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 andrew@str8ts.com.

Post a Comment or Question here...

Thursday 29-Dec-2011

... by: Bill, Florida

Can all solutions be found by logic or is it sometimes necessary to use a trial and error approach?

Andrew Stuart writes:

Theoretically it should be possible to use logic on all sudoku puzzles, but I don’t possess every Sudoku strategy, although I do try to incorporate new ideas as soon as possible. Currently, if I produce a large amount of random stock, about 0.5% will still be unsolvable, so it’s possible to produce many of these ‘unsolvables’. I don’t think there is a general solving strategy that is entirely logical, but people do post solutions which combine several strategies to get past bottlenecks and the solvers on the weekly 'unsolvable' do come up with some pattern based method for these examples. My job, when I get the time, is to work out what they have done and try and make a general principle I can add to the solver armory.

The problem is candidate density. If you look at my solver when it comes to 'Run out of known strategies' you will see most cells contain 3 or more candidates. Most of the advanced strategies and all the chaining ones require bi-value (2 in one cell) or bi-location (2 in one unit) to get anywhere. So there is plenty of room for more thought and ideas, which is the attraction of Sudoku – its very deep.

Sunday 11-Dec-2011

... by: Hache, UK but living in France

Hi Andrew,

Not sure whether this is a bug or not. I am using a MacBook Pro running Mac OS X and Safari. Pressing enter on the keyboard after 'take step' has no effect.

Also a question: Sometimes when I'm stepping through to check a solution I notice that although I have completed the grid correctly I haven't always arrived there by the same route as the solver. Does that mean that I have arrived at the correct solution by coincidence rather than true logic or are there multiple routes to solving a grid?

Fantastic site by the way - I use it every single day.

Many thanks

Andrew Stuart writes:

Great to hear you are enjoying the site. I don't have a Mac to test it on but I do have Safari for the PC which does work for me. Sounds like a browser bug if Take Step isn’t working...both mouse and keyboard ?

Yes there are many routes possible - unchecking some of my strategies will reveal alternatives, but even within a strategy it only returns the first or shortest example. I have tried to use logical and statistical techniques to order the strategies in terms of complexity and then return the easiest/shortest solve routes so I hope it is optimal, but humans also have intuition which I can't program. So you might find another way.

Hi Andrew This is an X Solver: After a few steps you say that Simple Colouring Rule 5 removes candidate 7 in B2, because it can see two different coloured 7's elsewhere. But it can only see two greens - if the solution was a purple 7 how does it eliminate the 7 in B2? What am I missing?

Andrew Stuart writes:

Hi Kate This is a nice example of Simple Colouring in Sudoku X - short because of the diagonal. The 7 in B2 can see two green cells - and importantly the blue 7 in G9 because of the diagonal. This is an excellent example of taking advantage of thee extra constraint.

In todays daily Sudoku (12/2), at the step prior to the simple coloring solution, I think there is a hidden tripple at A5,A8,A9. (1,3,7) If this is so, then the puzzel is easly solved without using the more advanced solutions.

Andrew Stuart writes:

It's A589 is neither a Naked Single of {1,3,7} because of the 5 in A5, nor a Hidden Single because of the extra 7 in A3. It can work out, however, if you strike lucky.

Tuesday 29-Nov-2011

... by: Lewy, Alaska, US

It would be truly wonderful to be able to add a color to a candidate within a cell. Your newer explanation of coloring leads me to want to try that method, but I really like being able to mark up a puzzle on the computer.

I'm sure that would be difficult to do, but it surely would be helpful.

And ... many thanks for all the work you have put into this page. Great job.

--Lewy

Andrew Stuart writes:

Complicated but perhaps doable. The user interface if already fairly complex but I'll give it a good think over

Thursday 17-Nov-2011

... by: triemers, Netherlands

Hi Andrew,

Today's Killer Sudoku has multiple solutions. Your solver (if you let it run unassisted) will only show one because of an assumption made regarding the possible combinations allowed in a cage: no number can ever occur twice within a cage. This however is not valid when the cage crosses Box boundaries. Specifically for today A3-A4-B4-C4=21, your solver shows as solution (4,9,1,7), when (4,6,4,7) is also a valid combination in this puzzle. It will actually lead to multiple valid solutions.

