Author Topic: Sudoku  (Read 3969 times)

Wowbagger

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Sudoku
« on: January 03, 2014, 06:40:28 pm »
I'm quite partial to the occasional puzzle but I wouldn't say I'm any good. I tend to do puzzles from the site http://www.websudoku.com and select any one of the four categories (Easy, Medium, Hard, Evil) depending upon how much time I've got. My average time on an "evil" puzzle is a little over 40 minutes.

I stumbled across a rather good sudoku solver a couple of days ago, http://www.sudokuwiki.org/sudoku.htm, which shows how it solves puzzles by eliminating the various numbers that are impossible, one step at a time. I found it very interesting that the solver employs up to 20 different strategies to eliminate numbers. I tried a few of the "evil' puzzles from websudoku.com and watched the solver at work. To solve an evil puzzle, it hardly ever has to use more than about the simplest 4 of the strategies available to it. I would imagine that this illustrates how much better at this kind of puzzle computers are than are humans.

I tried a "diabolical" puzzle from the solver's own stock and it was very interesting as such weird devices as "XY-wing" and "Simple colouring" were employed. Even though the logic was, apparently, laid out before me, it wasn't easy to see exactly what was going on.

Does anyone else have a dabble?
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mattc

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Re: Sudoku
« Reply #1 on: January 03, 2014, 07:07:21 pm »
I'm a little addicted to the "killer" variant. There are a lot more permutations than with the 'classic'.

I can't say solving these sorts of puzzles electronically appeals to me - I love paper-and-pencil, and scribbling notes in the squares and the margins. (So pretty much all my Sudokus are from The Times!)
Has never ridden RAAM
---------
No.11  Because of the great host of those who dislike the least appearance of "swank " when they travel the roads and lanes. - From Kuklos' 39 Articles

Wowbagger

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Re: Sudoku
« Reply #2 on: January 03, 2014, 07:30:39 pm »
I don't usually bother with notes - I hold the numbers in my head. That clearly limits me to the relatively simple solving techniques that have been revealed to me by the "solver": for some of the more advanced ones I think you would have to have all the possible numbers for each square pencilled in. With the amount of space you've got available in the average newspaper that's not really feasible.

Yesterday's Graun's puzzle (I actually bought a physical paper yesterday) claimed to be "hard" but I could do the lot without notes - just writing down the only possible answer as soon as it became obvious.
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Re: Sudoku
« Reply #3 on: January 03, 2014, 08:14:31 pm »
I used to do sudokus (I too was a note scribbler) but then turned to super-hanjie ........I find these a little more absorbing than your average sudoku because for me the sudoku is often a case of either doing it or not doing it but the SH I can keep wheedling away at all day, with one of the harder ones, and get there in the end.

But I now find that doing SHs in the evening results in headaches in the morning  ???

Re: Sudoku
« Reply #4 on: January 03, 2014, 08:34:32 pm »
To solve an evil puzzle, it hardly ever has to use more than about the simplest 4 of the strategies available to it. I would imagine that this illustrates how much better at this kind of puzzle computers are than are humans.

Yes, that's pretty much the case, and it's worse than you think: by restricting itself to solving techniques that can be explained to humans, the computer is hobbling itself. The fastest algorithms for solving Sudokus (for example, Donald Knuth's "Algorithm X") result in solutions that are utterly opaque to humans. Having written several programs for setting and solving Sudokus, I've come to think that Sudokus are really best thought of as puzzles by computers for computers!

Re: Sudoku
« Reply #5 on: January 03, 2014, 08:54:43 pm »
Doing the puzzles is the main reason I buy a paper (on a much-reduced price subscription), and I do them before reading the paper as a check my brain still works.

I used to write the possible numbers in and complete by a process of elimination, but can now do all but the very hardest without. I write the missing numbers on the outside of the grid against each row and column as a reminder, I can't hold them all in my head!

I also printed off some blank grids, so if I went wrong I could start again, but don't usually bother these days.

