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...

Sunday 3-Sep-2006

... by: CT, Belgium

It's the best solver I found on the internet. The good thing is, that it's a step by step solver.

I have a "Sudoko Genius" book with lots of difficult puzzles. If I'm stuck on one, I fill it in in your program to find the next step. It helps me to learn what I have overlooked.

My wife (Cathy) pointed this one out to me. She takes a much more global view of the board than I do.

This is an extension of the Singles in Row/Col/Box to Doubles. In the above puzzle, she noticed that E4 and E6 must be either a 2 or 6. This is because the 2,6 in rows D,F and column 5 eliminates 2,6 from the middle box except for E4 and E6. Hence, these cells must be 2 or 6.

In the long run, this didn't have much of an effect on how fast the solver solved this puzzle. Without this info, the solver takes 51 mouse clicks to solve the puzzle. With this info it took only 49 clicks. Not much different.

This is a more interesting example of the even/odd path length strategy.

Looking at the above board, we see a pair chain of 4,6 located at: E5,E7,F8,A8, since there is both an even and odd number of steps from E5 to A5, the 4,6 at A5 can be eliminated. Your puzzle solver eliminates the (4,6) at A5 by applying simple coloring twice.

Using the solver further, we eliminate 1 at B6 using:

X-CYCLE on 1 (Discontinuous Alternating Nice Loop, length 6): 1[r2c6]-1[r2c2]=1[r7c2]-1[r8c3]=1[r8c6]-1[r2c6]- - Discontinuity is two weak links joined at R2C6, 1 can be removed from that cell

Now we can apply the even/odd chain length strategy again to eliminate the 4,8 at B7. There is an even path from F6 to B7 (F6==>B2==>B7) and also an odd bath from F6 to B7 (F6==>F8==>E7==>B7).

At this point, the puzzle is easily solved.

Your solver uses Box-Line Reduction LBR: 6 exists only in box 3 and row A, can remove from B7 LBR: 6 exists only in box 3 and row A, can remove from C7

Followed by Simple Coloring SINGLES CHAIN (Type 2): Removing 4 from R1C4 SINGLES CHAIN (Type 2): Removing 4 from R1C8 SINGLES CHAIN (Type 2): Removing 4 from R2C7 SINGLES CHAIN (Type 2): Removing 4 from R5C7 SINGLES CHAIN (Type 2): Removing 4 from R6C6 SINGLES CHAIN (Type 2): Removing 4 from R7C5 SINGLES CHAIN (Type 2): Removing 4 from R8C3

This even/odd strategy may be a specialized case of the simple coloring methods, but it does look at the problem from a different perspective; i.e., counting the lengths of the chains.

Love your solver. Enjoy your holiday.

Saturday 2-Sep-2006

... by: Paul, New Jersey, USA

Why do some screens identify cells using letters and numbers format for columns and rows, whereas other screens use a numbers only format. The inconsistenct is confusing. Maybe a future update could fix this problem.

Andrew Stuart writes:

Going to change this soon. There are two rival coordinate systems (two main ones) and I restorted to the rXcY system for the complex strategies because most discussion on the two main forums used this - and it was useful to be compatible. Very soon you will be able to pick the system you prefer. I prefer the letter/number since its shorter.

Tuesday 29-Aug-2006

... by: unkx80,

I see that your "toggle uniqueness strategies" toggles empty rectangles (ER). Unfortunately, I don't think ER depends on the puzzle being unique.

Monday 28-Aug-2006

... by: Ed, Hollidaysburg, Pennsylvania, USA

I use your site daily, sometimes to print out puzzles from the daily papers to make them easier to solve manually, sometimes to solve them on your site, and sometimes to help us when we get stuck on a difficult puzzle. I've learned a lot about solving sudokus from your excellent descriptions of solving strategies. Thanks for a great site!

Sunday 27-Aug-2006

... by: Jo, USA

Andrew, Your online Sudoku Solver is fantastic! I have no complaints and have found no bugs. The step by step explanations and your documentation are superb. Use of your solver has taken me from beginner to where I am now able to solve even Diabolical puzzles. I know you have and are continuing to invest much time and energy into this program. And I do appreciate that, but...

Any possibility you might develop a similar step by step solver for Jigsaw Sudoku? I'm just beginning with the Jigsaw Sudokus and find some of the Jigsaw Gentle puzzles more difficult to solve than the "regular" Diabolicals. While I know the strategy is basically the same for both, the Jigsaw version is driving me mad.

Again, thanks for the great site! Jo

Andrew Stuart writes:

Hi Jo Yep I think that's got to happen. There is a problem as to how to design a user interface to allow the boxes to be drawn. Only real difficulty.

Opps, yes you are right. Corrected board is located above. Use it along with discussion of even/odd strategy below.

