sudokUI › Sudoku techniques › Swordfish
Swordfish in sudoku
3-fish · Naked Triple seen sideways · how it fits
Digit X has two or three candidates in each of three rows, all lying in the same three columns, or likewise with rows and columns swapped.
Why it works
Each of the three rows places X in a different one of the three columns, so all three columns get their X from these rows. Remove X from all other cells in those columns, or in those rows for the swapped form.
How to spot it
Filter on one digit and find three rows whose candidates together span only three columns: a row may use just two of them.
Worked example
Swordfish on 9: in columns 1, 4, 8, 9 can only go in rows 3, 4, 5 (r3c1, r5c1, r4c4, r5c4, r3c8, r4c8). Those 3 columns will use up 9 in those 3 rows, so 9 can go nowhere else in them. So 9 is removed from r5c2.
Practise it on a real puzzle. sudokUI generates a puzzle that needs Swordfish and takes you straight to the position where it applies. Hints draw the pattern on the board and explain the step.
In a puzzle's rating
Each time the solver needs Swordfish, the puzzle's difficulty rating grows by 150. Its class is Hard. It is needed in 1 in 81 puzzles that sudokUI generates. How the rating works.