Charlotte

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Re: Sudoku
« Reply #6 on: January 04, 2014, 12:12:21 am »
I prefer real ku.
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Re: Sudoku
« Reply #7 on: January 04, 2014, 09:31:30 am »
I don't usually bother with notes - I hold the numbers in my head. That clearly limits me to the relatively simple solving techniques that have been revealed to me by the "solver": for some of the more advanced ones I think you would have to have all the possible numbers for each square pencilled in. With the amount of space you've got available in the average newspaper that's not really feasible.

The trick for that is to use an imaginary 3x3 grid within the square and use dots to represent the possible numbers. I find standard sudoku unrewarding especially when they get easier towards the finish and end up thinking "so what?". Killer sudoku can be more interesting.
Quote from: tiermat
that's not science, it's semantics.

Wowbagger

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Re: Sudoku
« Reply #8 on: September 04, 2014, 11:57:13 pm »
I ask the solvers here present to have a crack at this one:-

http://killersudokuonline.com/archives/2014/5/11

It defeated me this evening to the extent that I keyed it into the solver at sudokuwiki.org and it defeated that as well! Could be that the techniques required were too incomprehensible for a computer to explain to a cerebrally challenged sexuagenarian, as per Gareth's comment above?

The chunks that I couldn't unravel were the 15 and the 14 in the top left block, the 31 and the 14 in the bottom left block, and the 10 and the 17 in the bottom right block. It seemed to me that there was more than one solution but my brain was overheating by the time I had finished.

I try to avoid trial and error, but I resorted to that at the end and that still didn't help me.
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Pippa

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Re: Sudoku
« Reply #9 on: September 05, 2014, 04:00:48 pm »
I got there in the end but it did involve trial and error. I ended up with the same unravelled "chunks" as you.

For the bottom L square for the 31 and 14, I had one number in the 31 (a 7 from the usual sudoku rules). It was clear that the key was around the combination of 25 or 34 in the 14. So, unlike you keeping all the numbers in my brain, into each square I entered the possible numbers.

There were a couple in this bottom L square where there were only 2 choices; one of these was a choice between 2 and 8; I figured with 2 being key this would be a good place to start with the trial and error approach as it would probably unravel the puzzle (equally there was 4/6 choice which would have worked as well I suspect). And it did. I got the choice right first time (but that was a purely 50:50 chance) so I did check the other option as well, just to be sure there weren't 2 solutions; there weren't.

mattc

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Re: Sudoku
« Reply #10 on: September 05, 2014, 06:53:18 pm »
Yup, same here. trial-n-error on the 14 being
1346
or
1258

<slides note-paper along desk to show workings to honoured opponent>

Nice site Wow - although I much prefer my scribbling-on-the-newspaper approach. It's very rare to have more than 3 options for a square, so there's usually space.
Has never ridden RAAM
---------
No.11  Because of the great host of those who dislike the least appearance of "swank " when they travel the roads and lanes. - From Kuklos' 39 Articles

Wowbagger

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Re: Sudoku
« Reply #11 on: October 26, 2014, 04:28:54 pm »
Quite please with myself today. Solved this

http://killersudokuonline.com/play.html?puzzle=D3s05ry3221&year=2014

with no guesses and no mistakes.
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Wowbagger

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Re: Sudoku
« Reply #12 on: November 16, 2014, 08:25:47 pm »
And another!

http://killersudokuonline.com/play.html?puzzle=D3s2now3242&year=2014

I still normally get stuck and guess on these harder ones. I don't think I guessed on this one, but there was a calculation at one point that I thought was logical and either it was or it was a lucky guess.
(click to show/hide)
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Re: Sudoku
« Reply #13 on: November 19, 2014, 07:24:24 pm »
I do one a week- Saturday Times Samurai 5 grids in one which I enjoy, when I don't make a mess of it

Suggested times for "difficult" are usually around 55 minutes.
I usually do it over two/three nights taking on average about 3 times as long in total
I must be slow and thicky :facepalm:

Wowbagger

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Re: Sudoku
« Reply #14 on: October 06, 2015, 05:45:52 pm »
Another seeming ambiguity here.

http://killersudokuonline.com/iphone/player.html?puzzle=D352d6q3566&year=2015

I'd be grateful for comments.

