dailysudoku.com

[Terry]

Why is the August 14 puzzle rated "very hard"? I finished it in 26 minutes, which is about as fast as I can type. I didn't even have to consider alternative possibilities for any cells. The thing just came together bing-bing-bing. Some puzzles rated "easy" have taken me longer than that. Just out of curiousity, how do you guys decide on the ratings for these things anyway?

BTW: My system is to save each puzzle as a picture and then insert it in a PowerPoint slide. Then I increase the size of the puzzle by an inch all around so I have more room in the cells. Then I create text boxes with the numbers 1-9 and copy them as needed to put in the cells in the puzzle. Then I put in all the obvious numbers - the ones that can only go in one cell. Then I put in the ones where I can tell which row or column they should go in within each small square, but not which cell. I color-code them (rows are green, columns are purple). Finally, I go back through every cell that doesn't have a definite number and put in every possible number that could go there (in very small numbers, of course). Then I look at each row, each column, and each small box, and by process of elimination I can eventually solve every puzzle. The more cells there are with multiple possibilites, the longer it takes, and, of course, if I make a typo, I'm screwed, but basically this works every time. I never have to guess. However, even with this system, a puzzle rated "very hard" usually would take longer than 26 minutes. So, again, what gives?

This little quibble aside, I love this site - great puzzles.
Terry

[Guest]

I found this one fairly easy as well. I think
http://www.boldts.net/blog/Sudoku
has more challenging puzzles.

[Guest]

I didn't get very far at all.

000 084 300
800 600 090
056 009 810

010 200 908
009 000 523
208 003 040

080 900 430
020 001 007
007 460 000

At this point I typed it into the hint program, and the provided hint is in red. But, I don't see why.

[geoff h]

You can start as follows;

There is a pair of 3 and 5 in Square Number 4 - the pairs are in cell r4c1 and r4c3.

You can then eliminate Nr 5 from r4c5 and r4c6.

This then leaves a unique pair of 6 and 7 in r4c6 and r4c8 meaning you can eliminate 6 and 7 from r4c7, leaving Nr 9 the only possibility.

Hope this helps.

[Guest]

Indeed it does. Thanks. Just started doing these yesterday and maybe I shouldn't have leapt into the very hard category without practice. I had seen the 467 triple in square 4 but somehow failed to delete the 4 from r4c3, so I didn't have the 35 pair, etc. I see why explaining the hint programmatically is not as easy as one would wish; it requires a bit of backtracking but without knowing where things have gone wrong.

[David Bryant]

[Philmac]

I have to admit that I found this hard!

I have managed to do every other Sudoku puzzle (in the month that I have been doing them) by filtering and looking at PAIRS and gaps in sequences. This includes other 'very hard' category puzzles. On this one, however, I run up against a brick wall and had to consider triples for the first time ever!

Of course, I have probably missed something easy as one often does.

[Philmac]

guest and Geoof's reply.

First of all, the guest has reached EXACTLY the same position as I did when I almost gave up, so I think that the guest has been using the same strategies as me.

Geoff: am I correct in thinking then, that the only way to eliminate the possible 4 in r4c3 is to find the three 467 triples alluded to above?

[someone_somewhere]

Hi,

From the position that was mentioned before, you have overseen the possibility:

6 in r8c7 - Sole Candidate

And than you can continue . . .

see u,

[Guest]

I think it's the sole candidate only after the 9 given by the hint is filled in.

[August 14 Puzzle]

Yes, agree that you can only put Nr 6 in r8c7 after the Nr 9 has been put in as per the hint.

In reply to PhilMac, you can eliminate the Nr 4 from r4c3 becaus of the pair of 3 and 5 in r4c1 and r4c3 - this naturally follows from the 4,6,7 triple in Square 4.

Cheers.

[aardy]

I think very hard puzzles have more than one solution.Has anyone ever gave a try.Kindly do try and post the result.

[naz]

Hi,

The method mentioned by Terry is very good because it surely takes you to the result.But I feel the method is not the best one for the following reasons. Firstly it is not fast. Secondly it does not provide you with mental exercise. Thirdly it takes away the joy of solving a puzzle. Fourthly you do not learn to be logical and reasonable. Fifthly you do not develop along the way as you do not try to search for the newer possibilities.

