Solver App for Android and iPhone
Strategies for Number Puzzles of all kinds
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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. 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 .

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

Thursday 9-Aug-2018

... by: St8fan, Michigan, USA

Thank God for this websire, and wifi on overseas flights! Your hints on extreme+ are awesome

Andrew Stuart writes (9-Aug-2018):

Glad to be of service!
Hope you're landing somewhere fun

Monday 30-Jul-2018

... by: Norris Barnes, US

I am at a mid-point solving an extreme puzzle and I want to skip ahead to 3D Medusa to help me solve it. I complete the puzzle but find I have an error. In reviewing your solution I see that 3D Medusa was not used to solve the puzzle.

Do I have to step thru each strategy in sequence to successfully complete the puzzle?

Andrew Stuart writes (31-Jul-2018):

There are numerous possible ways to solve any single puzzle and your choices are just as valid. As I wrote before, basic ones must come first but other than that, your choice. You have to step through each step but the solver will skip ahead to the first to si

Monday 30-Jul-2018

... by: Norris Barnes, us

I there a 'required' sequence of strategies to solve each Sudoku? For example, I am working on an Extreme puzzle and I get to a mid-way point where I need to work thru the strategies one at a time in the sequence you have presented BUT am I allowed to skip ahead to a strategy (e.g., 3D MEDUSA) instead of working thru each logical test?

Love your site - use frequently, much to my wife's chagrin !

Andrew Stuart writes (31-Jul-2018):

No you dont need to try the lost in order except for basic strategies, whihc should be exhausted before moving to anything higher. Within basic strats these can be looked for en mass or in a different order but I think as listed then are in a natural order of complexity. There is a lot of overlap between more advanced strategies and it is difficult to order them in terms of complexity - so my ordering is my best subjective ordering. You can skip some by turning them off. I've wondered about allowing the user to re-order them but it's a complex UI problem, not a sudoku strategy problem.

Thursday 5-Jul-2018

... by: david, south america

The board has stopped working. numbers entered on the small board no longer appear on the large board. when ''take step'' is pressed the following message appears: put some numbers in the small box first

Andrew Stuart writes (6-Jul-2018):

Very sorry! Made a change yesterday and didn't check all probably browsers.
[CRTL]+[F5] to refresh the page now

Thursday 24-May-2018

... by: Alfredo Trotta, Italy

Hey Andrew, sorry if a keep stalkering you, but since I discovered your site (very recently), together with your app (android in my case), the meaning of solving sudoku puzzles changed completely for me. I am really delighted in learning techniques one by one and being able to solve puzzles I never dreamt I could before.
What I find someway very difficult is understanding how can I better use the solver in helping me find the way by myself (without using the "take step" capability), in case techniques like x-cycles, xy-chain or 3d medusa should be used. Using the "candidates can be chained" option (with 3D functionality when needed), and the show bilocation links tool, the situation gets very confused and it is very difficult to spot the solution.
Is there something I am missing?
What I think could help very much would be developing a tool for highlighting specific candidates in green or red and following the chain manually from a cell to another, with the tool creating automatically the relevant link, discriminating if strong or weak.
That would be great!!!
Do you think it would be possible in the next future?
Waiting for your feedback
Thanks for your great job

Andrew Stuart writes (26-May-2018):

I get where you are coming from. Searching for a *any*pattern in the blizzard of numbers is difficult just with pen, paper and eyeball. I started this project hoping there would be simpler strategies (but still not trial and error) and was surprised, that in the difficult puzzles those I've documented really are the best solutions. I'm certain that few newspaper puzzles rarely have the need for them and we compilers have to look very hard for hard puzzles (the distribution is weighted to easy by a long way). The site tries to record all valid approaches even ones that are very academic (that is ones that are best handled by a computer) but I do get some people claiming they solve with these methods by hand. I don't know, is the short answer. I can see that some better tools on the solver and in the app could help. But I'd need a very detailed description of how a person would use them as I don't solve individual puzzles in the same way. My work is generalizing from a strategy to find algorithms that work across many puzzles.
The other problem is space. The app doesn't really have the space to fiddle with candidates. I could try a zoom or make it tablet only. I'll take a look at candidate highlighting and links in the next version then.

