I've received a lot of interesting comments and questions from Sudoku fans over the last few years and this page
is where I try to answer them. Please feel free to drop me a note on the side of the page or try the Facebook
comment box. Or you can email me directly at andrew@str8ts.com.
Post a Comment or Question here...
... by: gkelly, layton, ut
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I thought I understood coloring until your solved showed how to eliminate a 4 at H4 but showing a chain at C2,A3,C4,H3 how is it that from this you know to remove 4 from H4?
Andrew Stuart writes (19Oct2008):
Ok its an offchain elimination. Chain is H3>A3>C2>C4 and the reverse. Both ends can see H4 and both ends are difference color  so one with be a 4. so H4 can't be
That help?
... by: Geoff, Australia
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Hi, Andrew. This is a great siteit has turned me into a sudoku fanatic. I'm confused about xcycles where strong links are considered weak. Can all strong links be considered weak when convenient to achieve candidate elimination? In the above puzzle your solver eliminates 7 at d3 as it has weak links to f1 and g3. I think f1d3 is strong. Why am I wrong? Anyway, its all great fun. Geoff
Andrew Stuart writes (4Oct2008):
You are right, have a look at the bottom section of this page: http://www.scanraid.com/X_Cycles_Part_2 I explain all there
... by: RL Purple, UAS, Minnesota
Thank you. You have given a retired old professor a lot of insights and pleasure on taking up this hobby.
Andrew Stuart writes (4Oct2008):
Thank you sir! My pleasure
Best of luck
Andrew Stuart
... by: Larry orrick, U S
Would like to suggest you offer the option to print the "possibilities" along with the puzzle after buying the Puzzle Pack. This would save time as I don't like the job of doing that busy work. Thanks Larry
Andrew Stuart writes (29Sep2008):
I've thought of that as well. It would not be appropriate for gentle or moderate puzzles since it would give away too many solved cells, but for diabolical and extremes, it might work. It would mean doubling up the puzzle packs or making them twice the size. That could be done if there was sufficient demand. Otherwise, I could provide a text file with the puzzles to link to the solver, Take Step once would give you the same effect
... by: John Robinson, Western North Carolina
Would it be possible to make the daily sudoku saveable and to be able to print out the puzzle plus the possible candidates that I have inserted? Thank You, JR
Andrew Stuart writes (24Sep2008):
You can use the Email This Board function to save your sudoku at any stage of solving. Follow the instructions to copy into an email message and send the message to yourself.
... by: Mats Anderbok, Sweden
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It may be time to replace your brute force solution counter with a more efficient algorithm. It works fine in most cases, but the worst case is terrible. The maximum number of recursions for a valid sudoku is probably 1530 billion, which is about a thousand times more than your counter can handle before is produces a "timed out" error.
Solving singles first and then bivalue cells reduces the number of recursions considerably (but each recursion takes longer time). I will include a counter is the next version of my own solver (Java applet), but I haven't decided yet how to implement it.
The example is a permutation of no. 32632 from Gordon Royle's list (of 47793 puzzles with 17 given) which requires 14,724,893,706 recursions.
Andrew Stuart writes (24Sep2008):
Yes, the brute force algorithm could definitely be improved by having it solve as many squares as possible using basic strategies before applying the brute force approach. Your example solves completely using basic strategies, but times out using "Solution Count." As a workaround for the time out error, until "Solution Count" is made more efficient, I suggest using "Take Step" to solve a few squares and then try Solution Count again.
... by: PaulyPitts, New Jersey, USA
Would it be possible to add a button that would yield the instant puzzle solution ready for printout? While I'm working on a puzzle, like to verify my individual square solutions to check on my work. I also find this very instructive because it gives me instant feedback if my logic is faulty. If there was one button to click that would present the solved puzzle for printing, it would be of great help.
By the way, your Sudoku site is the very best. It just keeps on getting better and better. Keep up the good work.
PaulyPitts
Andrew Stuart writes (24Sep2008):
I don't plan to add an "Instant Solution" button. However, click "Solution Count" and copy the solution string. Open a second solver window and click Import a Sudoku. Paste the string and click OK. Click either the Print 1 or Print 2 button and then print the page with the solution.
Lastly, thanks for the compliment!
... by: pjf, Ontario, Canada
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I think you are missing a easy strategy
Consider 2 pairs; not in a box and not on any on any line. Those 2 pair define a square. The other two corners of the square cannot contain any possibilities of members of the pair.
Andrew Stuart writes (3Sep2008):
That's an interesting observation but I don't think it is always true as a general principle. I've picked a random diabolical to look for an example: D1 and E9 is the pair. E1 contains a 6/8 as well. It forms a short Remote Pair chain and E1 must contain a 6 or 8 as they are the only candidates left. Our pair must contain the same number (either 6 or 8). So while both opposite corners of the pair cannot contain BOTH numbers they can contain ONE of the numbers  which one though cannot be determined without other strategies.
... by: Arne Hajonides, Wilnis, Netherlands
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Hello Andrew
To start with, this is great site.
I have a question about "single's chains, type 1". I'm trying to solve the above puzzle, but I don't understand why the 8 is removed from D9 and not from the another of the cells in the chain (D2, G2, H3, H9)?
With another words, if I start the chain at D9>H9>H3>G2>D2, why is not the 8 removed from D2? (D2 = 8 in the solution and the solution count = 1)
Arne
ps. The puzzle is from a Sudokubook
Andrew Stuart writes (18Aug2008):
Do have a look at http://www.scanraid.com/Singles_Chains Which explains it more fully, but your example is a nice clear one. Either the two YELLOW cells will be the solution or the two BROWN cells will be. We don't know which was round. But that means any 8 that can see both colors can't be a solution. It can be removed. Type 1 works off the chain.
... by: Bernard Skehan, USA
This is the very best site on sudoku. It is better than any of the published books. I hope that someday you put all this in a book form, which would help when studying the strategies.
Please keep up this site, it is invaluable to beginners and advanced players.
Andrew Stuart writes (18Aug2008):
Thanks Bernard!
... by: Doug, Hong Kng
Your site is great. Have you produced a solver for the sudoku puzzles where the diagonals also have 1 9 in them?
Andrew Stuart writes (30Jun2008):
Yes I do have such a solver. Take a look at http://www.sudokuwiki.org/SudokuX.htm
