustrating step 23: Remove 5 from columns 2, 5 and 8 — Remove 5 from the extra cells in columns 2, 5 and 8: r5c2, r7c2, r5c5, r8c5, r3c8 and r7c8.

24

Result

Result after applying the technique.

Sudoku grid showing the result after applying the technique

Tips & Patterns

Swordfish requires careful tracking of candidate positions.
Look for candidates that are restricted to few columns across multiple rows.
Pencilmark highlighting greatly helps in spotting this pattern.