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. I'm also directing Str8ts feedback here. Please feel free to drop me a note on the side of the page. Or you can email me directly at andrew@str8ts.com.

Hi Andrew, First of all, congratulations for your website that's to me the ultimate sudoku reference ever on the web! Well I did not think I would have to do that but it seems to me there is a bug in the solver for the attached Sudoku. For some reason at step #11, the solver does not detect an obvious Unique Rectangle type 3 located CD45, the bi-value cell being D9 and thus eliminating candidates 6 in D1 and 9 in D2. Instead the solver catches a hidden UR involving the same cells, which is fine. However the UR should have been logically tracked before. The consequence in that case is 'dramatic' since it turns the Sudoku grade from tough to extreme! :-)

I am myself an IT programmer and I appreciate when bugs are submitted especially when the application is so elaborated and useful.

Thanks again for such an amazing tool!

Andrew Stuart writes:

Yes, you have identified a missing extension to URs, not a bug as such. I thought I'd covered the compliment to UR Type 3, but wasn’t looking for cells in the same box. So I have added this to the solver and the documentation. Great catch!

Sunday 3-Nov-2013

... by: David Eoll, New Hampshire, USA

The Solve Path button does not seem to work for me. I'm using Chrome 30.0.1599.101 m on Win7sp1 x64.

Great site!! I'll be back....

Andrew Stuart writes:

Works for me on chrome and Win7sp1 x64 which I also have. Can you check it's not opening in a window or tab that is already open and/or underneath something?

With the first 'grouped X-cycle' why is the 7 in A4 not removed?

Andrew Stuart writes:

I think it's safe to remove that 7. I'm pretty sure the way I've coded this the solver wont remove candidates it find in links coming out of a grouped cell. Probably being cautious. But logically there is no reason why its not a valid elimination.

Sunday 3-Nov-2013

... by: TomE, IN

You have a great solver. I have used it once in a while when I seem to be stumped....

However, in today's case, As I got futher into the step by step solution it turned out I had entered a number in the WRONG cell. I would like to go back to the beginning, make the correction and start over.

I have missed they way to do that. Or can you do that?

Thanks loads

TomE

Andrew Stuart writes:

I'd like to implement a full reverse history feature. The solver is designed to take any board at any stage of the solution process so it does not know when a puzzle is at the beginning or not. So there is no auto-restart. When entering clues you can tell it which are clues and solution but that only effects the color of the numbers. However, if you use it regularly you can use the [save] and [reload] before you kick off the solving steps. That will restore the puzzle. I usually keep puzzle as strings (using the [Email] button and copy/paste) if I'm working on several or many puzzles.

Sunday 3-Nov-2013

... by: Hassieb Wali, USA

Dear Mr. Stuart, I want to thank you for the most complete SUDOKU website you have developed and maintained. It is beyond my expectation and I often go to your website and seek help. I have a question and have not been able to find a good answer and that is about X-Cycle: How do you find a X-Cycle and what are the criteria for it. Does it have to be bi-value? There is no reference in the X-Cycle definition you have provided,; however, your examples imply that they are bi-values. Also when you say: X-Cycle on 5, I see a lot of 5's that have been ignored and that is confusing. Could you help me a little bit with this problem. I would really appreciate.

Regards,

Hassieb Wali

Andrew Stuart writes:

I've added a feature to the solver recently to help hunt for X-Cycles and other chaining strategies. Under the list of strategies is a set of checkboxes that you can use to highlight bi-location candidates (two left in a unit). If you suspect an X-Cycle you can check each number individually and perhaps build up a loop. You only have to alternate ON/OFF and ON implies an OFF link which doesn’t have to be bi-locational (since it removes all other instances). An X-Cycle has no start or end, you can construct it from any point, which makes it a little easier to spot. Not every X will be used because loops can be quite small (X-Wing is the smallest) but you can widen them to make them larger. Lots of X that never form part of a loop imply they are stranded in groups where they can't be linked in - there must be 3 or more in each unit. It is possible to have two or more isolated loops that cant be joined but this is rare.

I hope this is slightly helpful. Best is to look at examples found in the solver in the harder puzzles. They turn up a lot.

Saturday 2-Nov-2013

... by: Ms Love, Florida

Does it matter the order the numbers are placed when solving for a negative number in a set of cells.

Andrew Stuart writes:

Are you talking about Killer Sudoku? It's the only context I can think of that might have a negative number in a clue. If so, then the solution to the cells in the cage could still be in any order or combination. There are only a very small number of cage combinations that fit minus numbers and only 2-cell cages can have them.

Solver does not catch a Hidden Unique Rectangle in the below valid sudoku board ......5....4.18.9.3...79..6.6.9.2...795.8.243.......6...6.94.81..1..56......6.... Hidden Rectangle rectangle: [A2, A8, C2, C8] peak cell: C8[12] conclusion: 2 strong links on 1 in peak cell => removing [2] from A2

Andrew Stuart writes:

Interesting example, I'll take a look why this isn’t picked up. Thanks for sharing

This sudoku took well over daytosolve. I found that it took combination of clues to eliminate possibilities. bruteforce method worked finally. I was able to solve this using your program which identified some solver paths. v. v. difficult but good for a collection

Andrew Stuart writes:

Thanks for sharing it. Published for others to try and solve.

I was just messing around and put a sudoku from GNOME Sudoku here and wanted to see if the solver could solve one without some of the numbers.

