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!

Thursday 25-Jan-2007

... by: Tom, Manchester

Load Sudoku:

Hi Andrew, really like your site! It's been really useful as an instructional tool for learning the more advanced techniques. If I get completely stuck on a sudoku then I'll put it in and see what I've missed. When the solver finds the next move I'll then take a step back, switch that technique of and see if anything else works.

One thing I'd like to ask is how you choose the order of the techniques in the solver? Is it based on the difficulty of the technique? How often it occurs?

I might have a new technique for you too but I'm not sure. It's one I always use and I'd kinda assumed everyone else does to. I only really thought about this after doing Weekly Extreme No 18 from Once I'd moved to candidate elimination one use of the technique solved the puzzle. I'm going to wait until the weekly competition is over then post something about it on one of the forums.

I've put in the sudoku with all the numbers i'd found before having to move to candidate elimination.

Just a note on this, it wasn't particularly hard to get to this stage but the solver seems to really struggle and has to use 2 URs, an APE and a Forcing Chain to get close to this stage. I don't know if there's something missing from the solver or if this is a problem inherent in solving by candidate elimation. Any thoughts on this?

Tuesday 23-Jan-2007

... by: SF, USA

I learned something new about Sudoku strategies. Your site is extremely informative and easy to use. Prior to checking your site, I spent several days trying to solve a puzzle that required some of the advanced strategies described on your site. I simply never knew the strategies were possible. Thank you.

Andrew Stuart writes:
Thanks SF, always a pleasure!

Saturday 20-Jan-2007

... by: Dolores, Chicago

Thank you for the jigsaw solver.

On today's jigsaw puzzle the Law of Leftovers came up twice. I hope you have some time to show an explanation.

Also, I have been having a problem when changing canditates. I use Safari. Sometimes when I am clicking, it shuts down the browser or takes me to another place ( Sudoku solver).

Andrew Stuart writes:
Hi Delores, I also use Safari/Firefox. I'll check it again but that's very od behaviour. It used to do something similar when I had the Flash look advert, but I changed it to an HTML one within an hour, and that was months ago.

Saturday 20-Jan-2007

... by: Ron, Thornhill On Canada

I could not find todays's (jan 20) entitled Legoman in your table of shapes

Andrew Stuart writes:
I've no idea why, but in my Jigsaw Solver it's called 'Cyndie'

Friday 19-Jan-2007

... by: Charlie Katz, Florida/NH, USA

Load Sudoku:

When coloring first shows up, all squares with 2s and 5s, the solver
removes one number when it could remove both 2s and 5s from
the commonly situated square. Why did it only remove one ? Seems
like a bug to me.
I really like your solver. When I get a *** or a **** in my daily paper, the first thing I do is load it so that I can print out a larger version and then I let the solver go to see if it will solve and also if it involves X-wings, Y-wings or coloring, because I still have trouble understanding how to look for those.
One day, there was a totally weird algorithm, something like Bernie's Bingo. This left me speechless. "Because this one isn't 4 or 7, that one is 9". I thought, "Someone's on windowpane". But I didn't take the time to read your more detailed description of the algorithm. I guess you've added a lot since I printed out the doc last spring. Its mindblowing. Keep up the good work.

Also, if possible, please use coloring to help to determine what has changed. The originally given numbers would be one color. Then the numbers that are changing in this current move another color. Then when the next move is occurring, all non-original, past changed another color. Thanks for your consideration. I have no idea how hard this would be to code.

Andrew Stuart writes:
Hi Charlie, in this case you are correct, there are two identical single chains, one on top of the other. They are independent - the 2 and the 5 chains, although it could be re-expressed as an XY-Chain. The solver looks at numbers from 1 to 9 in that order so it's found the 2 first. It then quits and goes back to the start igoring the obvious 5. What you are seeing is the hightlighting for the 2 only, just so happens the 5 is also on the set of cells.

I could so arrange it to look for shorter chains before longer ones in which case it would get to the shorter 5-chain before the next longer 2-chain.

Monday 15-Jan-2007

... by: fdan, NE USA

I like your solver. I put puzzles it to check for a single solution and to get a printable version and larger puzzle. Are puzzles usually symetric with their exposed values?
Thanks for a great solver.

Andrew Stuart writes:
Puzzles are usually symetric but only at the discretion of the compiler who believes they are more aestetically pleasing. There is no logical reason for them to be symetrical. Our ones some times have the odd clue subtracted to make them a fraction harder which can make them slightly asymetric.

Monday 15-Jan-2007

... by: Richard Proctor, Utah

I read Walter Hudiburg's comments about the helper and he is right, it is possible to solve any puzzle quickly. But I use the helper to teach me what is happening and to see if I've made any mistakes. This is a great aid and in it's present form has taught me a lot. I would hate to see it changed if the change doesn't allow us to learn and see our mistakes.

Andrew Stuart writes:
Hi Richard, you will be able to use the solver on all puzzles - but I will make the change soon where the Daily and weekly ones are barred during their competition time. Thats only fair. You will be able to dig them out of the archive straight after the deadline and check and learn.

Friday 12-Jan-2007

... by: Fumigator, Utah

I just want to write a comment to gush about how awesome I think your solver is.

>>>>>>> G U S H <<<<<<<

Thursday 11-Jan-2007

... by: charlie katz, Florida, NH, USA

Noticed that when we start filling in the board all of the loaded numbers are red, but as the board is advanced by your program some of those oriiginal values are changing to blue. This is confusing if I try to keep checking which of the new values are accurate in my incomplete puzzle.

Thursday 11-Jan-2007

... by: William Radcliffe, Redmond, WA

This is just a feature request for the printed puzzles. I find that having the candidate numbers arranged in a 9x9 grid more appealing visually.

I realiize that this is just personal preference, but wonder there is a way to accomplish it - maybe using style sheets and sending the puzzle down as XML

I am one of those people who is very tech savy and even own a table PC, but I still like to solve my puzzles on paper for some reason.

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Thank-you everyone for all your questions and contributions.