Every dailysudoku.com puzzle should have only one logical solution.August 14 puzzle does not have a logical solution.It can be solved by other methods no doubt.It appears to me that Aug 14 puzzle is a good example to get more insight into sodoku

I will request Guest to differ once with the hint program if it does not give a logical hint.For a change try 6 instead of 9 in r4c7.

cheers
Iwill
I will

[someone_somewhere]

Hi,
I am solving Sudoku now for more than 5 monthes.
For me it puts a couples of challanges:

- "human solving". I try to solve it without any additional notes or tools.
Just think hard and fill in the next logical number. No tries, no corrections.
Up to level "very hard" and "fiendish" all can be solved by training and hard thinking, good observation, a little bit memory, ... and all other thinks that you got genetical and have trained to use it. Knowing the "combinatins" that are called from the simples one, like "naked single", "hidden single" (that all are using), to the medium ones, like for example "naked subset" and "hidden subset" and up to the professinal ones, like "X-Wing", "Swordfish", "XY-Wing", "Coloring", "Forcing Chains", "Nishio" will only help us to try to find more or less systematic for a LOGICAL solution. Unfortunate, I am using also a "visual" candidate table in order to be able to find this combinations. I will continue my training and some of them I will be able to perform in my head.

- "computer solving". To write a program to do the above task. For a programmer, this is nowaday not a very difficult challange. There are enough programming languages. The resulting programs will solve in about 3 to 5 seconds all the Sudoku's that can be solved by such a "combinations" (for the hardest ones, I still need about 1 hour). Still some remains and the programmers are looking at them to filter out new ways, combinations, algorithms to solve them.
(Of course, that when nothing else works, we can still use the TRIAL and ERROR to get to the solution).

- generation of Sudoku. By human, by programs.

- the (mathematical) proof that on the 9x9 there is no Sudoku starting with 18 numbers.

Feedback on other challanges are wellcomed,

see u

[Terry]

Dear Naz:
I'd be the first one to admit that my system (see 1st message in this thread) isn't perfect, but I think most of your criticism is a bit unfair.

You said:
Firstly it is not fast. (Two questions: 1. Compared to what? 2. Even if true, so what? This isn't a speed contest, is it?)
Secondly it does not provide you with mental exercise. (Why not? I just started doing these puzzles about 3 weeks ago. Coming up with any kind of system took a great deal of mental exercise.)
Thirdly it takes away the joy of solving a puzzle. (How so? Joy is entirely subjective. Believe me, when I solve a puzzle labeled "fiendishly hard" like some other sites have, I feel mighty pleased with myself.)
Fourthly you do not learn to be logical and reasonable. (Huh? I don't see how you reach this conclusion.)
Fifthly you do not develop along the way as you do not try to search for the newer possibilities. (Not sure what you mean by "newer possibilities." Anyway, maybe you wouldn't keep trying to get better, but I do. With each puzzle, I try to fill in more cells by just thinking them through - before I have to go to the system of putting in all possibilities and eliminating them that way. I'm getting better at this every day. )

Bottom line is that, before I figured out a logical and reasonable way to attack these puzzles, even the easy ones took me hours and hours and a lot of trial and error. I really couldn't learn or improve because I was just flailing around. But, now that I have a logical method that will ultimately solve any puzzle, I can concentrate on improving my ability to see what should go where without using my system. For years, I've been doing the Sunday New York Times crossword puzzle in ink - someday I'd like to solve Sudoku puzzles in ink, too.

Whatever system you use, have fun one and all!

Terry

[someone_somewhere]

Hi,

Can you try this one? And tell me how far you get?

000040030
980601000
000000200
000000001
004050700
600000000
005000000
000908076
070030000

see u,

[Guest]

Hi someone,
I could hardly crack two rows.Please give hint and justify.

000040030
983621457
000000200
000000001
004050700
600000005
005000000
002918576
070030000

Thanks,

[Terry]

Wow! What a great puzzle! Where did you find it? Or did you create it yourself? If the latter, I bow at your feet in awe. Thanks for all the fun!

This is the solution I got:
5 2 1 7 4 9 6 3 8
9 8 3 6 2 1 7 4 5
4 6 7 3 8 5 2 1 9
7 5 9 2 6 4 3 8 1
2 1 4 8 5 3 9 6 7
6 3 8 1 9 7 4 5 2
1 9 5 4 7 6 8 2 3
3 4 2 9 1 8 5 7 6
8 7 6 5 3 2 1 9 4

Terry

[someone_somewhere]

Hi,
No, unfortunate, it was not me.

But, if you want to get to almost the end of the 9x9 Sudoku Universe,
I can show you where to find it:

http://www.csse.uwa.edu.au/~gordon/sudokumin.php

There you can download the 19197 initial positions with 17 initial numbers.

P.S. it's better you write a program to see how many you can solve.
P.P.S. most of this "minimal" Sudoku are not having an symetric design, but how cares!
If that does not get you to the heaven, you can try the hell and create a Sudoku with 16 initial numbers on a 9x9 with unique solution !

see u,

[someone_somewhere]

Dear Guest,

That is also the point where I got stucked.
I still did not try the very advanced thechniques that I keep as a delicatese for later.

I think that Terry, who solved and enjoyed it so much, could disclose from his secrets, which technique he has used.

Terry, can you halp us? please.

see u,