zedoku

swordfish

level 5 - expert

like an x-wing but bigger - three rows and three columns instead of two. removes candidates from columns.

what it means

three rows each have a candidate number confined to the same three columns. since those three rows must place that number somewhere in those three columns, we can eliminate that candidate from other cells in those columns.

example 1: the swordfish pattern

look at these three rows. each row has candidate 5 in exactly three columns: 1, 4, and 7. the highlighted cells form the swordfish.

    5    
    5    
    5    
    5    
    5    
    5    
    5    
    5    
    5    
swordfish cells (contain 5)rows 1, 4, 7

example 2: not all rows need three cells

a swordfish can have 2 or 3 cells in each row, as long as all candidates are confined to the same three columns. here, rows 2 and 8 each have two cells with 3.

  3      
  3      
  3      
  3      
  3      
  3      
  3      

row 2 has 3 cells, row 5 has 2 cells, row 8 has 2 cells — still a valid swordfish!

example 3: elimination

the swordfish in rows 1, 5, and 8 means 7 must go somewhere in each of those rows within columns 3, 6, and 9. we can eliminate 7 from other cells in those columns.

before

      7  
      7  
      7  
 2    7 9
1    67  
      7  
      7  
      7  
   4  78 
      7  
      7  
      7  

7 will be eliminated

after

      7  
      7  
      7  
 2      9
1    6   
      7  
      7  
      7  
   4   8 
      7  
      7  
      7  

7 removed, new singles revealed!

swordfish vs x-wing

x-wing

  • 2 rows × 2 columns
  • 4 cells total
  • like two fish swimming together
  • eliminates from columns

swordfish

  • 3 rows × 3 columns
  • 6-9 cells total
  • like three fish in formation
  • eliminates from columns

same logic, just bigger!

the "three fish" analogy

imagine three fish swimming in formation across an ocean. each fish must stay in one of three specific lanes. since all three fish occupy those three lanes, no other fish can swim there!

x-wing is two fish in two lanes. swordfish is three fish in three lanes. the pattern scales — there's even a "jellyfish" (4×4) for truly evil puzzles!

pattern variations

swordfish can appear in different formations as long as the rules are met:

  • each of the three rows must have 2-3 cells with the candidate
  • all candidates must be confined to exactly 3 columns
  • each of the 3 columns must have candidates in at least 2 of the 3 rows
  • total of 6-9 cells forming the pattern

the columns become "full" because the three rows must place the number there — leaving no room for that candidate elsewhere in those columns.

don't worry!

this is a very advanced technique. most sudoku puzzles, even hard ones, can be solved without ever finding a swordfish. focus on mastering the simpler techniques first — you may never need this one!

when you do encounter one, look for three rows where a candidate appears sparingly and seems aligned in columns. the pattern often stands out once you know what to look for.

summary

  • find three rows with the same candidate confined to the same three columns
  • the candidate can appear 2 or 3 times in each row
  • eliminate that candidate from other cells in those three columns
  • same logic as x-wing, just with three rows/columns instead of two
  • very rare in most puzzles — don't stress if you can't find one!
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