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 30-Dec-2008

... by: RIZDINA, USA

This site is amazing. In just one day I was able to pick up several new techniques that I was unable to master at other sites. Keep up the great work. I am regular to the site now.

Andrew Stuart writes (1-Jan-2009):
Appreciate the kind words! Have a great new year :-)

Monday 29-Dec-2008

... by: julie, Australia

I have just spent many hours on a 5 star rated Sudoku in the local paper. Finally putting into a Sudoku solver I was frustrated to find it did not have a unique solution. You are right what a turnoff. Is there a way of determining if there is more than 1 solution.

Andrew Stuart writes (29-Dec-2008):
My guess is that some puzzle publishers donít understand the puzzle or donít know who to check their puzzles. Its also possible you might have transposed a clue incorrectly? Easily done. There is no way to determine how many solutions a puzzle has bar a brute force algorithm such as I employ with the "Solution Count" button.

Monday 29-Dec-2008

... by: Ken Rizzo, Ft. Mill, SC - Just South of Charlotte NC

What an incredible Sudoku Site you've put together. Very indepth and detailed in different solving techniques. When I have time I'm planning on reading about the different strategies that you speak about. Your site is so much better than any other sudoku site I've been on. Well done Andrew.

Andrew Stuart writes (29-Dec-2008):
Most appreciated Ken, looking forward to putting lots of new stuff in 2009

Friday 26-Dec-2008

... by: joconnor, US - Bailey, Colorado

Hi, In your section on Unique Rectangles it seems to me that you are not saying that the ambiguity results in invalid solutions but that the choices are not self-defined by the existing pattern. In your Type 1 Unique Rectangle you seem to be saying not that either choice of the "deadly pattern" would be wrong as a solution but that the puzzle is improper in allowing such a choice and you are selecting a third alternative (the additional characters after throwing out the deadly duo) as proper. Is this actually the only valid choice or is it simply a third alternative?

Andrew Stuart writes (29-Dec-2008):


Interesting question. It depends on your definition of a proper sudoku. If there are insufficient clues then you will have more than one solution, although any solution that fills the board and satisfies the rules could be said to be a completed sudoku. We make the choice that a sudoku should have only one solution to make it a satisfactory puzzle. Multiple solutions while valid in the logical sense are frsutrating to the puzzle solver who does not expect this nor wishes to find all solutions.



Unique rectangles take advantage of the implicit agreement between the puzzle maker and the puzzle solver that there will only be one solution. Sometimes its explicit - as in my introductions to puzzle books or in my packs. So you could incorporate the statement of one solution into the rules and therefore the deadly pattern leads to 'invalid' as opposed to 'alternative' solutions.



URs are very common in the harder puzzles and well worth looking into, but will break your solving route if the puzzle is faulty, which is why I included the "Solution Count" on my solver as a check.

Saturday 22-Nov-2008

... by: jbm, California

Many thanks for saving me from that first tedious, time-consuming, boring, mindless step that shows only the possible numbers in each square. Without that lovely shortcut I'd have given up on Sudoku long ago!

Andrew Stuart writes (23-Nov-2008):
Great to know its working for you :-)

Saturday 22-Nov-2008

... by: Bob Spessard, USA

Why are pointing pairs down the difficulty list. They seem to be much easier to spot than hidden pairs and triples?

Andrew Stuart writes (23-Nov-2008):

The order of the strategies is only roughly in order of complexity, there's a certain amount of user preference and thatís my list. I'd like to offer the chance of re-ordering them but itís a bit of a programming job. So I don't disagree with you

Saturday 22-Nov-2008

... by: Paul, London, England

Using your Sudoku assistant in Google Chrome it is not possible, after using the mouse to start the solution process, to then use the enter key to step through the remaining steps in the solution. The mouse must be used for all the steps - very tedious. Because of this I revert to IE5 when I need your wonderful tool, such as the Diabolical sudoku in yesterday's Daily Telegraph.

Andrew Stuart writes (22-Nov-2008):

I've just downloaded Chrome to confirm this problem. Previously was replying on user reports who said the solver worked fine in Chrome. I've done some research and testing - its not the page, its definitely a short coming with the browser. Button simple do not show focus when you have focus on them. Nor does enter key activate them. I hope they fix it with the next release. It is a very new browser.

Thanks for the alert



Saturday 22-Nov-2008

... by: suchita chaturvedi, india,asia

ur site is amazing.I've made a project on sudoku solver only with the help of ur site. It helps me a lot.It gives a very good idea about how the puzzle is solved stepwise.iI want to thank u as much as possible. THANK UUUUUUUUUUUUUUUUU

Friday 14-Nov-2008

... by: Pat Williams, USA

Load Sudoku:

I liked your site.

Andrew Stuart writes (14-Nov-2008):
thx!

Sunday 19-Oct-2008

... by: gkelly, layton, ut

Load Sudoku:

I thought I understood coloring until your solved showed how to eliminate a 4 at H4 but showing a chain at C2,A3,C4,H3 how is it that from this you know to remove 4 from H4?

Andrew Stuart writes (19-Oct-2008):
Ok its an off-chain elimination. Chain is H3->A3->C2->C4 and the reverse. Both ends can see H4 and both ends are difference color - so one with be a 4. so H4 can't be

That help?


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