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!

Monday 29-Jun-2009

... by: KM, US

I noticed in your new killer solver that the solver doesn't check the individual sums of the cages early on. For example, a cage of 2 blocks that needs to add up to 11, where one block has been narrowed down to 7 or 8, and the other to 7, 8 or 3, is essentially already solved, but the solver doesn't catch onto this until much later through some more complex strategies. Also, I had one where a cage of 2 blocks needed to add up to 7, and the one block was a 4. The second block should have been solved, but the solver didn't catch onto this until much later.

The solver definitely does the trick, but doesn't necessarily always take advantage of what I would consider already solved squares.

I've been solving killers by hand using the edit option in your regular sudoku solver for nearly a year (I'm too lazy to actually use a pencil and paper), but this definitely makes it easier since I don't need a second window open with the actual layout of the killer that I'm trying to solve. :-) Thanks!

Andrew Stuart writes:
I think that was a bug. I've loaded a new version now. You might have to clear cache (I hope now). I'd be very interested to know if the problem has cleared up. I realised there was a problem myself when the tough example didn't solve.

Bug reports like this most appreciated

KenKen 6x6 is in trial at the moment, hope to release soon :)

Monday 29-Jun-2009

... by: Bill Snedden, DeLand, Florida USA

When a site displayed Sudoku is solved it would be cool to make a sound or display a congratulatory recognition.

That would signify an accomplishment. For joining (and perhaps paying) members you might offer some kind of a score board. The scoreboard would be kept on the users computer, maybe by category; e.g. easy medium, difficult, Extreme, crossword, etc.

Not certain about the mechanics, but this might entice more paying customers to come back.

Andrew Stuart writes:
I think you are refering to the Daily Sudoku at ?

Yes, there's a great deal I'd like to add to the players. I used to run a competition site which did allow scores to be saved and lists of users to shown. It was very complicated and took a lot of time to manage. I have no plans to introduce a paying members part of the site, although a number of people have tempted me. I can just about support the time on the site from the book and the puzzle packs

Monday 29-Jun-2009

... by: Harold, England

Load Sudoku:

Has the current version of the Solver stopped printing all but the final solution? (and then doesn't clear it until the next solution appears).

With the example attached, the Solver responds "NAKED TRIPLE (Box): E4/E5/E6 removes 4 from F5" but is removing from several cells; also it doesn't mention the triple in D1/2/3 removing the 1s in box 5.

The next solution simply says "POINTING TRIPLE: 5s at J1/J2/J3 points to J6, removing 5" but doesn't mention the pointing pair in C7/8, nor in G1/2.


Andrew Stuart writes:
Yes I'd introduced a bug. Fixed now in todays patch (clear cache if necessary)

Appreciate the alert though, don't always spot them

Saturday 27-Jun-2009

... by: Cliff, Missouri USA

Your book states an empty rectange must have more than two candidates in a box. Should it be two or more? Look at example in Fig. 35.7. You note a hidden pair of 6's will eliminate any other 6's in columun 1. A single 6 would also eliminate any other 6's in the columun. You are consistant in that your solver does not find rectangles with 2 canidates in a box. Maybe just a technicality as with two candidates in the box a x-cycle (turbofish) makes the same elimination.

Andrew Stuart writes:
Second para start of chapter? The text is correct - more than 2 candidates - but I was vague about a cerain point. I was meant to be refering to Empty Rectangle Lines and the Empty Rectange Intersection. 2 candidates in a box will not give you a unique ERI in that box (and nor will 3 if they are all in a line). I wasn't refering to the elimination unit - which require strong links and hense 2 candidates.

But if there's ever another edition, I'd certainly want to improve that paragraph

Sunday 14-Jun-2009

... by: Don Carroll, Cape Winelands, South Africa

Today's (14 June) Killer sudoku has a problem - the dotted number grids do not link up and number is too large and in the center of the cell.

Thanks, Don.

Andrew Stuart writes:
Fixed. I'd broken that when I put in the new Killer Solver. Thanks for the alert

Saturday 13-Jun-2009

... by: Gus, usa

Updated Firefox to 3.0.11 and Solver now halts at x-wing. Can't solve Friday Jun12, 2009 in LA Times, without the advanced strategies.

Andrew Stuart writes:
I have radically changed the way the browser calls the server now. Can I ask you to first clear your cache and try again? I know it works for me but there could be a problem and I want to eliminate cache issues first. Let me know what happens

Friday 12-Jun-2009

... by: Larry Orrick, USA

I hope your puzzle packs are coming soon.

Andrew Stuart writes:
My bad, the month's now switch over automatically but I still have to remember to put them on the web site. Doh! All there now.

Sunday 31-May-2009



I am greatly impressed by the clarity of your logic and the examples you provide. When I ordered your Logic of Sudoku I waited for it like a kid waits for Santa Claus, and have loved being able to navigate so easily through a finely structured, well indexed reference.

My comment has to do with language. In seventh grade English class, Mrs. West demanded precision in our grammar. She still stands over me six decades later and clucks at imprecise syntax. The logic of your puzzles is impeccable; in Box/Line Reduction your language is not. In the following sentence "only" applies to "box 5" and "row D," which is not logical, Captain. [I also love your sense of humor.]

LBR: 1 exists only in box 5 and row D, can remove from E4

An accurate sentence would be:
LBR: 1 exists in row D only in box 5.

Most of all, Andrew, I am thoroughly impressed with the gentleness and respect for others you demonstrate in your responses to input. You are modeling in your field what our world leaders need to practice.

Thanks for your delightful, entertaining and exemplary work.

Andrew Stuart writes:
You comments are most appeciated, thanks for writing in :)

I believe the better sounding sentence you have proposed refers to the book? I've added it to my master document. I'm not sure if or when there will be a second edition, but it's great to have such feedback. I can easily correct the web site version

But this is shorter and different text, so think that stands


Friday 22-May-2009

... by: Janet Barclay, UK

I LOVE your Sudoku solver - I always use it when I get stuck, usually on the X-wing strategy, thanks for saving my sanity!
I'm just wondering whether there's any chance of you becoming interested in KenKen puzzles - I can't find a solver for them that gives the logical steps as your Soduku solver does. There are KenKen solvers that just give you the answer, which isn't any help at all. Here's hoping!

Andrew Stuart writes:
I have now been able to release a KenKen solver here. Enjoy

Thursday 14-May-2009

... by: Mrs Peel, Arizona

I have to say that every time I visit the solver pages I am very impressed by the improvements and delight in using these pages. The help has allowed me to progress to doing the most dillicult puzzles without relying on solver help. It's a tribute to what you've done and accomplished.
Many thanks from this part of the country.

Phil Zell

Andrew Stuart writes:

Thank you Phil, great to receive such positive comments.

Best of luck with your solving


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Thank-you everyone for all your questions and contributions.