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!

Wednesday 14-Jul-2010

... by: Reg, Australia

Ive never replied to anything on the Net - ever- but I just have to write and have to say this is a BRILLIANT site. Thanks so much.
I only looked for such a site because I got to the end of a 'medium' level book and was p'd off I couldnt solve the last one!
I thought people got to my level, of using maybe a dozen tactics and then had to start guessing to get a solution (and stuff that!) - not so! Theres so many more clever tactics to use!
Thanks again

Andrew Stuart writes:
Thanks Reg, glad my site is working well for you :)

Wednesday 7-Jul-2010

... by: Chris Queen, UK

Hi Andrew

A friend sent me The Times Killer # 1479 for Fri 18th June (it was marked as deadly and deadly it was!!).

I got hardly anwhere on it - but your Solver started off cheerfully enough. Suddenly it ran out of ideas and gave up.

I am pretty sure that I typed it all in correctly. Does this mean I have found a bug or is it too complicated for the solver - I should add that some of the cages contained 7 digits and were spread over 3 boxes.

I haven't sent it to you as I am not sure in what form you need to see it

Should I retire injured and tear the horrid thing up?

best wishes - Chris

PS I really admire the Killer entry Software

Andrew Stuart writes:
It might be a very hard killer indeed, I'm surprised the solver doesn't get it. When I make killer puzzles about 0.5% out of a large run will be still unsolvable by my grader. There are extremes which it can't solver logically - and these are interesting as there will be strategies to be dug up. I don’t think the solver can cope with cages more than 6 or 7 cells, but I don’t think the Times ones are often like that. Screen shot is the only way to send the killers at the moment, I need to devise a way to sending them by the website. Strategy-wise, Sudoku and killer, there's plenty more to find out

Friday 2-Jul-2010

... by: Steve, UK

I've tried to solve your first "Extreme" Str8ts and you're certainly right about the need for time and patience!!!
Just for my peace of mind, do you consider that there is a "logical" solution to this puzzle or is "trial and error" required?

Andrew Stuart writes:
The str8ts solver - and the puzzle generally - is at an early stage, so I expect to find some more logical strategies, hopefully from the solving community. But as it stands "trial and error" is required for the Extremes. This is the first time I've published a puzzle without a known logical solution, but I did so in response to Ulrich in Germany who is investigating minimal clues/black cells, and I wanted to throw a few such puzzles up to broaden the Str8ts spectrum.

Friday 2-Jul-2010

... by: pj90266, Manhattan Beach, CA

Load Sudoku:

so - this is too tough for your grader - but it has a unique solution that your program can find - came from los angeles times sudoku 7/2/10
~phil a

Andrew Stuart writes:
This can be solved using the solver - Bowman's Bingo is used. But that strategy is not used in the grader. If you keep using "Take Step" it will solve. Nasty puzzle though :P

Tuesday 29-Jun-2010

... by: Cathy, France

Thank you for this wonderful website.
It's really helpful to solve hard sudoku.
I'm buying the book definitively !!
Carry on

Andrew Stuart writes:
Appreciated, thx Cathy :)

Monday 28-Jun-2010

... by: HO, Geelong, Australia

I am using the 6x6 KenKen® Solver and am pretty sure I have solved a few puzzles, but it doesn't seem to recognise when it is solved.

I am using Mozilla Firefox as my browser.

Andrew Stuart writes:
There are some bugs I know of and it deserves a new version. I'm made some progress but its a question of time before I can put the new version up. Appreciate the prod though

Wednesday 23-Jun-2010

... by: Becca, USA

Thanks so much! Yours is the first solver that could explain the strategies to my right-brain thinking.

Andrew Stuart writes:
Awesome, appreciate the feedback

Tuesday 22-Jun-2010

... by: RGS, Hong Kong

From a novice - modelled Sudoku killer number 100 from The Times Killer Sudoku book 3 on the Sudoku solver, which came up with an error - '0' appearing in one of the boxes - any insights on what this means?

Andrew Stuart writes:
It means the solver eliminated all the candidates in that cell. That usually means the puzzle is broken, or you perhaps missed a clue or cage configuration. I'd be very surprised if the Times Killer was faulty as of itself.

Saturday 19-Jun-2010

... by: john lawlor, Dublin,Ireland

Hi Andrew
Thank you for keeping my sanity,I can sleep easy at night knowing a puzzle is solved.I Don't tell anyone you did it


Thursday 27-May-2010

... by: Brian, Australia

Hi there.
In your Xwing solutions etc. you offer explanations such as (I quote)
"- Contradiction: When D7 is set to 4 the chain implies it cannot not be 4 - it can be removed"
The words "cannot not" be something indicate that it must be that something. You then say to remove it. I don't believe that a double negative is appropriate or correct in the explanation.
Please correct me if I am mistaken here.
Brilliant program anyway.

Andrew Stuart writes:
"cannot not" is a typo and should simple be "cannot" - beg pardon. I have corrected it in the source code and will be released when I install the next patch. Appreciate the alert :)
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Thank-you everyone for all your questions and contributions.