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May 15 NVH?

 
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hughwill



Joined: 05 Apr 2010
Posts: 424
Location: Birmingham UK

PostPosted: Sun May 15, 2016 9:42 am    Post subject: May 15 NVH? Reply with quote

After basics (but not all):
Code:

+----------------+-------------+------------+
| 6   57    578  | 18  4   23  | 15 23  9   |
| 123 9     138  | 5   368 236 | 7  236 4   |
| 235 4     35   | 136 9   7   | 15 236 8   |
+----------------+-------------+------------+
| 137 1367  9    | 2   5   36  | 8  4   16  |
| 145 156   2    | 468 68  9   | 3  7   156 |
| 345 8     356  | 346 7   1   | 2  9   56  |
+----------------+-------------+------------+
| 8   23567 3567 | 9   236 45  | 46 1   37  |
| 137 12367 4    | 367 236 8   | 9  5   37  |
| 9   3567  3567 | 367 1   45  | 46 8   2   |
+----------------+-------------+------------+



Play this puzzle online at the Daily Sudoku site

A Check on the grade of the above diagram gets "Very Hard' despite
the obvious single 1 in C3. After entering the 1 the grade changes to Easy.
I suspect the bug in the algorithm is around perceiving certain Box-line
reductions as X-Wings (some are very similar). Any other ideas?

Hugh
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dongrave



Joined: 06 Mar 2014
Posts: 568

PostPosted: Sun May 15, 2016 3:58 pm    Post subject: Reply with quote

This one's even more surprising than the others! Hey! Wait a minute! No, it's not that surprising. I don't think it quite reached your grid. I think you're right about your Box/Line Reduction theory. I think it's not always correctly prioritizing the locked candidates (in this case, the locked 1's in row 2 of box 1) and is instead applying a Wing on the grid that you posted but with a 1 still in r1c23 and r3c13 (because I don't think it made the last move that you made to eliminate them). I think you're right. I think it's hit or miss when it comes down to a Box/Line Reduction or a Wing.

Update: By the way, I forgot to mention - in this case, the X Wing that it probably applied was the 1's in columns 4 and 7 which removes the exact same candidates as the locked 1's in row 2!
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hughwill



Joined: 05 Apr 2010
Posts: 424
Location: Birmingham UK

PostPosted: Mon May 16, 2016 11:10 am    Post subject: Reply with quote

I've always assumed that where there's a box-line reduction any X wing that
does the same job is not really an X-wing because basics take precedence.
However this distinction is arbitrary. On much rarer occasions I've noticed that
puzzles which can be done with basics include XY or XYZ wings though I don't
think the algorithm is ever fooled by these in the same way.
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dongrave



Joined: 06 Mar 2014
Posts: 568

PostPosted: Mon May 16, 2016 12:11 pm    Post subject: Reply with quote

Oh, I think you're definitely right! I should have said 'when it comes down to a Box/Line Reduction or an X-Wing' (not 'or a Wing'). Thanks Hugh!
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