BTW: I thoroughly enjoy my daily ration of puzzles. :-)

Andrew Stuart writes:

Not if you take into account the Killer Cage Convention, which I do. Have a read of this: The_Killer_Cage_Convention

Friday 4-Nov-2011

... by: Pete, cincinnati,oh,usa

thought it seemed kinda way hard

guessed my way to an answer

loaded it into your solver said solution count zero

the solver solved it too a bunch of steps with the extreme chains

Daily Jigsaw Sudoku for 4-Nov-2011 No 96, Diabolical

Andrew Stuart writes:

I think you're right, that one is too hard to be just diabolical. I believe the issues is that the spectrum is so wide at the tail end diabolical are in too great a range. I did some recalibrartion recently on the jigsaws and I wanted to extract the extremes by picking the hardest 10% or so, but that still leaves too many like these in the diabolical. What I need to do is tweak the heuristics to promote puzzles that rely on too many 'extreme' strategies. I'll give that a go in the next week or so

There is a problem with the solution count on the jigsaw solver. Its not working for me but is working offline. I'll have to figure that out first.

Saturday 29-Oct-2011

... by: JCS, Belgium

Why are there no Daily Kenken anymore ? It was fun !

Andrew Stuart writes:

Bad timing. The stock ran out just as I went away for a week. However, I have uploaded a new stock with four grades throughout the week and a link to the KenKen solver. Share and Enjoy

Friday 28-Oct-2011

... by: Tommy b, Scotland

Fantastic site. Been imagining this existing for weeks.....and here it is..learnt so much!

## Thursday 29-Dec-2011

## ... by: Bill, Florida

The problem is candidate density. If you look at my solver when it comes to 'Run out of known strategies' you will see most cells contain 3 or more candidates. Most of the advanced strategies and all the chaining ones require bi-value (2 in one cell) or bi-location (2 in one unit) to get anywhere. So there is plenty of room for more thought and ideas, which is the attraction of Sudoku – its very deep.

## Sunday 11-Dec-2011

## ... by: Hache, UK but living in France

Not sure whether this is a bug or not. I am using a MacBook Pro running Mac OS X and Safari. Pressing enter on the keyboard after 'take step' has no effect.

Also a question: Sometimes when I'm stepping through to check a solution I notice that although I have completed the grid correctly I haven't always arrived there by the same route as the solver. Does that mean that I have arrived at the correct solution by coincidence rather than true logic or are there multiple routes to solving a grid?

Fantastic site by the way - I use it every single day.

Many thanks

Yes there are many routes possible - unchecking some of my strategies will reveal alternatives, but even within a strategy it only returns the first or shortest example. I have tried to use logical and statistical techniques to order the strategies in terms of complexity and then return the easiest/shortest solve routes so I hope it is optimal, but humans also have intuition which I can't program. So you might find another way.

## Saturday 3-Dec-2011

## ... by: Kate, London, UK

Load Sudoku X: CLICK TO LOADThis is an X Solver: After a few steps you say that Simple Colouring Rule 5 removes candidate 7 in B2, because it can see two different coloured 7's elsewhere. But it can only see two greens - if the solution was a purple 7 how does it eliminate the 7 in B2? What am I missing?

This is a nice example of Simple Colouring in Sudoku X - short because of the diagonal. The 7 in B2 can see two green cells - and importantly the blue 7 in G9 because of the diagonal. This is an excellent example of taking advantage of thee extra constraint.

## Friday 2-Dec-2011

## ... by: kenton, Alexandria, VA, USA

Load Sudoku: CLICK TO LOAD## Tuesday 29-Nov-2011

## ... by: Lewy, Alaska, US

I'm sure that would be difficult to do, but it surely would be helpful.

And ... many thanks for all the work you have put into this page. Great job.

--Lewy

## Thursday 17-Nov-2011

## ... by: triemers, Netherlands

Today's Killer Sudoku has multiple solutions. Your solver (if you let it run unassisted) will only show one because of an assumption made regarding the possible combinations allowed in a cage: no number can ever occur twice within a cage. This however is not valid when the cage crosses Box boundaries.

Specifically for today A3-A4-B4-C4=21, your solver shows as solution (4,9,1,7), when (4,6,4,7) is also a valid combination in this puzzle. It will actually lead to multiple valid solutions.

BTW: I thoroughly enjoy my daily ration of puzzles. :-)

The_Killer_Cage_Convention

## Friday 4-Nov-2011

## ... by: Pete, cincinnati,oh,usa

guessed my way to an answer

loaded it into your solver said solution count zero

the solver solved it too a bunch of steps with the extreme chains

Daily Jigsaw Sudoku for 4-Nov-2011

No 96, Diabolical

There is a problem with the solution count on the jigsaw solver. Its not working for me but is working offline. I'll have to figure that out first.

## Saturday 29-Oct-2011

## ... by: JCS, Belgium

Share and Enjoy

## Friday 28-Oct-2011

## ... by: Tommy b, Scotland

## Wednesday 19-Oct-2011

## ... by: Ballooh, South Africa