Solver App for Android
Strategies for Popular Number Puzzles
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Feedback and Questions

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 .

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Many thanks to all the people who have helped improve the solvers and strategies with their feedback!

Sunday 22-Jun-2008

... by: Dabas, Australia

Your explanations are great!
The section on X-Cycles has been a great eye-opener and I keep finding ways of using these strategies!

Thanks for an excellent site

Sunday 1-Jun-2008

... by: Judy, USA


Thank you for your solver. It has been a great way for me to learn to do the more advanced boards. The clear instructions for each technique are the most clear I have found. I do have a question/comment. Why is the last row labeled J in your solver instead of I? This lead to a bit of confusion for me as I tried to follow the notes for each step. Please note this odd row labeling on the page if there is a reason for it.

Thank you for the great work.

Andrew Stuart writes:

The reason for that convention is because "I" can look like "1", which can be confusing when talking about cell I1 etc, J1 is simply visibly easier. You should not find "I" turning up in any output from the solver or in any of the documentation

Tuesday 27-May-2008

... by: est, usa

This morning there was no PRINTABLE version of the puzzle available, which I have always used to enlarge the puzzle. Please put it back.

Andrew Stuart writes:

Hi, there was an issue on Sunday/Monday in Firefox. Firefox is case-senstive to html element names and I introduced a bug when cleaning up the code. Fixed pretty quickly and printable versions are working in both browsers today. Do have another go

Tuesday 6-May-2008

... by: J McKee, Virginia, USA

Great site! Especially for a non-mensa like me. I have a question about your example puzzle for Sashimi Finned X-Wings. Would not the 4 in D2 have to be absent for this to work? I'm assuming that the fin is the only allowable 4 candidate deviation in the X-wing rows. If the 4 in D2 is actually not a problem, could not E6 and F6 just as easily be considered a fin? Straighten me out, please.


Andrew Stuart writes:

Really glad you wrote in since there was a problem with my diagram. Not sure how but the screen snap shot I took was of a position a few steps before the correct situation. Have a look at the page now. Even I was struggling to make sense of the example because there were stray 4s in D2 and D3. But having removed them it makes sense now.

So, correct, the 4s in D2 and D3 do have to be absent for it to work.

These are recent pages, and I suffer "wood from the trees" syndrome when I check what I write and do.


Saturday 3-May-2008

... by: Relaymsn, United States

Great site. I was looking for a site for solving some unsolvable (to me) Sudokus that I had accumulated. Some of the techniques will take me a while to do with pencil and paper but I can master others quickly.

By the way, the site worked great on Safari 3.1.1.

Andrew Stuart writes:

Thanks for the feedback - appreciated. And good to know its works on Safari as well!

Friday 2-May-2008

... by: Rajk, Netherlands

Escargot does not have a solution?
Run out of known strategies.

Andrew Stuart writes:

Yep. That is a semi-famous sudoku invented by a Finnish mathematician who claims it is the hardest sudoku ever. Currently aboyt 0.2% of randomly produced sudokus can't be solved with my logical strategy set so there is plenty of work still to do...

Sunday 6-Apr-2008

... by: Murray, Australia

Load Sudoku:

Hi Andrew, in solving the above problem I encountered the Diabolical strategy called "20 Bug". there is no explanation for this when I click on the link. Perhaps this is an accidental omission.



Andrew Stuart writes:

Please load your sudoku and click Solution Count. Your sudoku has 5 solutions. When it Takes Step, the solver does not check for multiple solutions. Due to use of strategies that depend on the sudoku having a unique solution, the solver may produce strange results for sudokus having more than one solution. Furthermore, Take Step may find a solution without giving any indication that other solutions exist.

Wednesday 2-Apr-2008

... by: Andy Fraser, UK

Excellent solver. Hopefully I will learn to spot these patterns eventually.

I have a slight problem though:

Using the back button "<<" takes it back more than one step; sometimes even removing correct numbers I have put in manually.

I am using Firefox in WinXP home.

Andrew Stuart writes:

There is an issue with entering numbers and using the back button. Back really only works after you have "taken step" and it is remembering the state the board was in when you pressed that button, not when you entered something. I'd avoid that button for un-doing manually entered numbers

Tuesday 1-Apr-2008

... by: JohnR, USA

Now that seems to have died, I believe there will be a population wanting to continue a daily-weekly competition. Thoughts?

Andrew Stuart writes:

I have started posting a daily sudoku on my site at So far, I haven't set up a competition. It does look like the current administrators at have worked out the glitches and are keeping the competitions alive. Since that site is outside my control, I won't speculate on what changes they may implement. However, the problem of experts lists that are top heavy with perfect scores, that I addressed last year, remains. The spread of scores, that we originally had, is long gone. The number of perfect scores continues to grow, resulting in ever more lengthy experts lists. How the new administrators will handle the problem remains to be seen.

Monday 31-Mar-2008

... by: D. Meyer, Billings, Montana

Load Sudoku:

How come your solving page states. "run out of known strategies" for this particular board. But the newspaper prints a solution which I can send to
you if you wish?

Andrew Stuart writes:

I clicked Solution Count and the solver found 14 solutions. The Unique Rectangles strategy is only valid for sudokus that have exactly one solution. Unique Rectangles may remove all solutions from a sudoku that has multiple solutions.

Commonly, multple solutions are the result of omitting one or more givens when entering the puzzle. If you are part way through the solution process when you discover that you left out a given, it is best to Clear and re-enter the whole sudoku. The solver may have already eliminated a candidate that is part of the solution and, thus, no longer be able to find the solution.
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Thank-you everyone for all your questions and contributions.