wkrfresno wrote:Double swordfish are very common. I suspect people find one and go on without looking for more.

Puzzles *requiring* finding two swordfish are not common -- assuming that you always apply simpler tactics first (x-wings, locked candidates, naked and hidden subsets, singles -- your selection may vary)

At the same time, the meaning of "common" depends on where you get your puzzle. If you use Simple Sudoku to generate "extreme" puzzles, most of them will have a Swordfish -- again, depending on you solving methods. One could apply much more complex tactics to circumvent anything simpler. If on the other hand, you use Pappocom Sudoku, you will never, ever need a Swordfish. "Random" Sudoku's composition depend an the construction methods, which vary, but generally, random puzzles are pretty easy and fish-free. But you can solve even the simplest puzzles by filling in the candidate lists and looking for swordfish right off the bat.

CathyW wrote:Have been using Simple Sudoku to practise some techniques (selecting 'Extreme' difficulty) and found a double swordfish (1s) in this puzzle:

And after the eliminations from that you get a second swordfish as follows:

- Code: Select all
`{9} {+1246} {2467} {1268} {+167} {2678} {5} {3} {+12} `

{8} {5} {1267} {4} {3} {267} {12} {67} {9}

{26} {3} {267} {1269} {5} {2679} {8} {67} {4}

{5} {+1467} {9} {16} {8} {3} {467} {2} {+17}

{246} {+12467} {3} {1256} {+146} {2456} {9} {458} {+178}

{1246} {8} {1246} {7} {9} {2456} {146} {45} {3}

{7} {46} {8} {369} {2} {469} {34} {1} {5}

{3} {246} {2456} {568} {467} {1} {247} {9} {278}

{124} {9} {1245} {358} {47} {4578} {2347} {478} {6}

... except that there only one place for a 1 in row 3 at this point. Though it's valid to make your exclusions from this second swordfish, nothing but singles is required after the first one.