Saturday 26-Aug-2006

... by: Paul Akehurst de Visme, Portugal

While waiting for your reply on the html for my puzzle, I noticed that in the 'X-Wings' Strategy example, the rows and columns are both designated by numbers, rather than letters and numbers, making the explanation "This is good news because this leaves only a 9 at G9 and we can complete." not correct. I know it is only a small matter, but playing Sudoku encourages exactness!! Regards Paul

Wednesday 23-Aug-2006

... by: Michael, UK

Any reply or comment on my query of 13th August?

Andrew Stuart writes:

Sorry Michael, yes I have a comment. Will be writing shortly (travelling at the moment)

## Sunday 3-Sep-2006

## ... by: CT, Belgium

The good thing is, that it's a step by step solver.

I have a "Sudoko Genius" book with lots of difficult puzzles. If I'm stuck on one, I fill it in in your program to find the next step.

It helps me to learn what I have overlooked.

Great program!

## Sunday 3-Sep-2006

## ... by: Arthur Lurvey, Tucson, AZ, USA

Load Sudoku: CLICK TO LOADThis is an extension of the Singles in Row/Col/Box to Doubles. In the above puzzle, she noticed that E4 and E6 must be either a 2 or 6. This is because the 2,6 in rows D,F and column 5 eliminates 2,6 from the middle box except for E4 and E6. Hence, these cells must be 2 or 6.

In the long run, this didn't have much of an effect on how fast the solver solved this puzzle. Without this info, the solver takes 51 mouse clicks to solve the puzzle. With this info it took only 49 clicks. Not much different.

## Saturday 2-Sep-2006

## ... by: Arthur Lurvey, Tucson, AZ, USA

Load Sudoku: CLICK TO LOADThis is a more interesting example of the even/odd path length strategy.

Looking at the above board, we see a pair chain of 4,6 located at: E5,E7,F8,A8, since there is both an even and odd number of steps from E5 to A5, the 4,6 at A5 can be eliminated. Your puzzle solver eliminates the (4,6) at A5 by applying simple coloring twice.

Using the solver further, we eliminate 1 at B6 using:

X-CYCLE on 1 (Discontinuous Alternating Nice Loop, length 6):

1[r2c6]-1[r2c2]=1[r7c2]-1[r8c3]=1[r8c6]-1[r2c6]-

- Discontinuity is two weak links joined at R2C6, 1 can be removed from that cell

Now we can apply the even/odd chain length strategy again to eliminate the 4,8 at B7. There is an even path from F6 to B7 (F6==>B2==>B7) and also an odd bath from F6 to B7 (F6==>F8==>E7==>B7).

At this point, the puzzle is easily solved.

Your solver uses Box-Line Reduction

LBR: 6 exists only in box 3 and row A, can remove from B7

LBR: 6 exists only in box 3 and row A, can remove from C7

Followed by Simple Coloring

SINGLES CHAIN (Type 2): Removing 4 from R1C4

SINGLES CHAIN (Type 2): Removing 4 from R1C8

SINGLES CHAIN (Type 2): Removing 4 from R2C7

SINGLES CHAIN (Type 2): Removing 4 from R5C7

SINGLES CHAIN (Type 2): Removing 4 from R6C6

SINGLES CHAIN (Type 2): Removing 4 from R7C5

SINGLES CHAIN (Type 2): Removing 4 from R8C3

This even/odd strategy may be a specialized case of the simple coloring methods, but it does look at the problem from a different perspective; i.e., counting the lengths of the chains.

Love your solver. Enjoy your holiday.

## Saturday 2-Sep-2006

## ... by: Paul, New Jersey, USA

## Tuesday 29-Aug-2006

## ... by: unkx80,

## Monday 28-Aug-2006

## ... by: Ed, Hollidaysburg, Pennsylvania, USA

## Sunday 27-Aug-2006

## ... by: Jo, USA

Your online Sudoku Solver is fantastic! I have no complaints and have found no bugs. The step by step explanations and your documentation are superb. Use of your solver has taken me from beginner to where I am now able to solve even Diabolical puzzles. I know you have and are continuing to invest much time and energy into this program. And I do appreciate that, but...

Any possibility you might develop a similar step by step solver for Jigsaw Sudoku? I'm just beginning with the Jigsaw Sudokus and find some of the Jigsaw Gentle puzzles more difficult to solve than the "regular" Diabolicals. While I know the strategy is basically the same for both, the Jigsaw version is driving me mad.

Again, thanks for the great site! Jo

Yep I think that's got to happen. There is a problem as to how to design a user interface to allow the boxes to be drawn. Only real difficulty.

## Sunday 27-Aug-2006

## ... by: Arthur Lurvey, Tucson, AZ, USA

Load Sudoku: CLICK TO LOADOpps, yes you are right. Corrected board is located above. Use it along with discussion of even/odd strategy below.

## Saturday 26-Aug-2006

## ... by: Paul Akehurst de Visme, Portugal

Regards

Paul

## Wednesday 23-Aug-2006

## ... by: Michael, UK