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!

Friday 3-Nov-2017

... by: George, USA

Why doesn't your solver utilize multi chain coloring techniques? I see singles chaining and bivalue cell color chaining (3D Medusa). I'm new to Sudokus, and wondering if multicoloring is a poor technique, or if there are better techniques that take place of multicoloring? Thanks, you step by step sudoku solver with explanations, this has made learning the advanced techniques easy.

Andrew Stuart writes:
I'd have to see a good definition or specification for "multi-chain coloring" to assess it. Coloring overlaps with alternating interference chains a good deal, and may be the same thing to some people. I do have duel, triple and quad AICs all coded up and ready to be used at the end of the strategy list. Try some of the examples and tell me if that’s what you mean.

Sunday 29-Oct-2017

... by: Thom Mitchell, United States

I have enjoyed the Solver for several years:
(1) to check my answers for puzzles-without-solutions;
(2) help me improve my solving ability;
(3) nudge me forward when I get stuck with a particular puzzle; and
(4) check Solution Count.

Today I solved a puzzle, checked my solution and found my solution missing in Solver's Solution Count. This suggests an obvious question: Solution Count isn't necessarily exhaustive, correct?

Andrew Stuart writes:
Sorry for the late reply. What do you mean " found my solution missing" ? That you solution didn't match the solution count? Were there more than one solution? There should only be one for a valid puzzle, I'm sure you know

Solution Count is perfectly and 100% exhaustive, I can assure you. For low numbers of clues it can take a long time (and time out since server resources are throttled) and for multiple solution, it will bail at 500 solutions, since more are uninteresting

Tuesday 10-Oct-2017

... by: Karthik Raj, India

Dear Stuart,
I have been using your ios app for quite sometime now. Due to IOS 12 upgrade, I am not able to use it. Can you upload a new version with 64 bit support for people who have purchased it earlier.

Karthik Raj

Andrew Stuart writes:
I'm pleased to let you know that the iPad '5 Daily Puzzles' upgrade is complete and released. This will work on iOS 9, 10 and up.

UK store link
US store link

If you still have the app on your device, it should automatically update

Thursday 4-May-2017

... by: Michel, Canada

There is a Killer Sudoku board I would like you to look at

Click on this link:

I am at my wit's (such as it is) end having spent far too much time on this one.
It comes from Stefan Heine Killer Sudoku Hardcore #4.
If you can suggest an approach or another site where I might be able to solicit assistance, I would very much appreciate it. Thank you.

Andrew Stuart writes:
Sorry for the late reply, been on a vacation.

Yes I don’t have enough strategies for very hard Killer, I'm at about 50% at the moment, compared to 1/10,000 normal Sudoku

Sorry It could not be solved, but love to know how it can be solved logically

Friday 17-Mar-2017

... by: Doug, USA

Hi Andrew. Are there theoretical limits to how few numbers can be present initially to produce a unique solution? Are there algorithms for producing solvable sudokus, or is a trial and error approach necessary? More generally, has the game been thoroughly analyzed mathematically?

Thanks for your great site and for maintaining it over the years!

Andrew Stuart writes:
Good questions. 17 is the smallest but this has only been proven computationally not mathematically. There is a triangular relationship between making, solving and grading puzzles. Have you read this paper on the site?

Friday 17-Feb-2017

... by: Ed, USA

I really enjoy sudokuwiki. I doubt if I will ever get good at sudoku, but I try. There are a lot of things I don't understand in the strategies section, but I do have a question: What is meant or intended by "Show Possibles"

Thanks for the great games!!

Ed Kendall

Andrew Stuart writes:
This step looks at the clues (or recently solved squares) and runs along the rows, columns and boxes removing candidates that can't exists because of the clues (or solutions). It’s a clear-off step. Actually the word "possibles" harks back to the first year of Sudoku (2005) when people were making up names for everything and nothing was very fixed. I probably chose that as one of the friendly words at the time to denote the notes we make in the puzzle. Since then I've settled on 'candidates' in all my documents, so it’s a bit of an archaism.

Since the solver is a "step by step" solver I break down the moves into
- Strategy removes some candidates
- cells with one candidate left are changed into big numbers
- "show possibles" clears of candidates implied by that solved cell

Otherwise stuff would change too much for people to follow

Friday 3-Feb-2017

... by: dave, canada

Do you have puzzle packs that are made up of just one type of sudoku? I'd like to buy some puzzle packs that are only of the traditional sudoko type instead of packs that consist of a variety of puzzle types.

Andrew Stuart writes:
All the currently Sudoku packs are just the normal vanilla sudoku. There is one of each grade plus one with mixed grade. I used to do other Sudoku variants but not this year.

Tuesday 10-Jan-2017

... by: ynotme, France

When using the Take Step button, Sudoku Solver indicates that results (such and such a square set to such and such number, unique in row etc) will appear in the box below the grid, but in fact this happens only sporadically if at all. Is there some menu command or routine I'm missing or is it a bug ?

Andrew Stuart writes:
The reports in the box below the board only occur when something interesting happens. For very easy puzzles you may not need a strategy at all. Simple removal of candidates and the solving of cells as numbers are cleared are not reported individually (since there may be many in one step) and the same information is displayed on the board with colors.

Monday 9-Jan-2017

... by: Kay, USA

Thank you for this "solver" site!! I get stuck on some of the more difficult puzzles and simply can't see a solution. This will help me understand where I go wrong or add to my understanding of some of the more elusive strategies.

Andrew Stuart writes:
You're very welcome! Enjoy

Monday 2-Jan-2017

... by: Diane, Australia

Str8ts in 5 Daily Puzzles has not been loading on iPad for last 2 days. Other 4 puzzles load okay.

Andrew Stuart writes:
Yes I needed to add some more puzzles for 2017 for Str8ts
Thanks for the alert!
Have a great new year
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2015 2014 2013 2012 2011 2010 2009 2008 2007 2006
Thank-you everyone for all your questions and contributions.