# 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 andrew@str8ts.com.
**Post a Comment or Question here...**

## Monday 27-Oct-2014

## ... by: Uhm, Netherlands

Load Sudoku: CLICK TO LOADHere is an picture of Simple Sudoku were i painted the ape your solver did not find.

The blue cells are the pair from ape. combination 6,3 cannot be possible because of the purple colored cell. Combination 6,9 cannot be possible because of the triple colored in orange. So 6 can be excluded from the first blue cell.

This picture is taken right after the first found ape.

About the finned x-wing and grouped x-cycles: finned x-wing stands on place 23 where as you grouped x-cycles stand on place 21, so grouped x-cyles gets executed first so finned x-wings will never get found i think.

Outlook does not want me to send emails, so here again.

[edit]

OK the second APE is now showing up in the solver. I detect about 500 more APEs in my test library of 50,000 puzzles (total APE found is around 25,962) so it’s a small but decent increase but there's a 20% increase in time spent looking for APE, but not significant compared to some strats.

Good to be complete (or more complete)

I'll look at the finned x-wings next

[edit]

Uhm can change your reactions

## Wednesday 22-Oct-2014

## ... by: Uhm, Netherlands

Load Sudoku: CLICK TO LOADThis is in the examle of AIC weak link.

And am i right that there is an overlap between finned x-wing and grouped x-cycles, maybe its better to place the finned x-wing above grouped x-cycles.

Can you provide a screen shot of the second APE? Or a scan? Like to see the full pattern.

There is an overlap, but the solver has very different algorithms for both strategies and I've not investigated the exactness of the overlap. But finned x-wing are above grouped x-cycles in the order, no?

Do leave an email address next time

Cheers

## Monday 13-Oct-2014

## ... by: Francois, Canada

Load Sudoku: CLICK TO LOADMany days to get to the solution.

Checked most solvers on the internet, and only Brute Force works if at all.

Have you come across this one before, and is there an elegant way to tackle it?

http://www.sudokuwiki.org/Arto_Inkala_Sudoku

Relatively famous

## Monday 6-Oct-2014

## ... by: Ken Marks, USA Portland Oregon

Do the cookies look like they are doing their job?

There are two on the solver:

1) is automatic and saves the board when you enter a puzzle or 'take step'. That is used to restore the board in case you accidentally close the windows, or just when you come back.

2) there is a manual one - you have to press "save" and "reload" to get that. The board must have changed in order for you to notice a difference when you reload.

Some minor cookies to remember check boxes as well

## Tuesday 30-Sep-2014

## ... by: John Kuhn, Wisconsin, USA

Your solver has been a great learning tool and reference for me, and I also found your book most useful. However, as I'm working through Frank Longo's book level 4 of 'Absolutely Nasty Sudoku', I have found something that makes me question your sequence of technique application. You are having the computer check for x-cycle before unique rectangle, and this doesn't make sense to me, because a simple unique rectangle is extremely easy to spot compared to an x-cycle. So for example puzzle #55 in Longo's book was solved with two unique rectangles as the only advanced technique, and your solver used the much more difficult xcycle x2, xyzwing, unique rectangle, simple coloring, ywing, xychain. So, just a suggestion on the ranking of difficulty. Thank you for all your work benefitting puzzle lovers!

## Monday 22-Sep-2014

## ... by: Uhm, Netherlands

But i could not find how you can program those strategies.

Did you ever thought about creating some pseudocode for on you website?

The more advance strategies are very hard to program.

I have now basic stragegies, xy-wing, xyz-wing, simple coloring, Fish-family and Guess.

But i dont know if i implemented guess and naked,hidden pairs->n with an optimal way.

Last time i tried to solve and 64x64 and it took a long time to get past all the naked,hidden pairs, triple up to 64.

But its get harder when the strategies gets harder. Its hard to come up with an good way to solve those.

Even if a pseudo-code is possible the programmer would never be sure that their implementation was complete and consistent since there is a loss of information in going from code -> pseudo-code. Also pseudo-code prevents the programmer from having a deep understanding of the problem being solved and therefore knowledge of completeness/consistency. So it can actually be harmful, in a sense, they would be writing a user-interface to a black box. Really, the best way is to start from the ground up, and being all your own work, is very satisfying. Expect to go through many incremental versions. I am over a 100 now. The recipes are the descriptions on the website and these contain examples. You can test your progress using sets of difficult puzzles, eg Ruud's top50k. I don’t mind providing those.

Thirdly, the software is commercially sensitive and an asset of the company I work for, as we try and scratch a living on the periphery of a market dominated by a few big players.. ; )

## Saturday 20-Sep-2014

## ... by: jackeith, usa

Load Sudoku: CLICK TO LOAD## Friday 19-Sep-2014

## ... by: Chuck, Virginia

It's been a while since I last wrote to you. I'm not writing with a question, just a suggestion. I would like to see a book that explains step, by step, how to find a given strategy. I have your "the Book", but like most other Sudoku books, it doesn't show you how to find the strategy, only how to use it. I don't have a problem with most of the simpler ones like X-Wing, Y-Wing, Unique Rectangles, and some others. But, Sword-Fish sometimes eludes me, and X-Y-Chain, well, that's almost impossible.

I've looked around on the web, and found some things that help, but in most cases, I can't find a true step-by-step procedure. Trust me, if you write such a book, I will buy it.

I might have written you on this subject before. If I did, well, here I am, at it again.

Thanks for listening

Chuck

What I'm doing with the book and the solver/website is to collect all the known valid ways of using pattern based (or 'state deterministic') ways of logically removing candidates and some are pretty esoteric to say the least. A computer is ideal for searching for them - as a coder I've gotten great satisfaction from devising optimal ways for catching all instances of a strategy using the least amount of time/memory - but that is a different activity from a pen/paper solver trying to solve a specific puzzle.

As you investigate a board and map out all the links in chains it's not really possible to know in advance which strategy will be most useful next. The solver gives the illusion of knowing but only because it is so fast. If it picks Medusa Rule 3, it does so only because it has exhaustively searched for and failed to find any instance of a simpler strategy. We humans can't do exhaustive searches, we have to use hunches and intuition. We will probably miss a simpler path when we use a complex one - but there are many, many paths (usually).

I would say, though, that the most fruitful area is the family of chaining strategies - you can build up a series of links between bi-value and bi-location candidates - these are the building blocks of all those strategies. Merely having an awareness of the tricks that each strategy will, in time, seep into your image of the board state and you'll get the Eureka moment.

I'd like to hear from pen and paper solvers who have tips and 'rules of thumb' they use. I'm sure there are a lot of interesting approaches. Some people swear by using dots in a 3x3 grid rather than small numbers, for example.

## Wednesday 17-Sep-2014

## ... by: Roger, USA

## Tuesday 16-Sep-2014

## ... by: Nigel Ogilvie, England