basic question about x wing

Advanced methods and approaches for solving Sudoku puzzles

basic question about x wing

Postby snapdragon » Fri Aug 19, 2005 3:41 pm

I'm trying to get the hang of using an x wing. Does each corner of the x in an x wing have to sit in a different 3x3?
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Postby george-no1 » Fri Aug 19, 2005 3:45 pm

Nope. And if you think about it logically, why would it? An X-Wing works when the (filtered) candidate forms a rectangular pattern with no other possible cells for it in either the row or the column, meaning you can delete this candidate from other cells in the column or row respectively. There is even no reason why all 4 corners cannot all lie in the same 3x3 box.

G:)
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Postby PaulIQ164 » Fri Aug 19, 2005 3:47 pm

Hang on -- you can't have an X-Wing with all 4 'corners' in the same box can you? I can't see how it would work. An example, maybe?
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Postby snapdragon » Fri Aug 19, 2005 3:54 pm

My limited brain says it stops being an x wing if all 4 are in the same box, because you no long have 2 rows (or columns) that are "sharing" the 2 digits, (though I'm not really sure what the definition is).
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Postby george-no1 » Fri Aug 19, 2005 3:59 pm

Paul, what about the box

2 469 5
3 167 189
68 14 146

There would be an X-Wing in the 1s if there were no more possible cells for the 1s in rows 2 and 3 *or* columns 2 and 3 (obviously not both, as there would be no point in calling it an X-Wing).

G:)
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Postby PaulIQ164 » Fri Aug 19, 2005 4:05 pm

george-no1 wrote:Paul, what about the box

2 469 5
3 167 189
68 14 146

There would be an X-Wing in the 1s if there were no more possible cells for the 1s in rows 2 and 3 *or* columns 2 and 3 (obviously not both, as there would be no point in calling it an X-Wing).

G:)


But there can't be 'no more possible cells for the 1s in rows 2 and 3', surely? If the 1 in this box is in row 2, there has to be another place the 1 can go somewhere else in row 3, and vice versa if the 1 is in row 3 in that box.
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Postby george-no1 » Fri Aug 19, 2005 4:13 pm

Oh, ok, should have thought of that before making brash statements. You're right, well done.

G:)
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