However, when I put the numbers in without the last one (2 in bottom-right corner) the solver mistakenly thinks that number must be 3 and after that makes mistaken assumptions. I understand that the solver might not be able to figure out a solution but a wrong number leans towards a wrong strategy or a wrong implementation of it. Just thought you should know.

Andrew Stuart writes:

That behaviour is by design. The solver *could* check if the puzzle has only one solution but that would be a huge overhead especially as it has no memory of previous steps and this would have to happen every time someone presses "take step". So we assume the puzzle has only one solution. However, logic can only work if the puzzle *has* one solution. It will find some steps logically but ultimately it will get in a mess and create a contradiction. If you don’t think this approach is correct think about a puzzle with just one number - how would it 'logically' proceed?

While trying some minimum Sudoku-X puzzles from http://www.sudocue.net/sudoku-x-12-7193.sdm I found the SudokuX solver fails for 000000000000000000000001000000000100000000000020340000100000053600007000000008020

The puzzle does require (fairly extensive) backtracking to solve but it has only one solution.

The key point at which is fails appears to be in applying the Grouped X-cycles deduction rule. Disabling those rules and going straight to Solution Count fails with the error that it needs at least 17 givens. That's the minimum for normal Sudoku. The minimum for Sudoku X is no more than 12 (the URL above).

Andrew Stuart writes:

Thanks for this email. I've corrected the message to say 12 clues, but I'm afraid the server will not return a solution count with so few clues. I have to throttle the services since it's a busy machine and the brute force tester sucks CPU. I can confirm my offline solver finds one solution. I have an article about this puzzle.

I'd be interested in the first move you found for Grouped X-cycles. Would be nice to make further progress in that area.

## Monday 4-Nov-2013

## ... by: Hervé Gérard, France

Load Sudoku: CLICK TO LOADFirst of all, congratulations for your website that's to me the ultimate sudoku reference ever on the web!

Well I did not think I would have to do that but it seems to me there is a bug in the solver for the attached Sudoku. For some reason at step #11, the solver does not detect an obvious Unique Rectangle type 3 located CD45, the bi-value cell being D9 and thus eliminating candidates 6 in D1 and 9 in D2.

Instead the solver catches a hidden UR involving the same cells, which is fine. However the UR should have been logically tracked before.

The consequence in that case is 'dramatic' since it turns the Sudoku grade from tough to extreme! :-)

I am myself an IT programmer and I appreciate when bugs are submitted especially when the application is so elaborated and useful.

Thanks again for such an amazing tool!

## Sunday 3-Nov-2013

## ... by: David Eoll, New Hampshire, USA

Great site!! I'll be back....

## Sunday 3-Nov-2013

## ... by: Gerrit Altena, Netherlands

Load Sudoku: CLICK TO LOAD## Sunday 3-Nov-2013

## ... by: TomE, IN

However, in today's case, As I got futher into the step by step solution it turned out I had entered a number in the WRONG cell. I would like to go back to the beginning, make the correction and start over.

I have missed they way to do that. Or can you do that?

Thanks loads

TomE

## Sunday 3-Nov-2013

## ... by: Hassieb Wali, USA

I want to thank you for the most complete SUDOKU website you have developed and maintained. It is beyond my expectation and I often go to your website and seek help.

I have a question and have not been able to find a good answer and that is about X-Cycle: How do you find a X-Cycle and what are the criteria for it. Does it have to be bi-value? There is no reference in the X-Cycle definition you have provided,; however, your examples imply that they are bi-values. Also when you say: X-Cycle on 5, I see a lot of 5's that have been ignored and that is confusing. Could you help me a little bit with this problem. I would really appreciate.

Regards,

Hassieb Wali

I hope this is slightly helpful. Best is to look at examples found in the solver in the harder puzzles. They turn up a lot.

## Saturday 2-Nov-2013

## ... by: Ms Love, Florida

## Monday 7-Oct-2013

## ... by: doman, PL/BE

Load Sudoku: CLICK TO LOAD......5....4.18.9.3...79..6.6.9.2...795.8.243.......6...6.94.81..1..56......6....

Hidden Rectangle

rectangle: [A2, A8, C2, C8]

peak cell: C8[12]

conclusion: 2 strong links on 1 in peak cell

=> removing [2] from A2

## Friday 4-Oct-2013

## ... by: guru, India

Load Sudoku: CLICK TO LOAD## Friday 4-Oct-2013

## ... by: Svavar Kjarrval, Iceland

Load Sudoku: CLICK TO LOADHowever, when I put the numbers in without the last one (2 in bottom-right corner) the solver mistakenly thinks that number must be 3 and after that makes mistaken assumptions. I understand that the solver might not be able to figure out a solution but a wrong number leans towards a wrong strategy or a wrong implementation of it. Just thought you should know.

## Tuesday 1-Oct-2013

## ... by: reoiutporewiu, home

Load Sudoku: CLICK TO LOADThe puzzle does require (fairly extensive) backtracking to solve but it has only one solution.

The key point at which is fails appears to be in applying the Grouped X-cycles deduction rule. Disabling those rules and going straight to Solution Count fails with the error that it needs at least 17 givens. That's the minimum for normal Sudoku. The minimum for Sudoku X is no more than 12 (the URL above).

Thanks for this email. I've corrected the message to say 12 clues, but I'm afraid the server will not return a solution count with so few clues. I have to throttle the services since it's a busy machine and the brute force tester sucks CPU. I can confirm my offline solver finds one solution. I have an article about this puzzle.

I'd be interested in the first move you found for Grouped X-cycles. Would be nice to make further progress in that area.