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 18-Jul-2011

... by: Pete, Enagland

Load Sudoku:

Hi - I love your site and have used it extensively - thankyou. It has taught me a number of methods and your ability to explain things is excellent.

I'm a little confused though with the use of the simple coloring and wondered if you would just take a quick look at the following. It's almost complete and I was trying to apply simple coloring to 6s in the puzzle.

It appeared to me that coloring we can set the 6 in F2 to green, then H2 must be blue - if so H3 is green.
If F2 is green, then F5 is blue (column 5 only has two 6s) and then E5 must be green - which means E3 must be blue (only two 6s in row E).
The 6 in F3 can therefore see a green 6 at F2 and H3 and a blue 6 at F5 and E3.

Does this not mean we can eliminate 6 from F3 ?

Therein lies my misunderstanding of this technique.

Any help much appreciated.

Thank you again.

Andrew Stuart writes:
I'll try and help. In that little cluster of 6s you don't have many chains. These are where only two 6s are left in a unit. You have F2 to H2. H2 to H3. But these don't join up to the remaining: E3 to E5 and E5 to F5. So there's not enough there to work with. Being unconnected there can be no conclusions drawn between or over them as they are separate clusters.

There are a couple of Medusa eliminations further on which is the same principle as simple colouring but using all numbers.

Tuesday 12-Jul-2011

... by: Dean Finney, USA

Why are Sudoku puzzles symmetrical around the center? Is this always true?

Andrew Stuart writes:
The only reason is aesthetic. They look better in print. It is entirely possible to make non-systemtic puzzles but they look rather untidy and thrown together. Occasionally mine will have one number missing that breaks perfect symmetry but that just because I could remove one more number. Doesn't happen often.

Non symmetry is more fun and looks good in a puzzle like Str8ts, contrast the upper and lower puzzle on this page

Sunday 10-Jul-2011

... by: Cricket, USA

using online solver for KenKen
not sure how to send puzzle. it was created from
get message:
No strategy request exists for this number: 0 (MXW)
6/6 KenKen

Andrew Stuart writes:
Not quite sure what happened but the kenken scripts didn’t seem to be the latest version. MXW is a strategy I junked some time ago. I've updated it now and it should work as intended. I have also added an "Email" button to make sharing puzzles easier. Do use that if you'd like to send me a puzzle.

Friday 8-Jul-2011

... by: Herbert Jensen, HONG KONG

In your "Grader" feature for the Sudoku Solver, what is the difference between the numerical score (e.g. 119) and the verbal evaluation (e.g. tough)? Also, how are each calculated or determined.

I have not found this information on your website. If it is there, please direct me to its location.

Andrew Stuart writes:
All Sudoku puzzles exist along a scoring spectrum. I quartile this to give the grade name, although there are other rules and heuristics which may bump the grade up or down. A good discussion of the process for creating and grading Sudoku can be found in my paper here:

Saturday 2-Jul-2011

... by: Marc Linder, USA

in the past i have been able to use your solver to check my worksheet. your new change which jumps to the next usable technique tells me more than i
want to know. coudn't you put an 'enable' - 'disable' option for people like me.
otherwise keep up your great work.

Andrew Stuart writes:
I am probably going to do that very soon. On the Killer solver the number of click could be enormous which is why I implemented it, but a number of people has written in to say they use the solvers in a slightly different way

Saturday 2-Jul-2011

... by: Timothy W. Reynolds, Anderson In

When using take step then back arrow the buttons become
black and the solver dies.

Andrew Stuart writes:
Can I check you are seeing v1.75 under the big board? There was a new bug introduced with 1.74 that caused the back button not to reset correctly and broke Take Step. It should have been fixed this last week.

Monday 27-Jun-2011

... by: Lisa Higgins, Worksop, England

I don't usually comment on wesites I have visited but I just really felt the need to do it on this occasion, I have just recently started playing sudoku and I am now addicted, I have been trying harder puzzles of late and have looked at various wesites to try and teach myself the various stages of solving a puzzle using different processes and I have to say a BIG, BIG thank you as not only does it explain the process but with the added fact that I can actually out my puzzle in when I get to a point where i,ve been stuck for hours is absolutely fantastic. Thank you so so much, BRILLIANT.

Andrew Stuart writes:
Thank you Lisa :)

Sunday 26-Jun-2011

... by: dot, USA

Am disappointed with the latest update.
Your site has been unique and special in that it is a solver site that works with you.
So many other sites out there you give it some numbers and out pops a solution.
With your site you could work your way along and press "take a step" when you got to a rough patch.
Your new update changes this. One used to be able to confidently clip through the elementary steps without the solver taking off into the world of x-wings and coloring. Now there is no way to limit yourself to basic help.

And it seems, it is not always solving the puzzple correctly. For me this is a hard thing to prove to my satisfaction, much less to yours.

And, I had noticed in recent weeks, before the last update, but more prevalent now, that solved numbers occasionally vanish from the board. They will be in the small square on the upper left hand side, but stop appearing in the board. I tried e-mailing a board like this to see what it would put in the vacant spot and the emailed string was complete. A small consolation after a little confusion.

Still waiting for the iPad version.

Thanks for having made this site available for all these years.

Andrew Stuart writes:
Appreciate you're feedback, I'll see what I can do. I know many people have gotten used to the way the take step works, especially as it hasn't changed for many years. The feedback i'd had on the killer solver, for example, was that it took way too many clicks to get through a solution since for a lot of the time the basic strategies were not useful yet there was a pause at every stage. So all I have changed is to skip failed strategies on the basic ones. However, if you are used to clicking fast you will click over the replacement of single candidates changed to solved numbers and the bit at the top which restarts the order after success. You need fewer clicks and slower, since each step now produces some kind of result.

The other feedback I've had on this change is that some people appreciate the pauses since it allows them to consider each basic strategy in turn before the solver does. So the next update I'll add a tick box "always skip failed strategies" so everyone can decide if they want the new or old style. Just a matter of fitting it in somehow

As for numbers vanishing, I'm not sure how this can occur except legitimately. If you spot this, can you "go back" and take a couple of screen shots before and after? Always good to nail a bug.

Tuesday 14-Jun-2011

... by: vysnc, israel

Load Sudoku:

Hi. great website, been using it for a while.
why is this Sudoku graded 53 - easy, if it uses some Tough Strategies to solve?

Andrew Stuart writes:
It's a puzzle which solves using basic strategies almost entirely except for one bottleneck - and the score for that one tough strategy is simply not enough to get the overall grade above easy.

Thursday 2-Jun-2011

... by: Cruetsonly, USA

Is it possible to show only possible candidates that I enter myself? I want to be able to enter guesses, as I would do with paper and pencil, but this does not seem to allow it. For example, I want to be able to show that I have eliminated two squares down to 5 & 6 without showing all the other candidates elsewhere on the board. Other than that, I love the site.

Andrew Stuart writes:
The solver needs to know what candidates are available at any given stage; so an empty cell is wrong since it contains no options. It would create too many ambiguous states to have the cells partially filled by the solver and partially the player. However, you can remove candidates if you are confident of your eliminations, by using the "Candidates can be Edited or Highlighted" options above the main board. The Puzzle Players, eg the Daily or Weekly puzzles allow you to enter positive candidates rather have negative removals, but I don't have a system whereby you can import your own puzzle into a player, only a solver. I'll think about what can be done.
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Thank-you everyone for all your questions and contributions.