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!


Tuesday 6-Mar-2012

... by: Joseph, US

I am having a problem understanding 3D Medusa Rule 6. I understand the rule; however, I don't see what makes the 1 in cell E6 the color green? Shouldn't there be a blue 1 in the same unit (row, column, or box) that therefore would then make the 1 in cell E6 green? If that is the case, then I think I understand why the 1 in E6 and the 7 in E5 can be removed, but in order to do this, there must be a blue 1 that can see the green 1. Perhaps I am missing something or just not understanding this?

Andrew Stuart writes (9-Mar-2012):
The 1 in E6 is green because the 6 in E6 is blue. E6 is a bi-value cell which means it can be a link in a chain. The chain (or net) rather got to the 6 in E6 first and turned it blue. Then it continued to the 1 turning it green. You are not taking into account bi-value cells basically, you are just looking in units (which is valid for Singles Chain, the 2D version of Medusa)

Monday 5-Mar-2012

... by: Eric, Tennessee, USA

I just want to say this is a fantastic site. I appreciate the design which not only allows me to attempt to solve puzzles by letting me put in (and then eliminate) my own possibles as I work but also will step me through to the solution if I get stumped.

When this site was unavailable for a time, I went looking for another site which would allow me to "work" the puzzle myself and not hand-feed me all the possibles. Finding none, I eventually just gave up looking and returned to pencil and paper. I was relieved when I saw your site back on-line.

You've done an amazing job with THE top-notch site. THANK YOU.

Andrew Stuart writes (9-Mar-2012):
Thanks Eric. I've had good luck over the last couple of years with this server, but mid week this week the whole ISP got taken down by hackers sending spam. We share this planet with some unkind humans.

Monday 5-Mar-2012

... by: Chuck, Virginia

I have an idea for another feature in Sudoku Solver. When I'm stumped on a Sudoku, I like to import it into the solver and then step through it with only the stratagies I use enabled. Since there is no way to turn off the EASY stratagies, I have to step through all of those until one of the harder stratagies are needed. How about adding another check box, in the EASY stratagy area, which would let the solver run through them at speed when "step" is selected. The solver would then stop at the next harder stratagy needed or with the "run out of stratagies" message if the needed stratagy was not one of those selected.
Thanks for taking time to listen to me ramble on.

Andrew Stuart writes (9-Mar-2012):
The lack of a check box on the easy strategies is deliberate I'm afraid. I noticed early on when developing the solver and the software that these easy strategies are essential for clearing off candidates and preparing for more complex one. Basically some harder strategies will mess up the board rather badly if I don't allow the easy ones to run immediately after. This is why I return only one instance of a harder strategy even though it might identify many. The consequences of a hard strategy need to be played out completely from the top. This also emphasises the 'minimum path' philosophy of the solver.

Monday 20-Feb-2012

... by: Jean-Claude, Paris, France

Please, Andrew, give usour weekly unsolvable !

# 42 is missing !

Thanks

Jean-Claude

Andrew Stuart writes (20-Feb-2012):
42 is back. I'd put two number 43s in, sorry about that!

Thanks for the alert


Sunday 22-Jan-2012

... by: mx8man, UK

Is there any way to "re-visit" a killer puzzle that I've typed in and that has been accepted by "solver"..... only to find that I've missed deleting a line, effectively giving a "zero" square. Otherwise have to re-type the lot....

Thanks for an EXCELLENT site!
Rgds.

Andrew Stuart writes (23-Jan-2012):
yes I'm looking for ways to improve the state saving on puzzles. I'll bear this in mind.



Saturday 21-Jan-2012

... by: HezzieQ, Australia

A great tool. Using your site, I proved that my local paper's soduko from reubenspuzzles. com.au ( no.3895) was insolvable. And here I was tearing my hair out wasting a couple of hours. Thanks.

Andrew Stuart writes (23-Jan-2012):
Glad it's working for you!

Friday 20-Jan-2012

... by: PeteTy, cincinnati,oh,usa

Load Sudoku:

checked out the article with 17 clue sudoku proof

i find one solution and its an ez puzzle

there are some textbooks like this where the author doesnt set the type and add the pictures the one on the pdf link seems fine

Andrew Stuart writes (23-Jan-2012):
I figured it out. Nature's article tries to reproduce the 17 Clue example from the paper but they get the 43 in the wrong place, giving an invalid puzzle with 9734 solutions. The Sudoku in the paper is correct. I have updated my article to reflect this.

Saturday 7-Jan-2012

... by: grok, Si Valley

Hello Andrew,

I wonder if you noticed this article about a mathematician who claims to have proven that a valid Sudoku puzzle requires at least 17 numbers to be given.

Andrew Stuart writes (9-Jan-2012):
Hi Grok, excellent head's up and a fascinating paper. I've made a 'news article' page about it here.

Thursday 5-Jan-2012

... by: Chuck, Louisville, Kentudky, USA

Andrew, I continue to admire your modesty, and how respectful you are of your fellow puzzlers. So often very bright people get too insecure to acknowledge contributions of others. Thanks for your openness and accessibility.

Andrew Stuart writes (9-Jan-2012):
Thanks Chuck, warm words, most appreciated.

Thursday 5-Jan-2012

... by: Bob Tuck, Canada

It would be nice if I could delete numbers your solver inserts. I see a strategy and want to eliminate choices, but I have to keep them in until I can either insert a number in that cell or eliminate it by taking the steps. I love your site and use it often, not to solve the puzzle, but because you neatly put possible numbers in I can readily look for a strategy. When I see one, I would like to take out the numbers the solver has inserted.

Andrew Stuart writes (9-Jan-2012):
I'll take another look at that aspect of the solver. I'm sure I thought there was a reason to 'protect' clues but I'm thinking its overrated as well

Tuesday 3-Jan-2012

... by: Serge, Russia

stop on naked quads... no progress

Andrew Stuart writes (4-Jan-2012):
Lots of email alerts from people discovering the solver is crashing, last three days. Had to happen when I was in a plane all day yesterday which was awkward, but on the case now. I restarted the solver last evening (UK time). Update I made on 1st probably has a problem I'll have to track down. But I always appreciate alerts. Thanks everyone.




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