Sunday 20-May-2018

... by: , Italy

Hei Andrew, why don't you add the little tool to show chains to the android app also?

Andrew Stuart writes (26-May-2018):

I will attempt to do that next version

Friday 23-Mar-2018

... by: Murray Goold, Perth, West Aust

Incredible: so much info!
Your Pattern Overlay example is prob missing a 3: G2 has to be 3 coz it’s the only 3 on Row G.
Saw several typo.s but only remember 2;
Somewhere around Medusa you’ve typed:
1. “forth”, should be “fourth”
2. “vis versa”, but appears ok as “vice versa” in intro section
Thought I had an “unsolvable” from Iversoft in Apple Store, but I’m sure Loops or Chains will “solve” it - to me (at this stage) that’s not really solving, its trial-and-error.
BUT, big thanks for making this info available. NB, I’ve only just got internet (and iPhone).

Andrew Stuart writes (27-Mar-2018):

Welcome to the Internet. Thanks for the typos. Believe me some very eagle eyed people have been reading these documents for over a decade (the older ones) and sending me corrections, so it still amazes me there are typos to be found, so well spotted.

Thursday 22-Mar-2018

... by: Prime97, England

Andrew like your site and i have found it most helpful and instructive. I cannot claim that I understand everything you write about but I am tackling much harder puzzles than previously. Your emphasis seems to be centred on the computer logic and I find it takes a lot of hard study for me to understand how to use it. I would like it better if it led me gently from the things to look for in starting on a technique/strategy and the method of using it manually, but your examples are a help. Your site is my preferred learning site. I would love to be able to consign Bowman's Bingo to the archive shelf.
Thank you.

Andrew Stuart writes (27-Mar-2018):

Yes, you are correct in identifying a bias. To me the puzzle is how we can crack Sudoku puzzles generally rather than specific instances. I've struggled to follow the discussions and strategies on the forums (back in the day, some of these forums are sadly lost now). Partly this is because some people, clearly very clever people, were nevertheless poor at describing their ideas and I was often reduced to studying a bare example to see if/how it could be generalised and picking through the esoteric terminology. The process of documenting it at a level I could understand was part of the way to understanding it for me and that helped make an algorithm to detect all cases. I try and illustrate the scope of a strategy with exemplars and get all the variations which must come from the algorithm. Sudoku is very deep in that there are still example puzzles with bottlenecks that will never be solved with simpler strategies, that's just it's nature. Some strategies are far from pen and paper solutions and are rather academic, I agree, but one of the aims of getting them out in public is to research ways to replace them with better, simpler strategies, if that is possible. Which is why it's such an exciting puzzle to ponder.

Thursday 22-Mar-2018

... by: PP, NC, USA

Best solver I've found! I learn from it, which is the best of all for me. Thank you!

Andrew Stuart writes (27-Mar-2018):


Sunday 7-Jan-2018

... by: Pete T., USA- Michigan

Still the best Solver on-line. When I get stumped I come here. When I want to learn or understand a solving strategy I come here. The only suggestion I have is to have each solved number show initially in a different color (maybe yellow) and then change to blue at the next "show possibles". It is sometimes difficult to notice all the solved squares in reviewing the board before they join the other blues. Likewise when using the 'back' button, the latest solved squares could be highlighted to highlight what just happened.
Thanks for a great solver

Andrew Stuart writes (7-Jan-2018):

That’s a sound idea and you catch me in an idle moment. So refresh the solver. I've given new solved cells a blush. I can't see to do yellow since the shadow color seems to blend with the font color which is blue. Odd but otherwise ok I think
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