![]() ![]() The gray square must then contain a 2 and the rest of the puzzle falls apart after that. This pairs up with the underlined 78 in the same row, meaning that the tan square must contain a 9. Aftere the 9 is eliminated, you are left with only 78. Once you do that, note the blue cell with the underlined 789. You can then remove the 9s from the blue squares. This pattern ensures that a 9 cannot exist anywhere else in those columns. These squares are lined up in three columns (blue arrows). Notice that the yellow squares are the only possibilities in their rows where a 9 can exist (red arrows). Here is a complete puzzle adapted from Sudopedia. However, the contraint is based on the 3 x 3 rectangle and it is possible that one, two, or even all three rows may have the key digit. It is probable that when you do find your first Swordfish on your own (and a thrilling moment that will be!) that it will follow the classic pattern. The "classic" position consists of three sets of doubles filling out two rows/columns in the 3 x 3 rectangle. That situation is impossible in the exact diagram shown above, but it is easy enough to construct a teaching diagram where it does (see below). If a 5 appears in these squares, they cannot be eliminated as they form part of the 3 x 3 rectangle. If you are paying attention, you probably want to ask me about the blue squares with the question marks. I could have constructed a similar pattern with the 5 restricted to three columns, lined up in three rows. Thus the green squares cannot contain a 5. This pattern ensures that the 5 in those columns can only appear in the yellow squares. Note that the 5 is limited to the three rows (red arrows) such that it is also lined up in three columns (blue arrows). The image below shows the "classic" Swordfish position, based on the digit 5. They are not easy to spot and not easy to keep in your head. When you are working with triples, however, there are six combinations. When you are working with doubles, there are only two combinations. Swordfish is formed from the corners, midpoints, and center of a 3 x 3 rectangle.ĭon't start looking for Swordfish until you can do X-Wings. X-Wing is formed from the corners of a 4-point rectangle. A Swordfish position exists when a single number exists in one of three rows (or columns) such that the only possibilities line up into exactly three columns (or rows). See my X-Wing puzzle if you don't remember. Remember that the X-Wing position exists when a single digit must belong in one of two rows such that the only possibilities form the corners of a rectangle. Whereas X-Wing is based on the principle of doubles, Swordfish is based on triples. Swordfish is the next step up from X-Wing. I guess, this series is having some benefit after all. Instead I am improving the quality of their auto-solving programs. I have come to terms with the idea that I am not teaching these people anything. However, *some* people seem to interpret this statement as a challenge. I originally wanted to arrange these caches so that you actually have to understand and use the technique and not simply throw it into a solver. This is another in my series of advanced sudoku strategies. ![]()
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