dailysudoku.com

[Donna]

Please help with this one?

3****9*5*
*67*1****
2*****7**
*7*9*****
*9*4*1*3*
*****6**8
**4*****9
****6*82*
*5*3****1

[tushar]

a 9 at column 5 and row 9

[tushar]

7 at column 9 and row 5

[tushar]

4 at column6 and row8

[David Bryant]

[The Fugitive]

I think it's fairly obvious that the puzzle is invalid. As it currently stands, it's impossible to solve.

Either the 7 in D2 or the 8 in F9 are incorrectly placed -- as they violate the rules of symmetry.

Moving the 7 from D2 to D1 will certainly resolve the puzzle, but it can't be solved logically. It can only be solved with trial and error.

However, leaving the 7 in D2 and moving the 8 from F9 to F8 produces a 100% logical solution, as follows (note that the term grid applies to the 3x3 squares):

I5 has to be 9, because it is unique to the grid
F7 has to be 9, because it is unique to the grid
B1 and C3 must be 5 and 9, because this pair is unique to these cells
G7 and H9 must be 3 and 5, because this pair is unique to these cells
H6 has to be 4, because it is unique to the row
F2 cannot be 4, because it must be in D1 or F1
A7 cannot be 4, because it must be in B7, B8 or B9
A9 cannot be 4, because it must be in B7, B8 or B9
C8 cannot be 4, because it must be in B7, B8 or B9
C9 cannot be 4, because it must be in B7, B8 or B9
D1 cannot be 1, because it must be in F1, F2 or F3
D3 cannot be 1, because it must be in F1, F2 or F3
G4 cannot be 7, because it must be in G6 or I6
G5 cannot be 7, because it must be in G6 or I6
H4 cannot be 7, because it must be in G6 or I6

From this point on the grid is self-solving...
H1 has to be 7, because it is unique to the row
B1 has to be 9, because it is unique to the column
C3 has to be 5, because all other potentials have been eliminated
B8 has to be 4, because all other potentials have been eliminated
C8 has to be 9, because it is unique to the grid
A7 has to be 1, because it is unique to the grid
A3 has to be 8, because all other potentials have been eliminated
A2 has to be 4, because all other potentials have been eliminated
C2 has to be 1, because all other potentials have been eliminated
H2 has to be 3, because all other potentials have been eliminated
F2 has to be 2, because all other potentials have been eliminated
G2 has to be 8, because all other potentials have been eliminated
I1 has to be 6, because all other potentials have been eliminated
G1 has to be 1, because all other potentials have been eliminated
H3 has to be 9, because all other potentials have been eliminated
I7 has to be 4, because all other potentials have been eliminated
I8 has to be 7, because all other potentials have been eliminated
G8 has to be 6, because all other potentials have been eliminated
D8 has to be 1, because all other potentials have been eliminated
I3 has to be 2, because all other potentials have been eliminated
I6 has to be 8, because all other potentials have been eliminated
C6 has to be 3, because all other potentials have been eliminated
H9 has to be 5, because all other potentials have been eliminated
G7 has to be 3, because all other potentials have been eliminated
B7 has to be 2, because all other potentials have been eliminated
B6 has to be 5, because all other potentials have been eliminated
B4 has to be 8, because all other potentials have been eliminated
C4 has to be 6, because all other potentials have been eliminated
C9 has to be 8, because all other potentials have been eliminated
A9 has to be 6, because all other potentials have been eliminated
B9 has to be 3, because all other potentials have been eliminated
C5 has to be 4, because all other potentials have been eliminated
D6 has to be 2, because all other potentials have been eliminated
G6 has to be 7, because all other potentials have been eliminated
D9 has to be 4, because all other potentials have been eliminated
F9 has to be 7, because all other potentials have been eliminated
E9 has to be 2, because all other potentials have been eliminated
F4 has to be 5, because all other potentials have been eliminated
G4 has to be 2, because all other potentials have been eliminated
G5 has to be 5, because all other potentials have been eliminated
H4 has to be 1, because all other potentials have been eliminated
A4 has to be 7, because all other potentials have been eliminated
A5 has to be 2, because all other potentials have been eliminated
F1 has to be 4, because all other potentials have been eliminated
F5 has to be 3, because all other potentials have been eliminated
D5 has to be 8, because all other potentials have been eliminated
E5 has to be 7, because all other potentials have been eliminated
D1 has to be 5, because all other potentials have been eliminated
E1 has to be 8, because all other potentials have been eliminated
E3 has to be 6, because all other potentials have been eliminated
D3 has to be 3, because all other potentials have been eliminated
F3 has to be 1, because all other potentials have been eliminated
E7 has to be 5, because all other potentials have been eliminated
D7 has to be 6, because all other potentials have been eliminated

[someone_somewhere]

Hi,

Fugitive, it looks like you wrote a nice program.

see u,

[Guest]

Hi someone_somewhere,

Thank you for your comments. I wrote the main "solver" in C++ (for Windows) over a couple of nights. It still needs some work and more than a little optimising, but so far it can resolve all of the Daily SuDokus and it fares no better or worse on the more "extreme" puzzles dotted around the forum. I'm particularly pleased with the output, as it clearly shows the primary logic behind these puzzles. Glad you like it too.

[The Fugitive]

Sorry - I forgot to log in first. I am the guest, above.

[someone_somewhere]

Hi,

There was a contest of Sudoku programs, but it is closed now.
Did you submit your program there?
Probable not.
I don't know the results, not published yet.
The main goal was to solved as much as possible, without the brute force of trail and error.
It looks like a lot of good programers wrote a lot of stuf.

What techniques did you program, which of them not yet?

see u,

P.S. the messages could be much better, but forget it, this is not important.

[Guest]

Hi,

I missed the competition, unfortunately. I only started doing SuDoku a few weeks ago. :-(

I'm not sure what the correct terms are for the techniques I use, however my implementation seems to be partially based upon the techniques described on the SadMan Software website.
http://www.simes.clara.co.uk/programs/sudoku.htm

Rule 1. Single
A combination of Naked Single and Hidden Single. I also refer to it as the self-solving rule, because my solver implements these "basic" rules by default (updating itself as clues are entered). Easy and medium puzzles are often resolved even before the last clue is entered -- hence they are self-solving. Also, all other rules invoke this rule by default, so all puzzles become self-solving at some point or another.

Rule 2. Subset
A combination of Naked Subset and Hidden Subset. Most puzzles beyond easy usually hit this rule at least once.

Rule 3. Interact
A combination of Block and Colum/Row Interactions, and Block/Block Interactions. Hard usuallys hits this rule at least once.

Rule 4. Extreme
Currently implements X-Wing and Swordfish. Very hard usually hits this rule at least once.

I hadn't planned to add any other techniques, but having seen the potential at SadMan Software I may spend some more time on them.