(click to show/hide)
Eating's a serious business. Don't bollocks around wagging your tail.

Re: Sudoku
« Reply #15 on: October 06, 2015, 06:45:23 pm »
(click to show/hide)
"Whoever fights monsters should see to it that in the process he does not become a monster.
And when you look long into an abyss, the abyss also looks into you." ~ Friedrich Nietzsche

Wowbagger

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Re: Sudoku
« Reply #16 on: October 06, 2015, 06:46:47 pm »
I keyed what I' solved into the solver mentioned upthread and it claimed there were two solutions!
Eating's a serious business. Don't bollocks around wagging your tail.

Re: Sudoku
« Reply #17 on: October 06, 2015, 06:56:45 pm »
My full solution:
(click to show/hide)

EDIT: The way I was working it, I couldn't see how there could be two solutions, unless when you get to the ambiguous state as described in your post, one tries the options in a different cell block to the ones I tried the alternatives for. And life's too short to try solving a killer sudoku more than once!  ;D
"Whoever fights monsters should see to it that in the process he does not become a monster.
And when you look long into an abyss, the abyss also looks into you." ~ Friedrich Nietzsche

Wowbagger

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Re: Sudoku
« Reply #18 on: January 07, 2016, 10:35:28 pm »
Either I am being especially dumb this evening or this is a lot harder than moderate!

http://www.killersudokuonline.com/play.html?puzzle=D2ksy103659&year=2016
Eating's a serious business. Don't bollocks around wagging your tail.

Re: Sudoku
« Reply #19 on: February 28, 2016, 03:38:50 pm »
Either I am being especially dumb this evening or this is a lot harder than moderate!

http://www.killersudokuonline.com/play.html?puzzle=D2ksy103659&year=2016

You're not, that was quite tricky - and not helped by there being more than one correct solution:

(click to show/hide)

And only one was accepted as correct! Grrr.
Quote from: tiermat
that's not science, it's semantics.

Wowbagger

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Re: Sudoku
« Reply #20 on: February 28, 2016, 05:07:15 pm »
That appears to have only one possible correct solution in my book: the bottom RH block of 9 has a 16 which already contains a 6 and a 2. The other number must be 8, so then you can solve the other three.
Eating's a serious business. Don't bollocks around wagging your tail.

Re: Sudoku
« Reply #21 on: February 28, 2016, 05:08:52 pm »
Doh!  :facepalm:


 :-[ too much red wine...
Quote from: tiermat
that's not science, it's semantics.

ElyDave

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Re: Sudoku
« Reply #22 on: February 28, 2016, 08:16:19 pm »
Not often these days, although I do like the Killer variants.

It's all about practice.  I once got to the point where I could consistently do about half-2/3 of the times cryptic crossword, no chance these days.
“Procrastination is the thief of time, collar him.” –Charles Dickens

Basil

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Re: Sudoku
« Reply #23 on: February 28, 2016, 08:29:01 pm »
I've always had difficulty with the killer version because I've never been sure if  you can use a number twice in a sum area if that area is across more than one block.

I've not explained that well.  Do you see what I'm asking?
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Re: Sudoku
« Reply #24 on: February 28, 2016, 09:04:11 pm »
I've always had difficulty with the killer version because I've never been sure if  you can use a number twice in a sum area if that area is across more than one block.

Short answer is no, you can't. If you find yourself repeating a number inside a sum area, you've made a mistake somewhere.

Hope this helps.

Yesterday's killer Sudoku in the Times was singularly mis-named as "deadly" - they reckoned on 56 minutes, and I rattled it off in around 30, and that included catching and correcting a mistake. ;D
"Whoever fights monsters should see to it that in the process he does not become a monster.
And when you look long into an abyss, the abyss also looks into you." ~ Friedrich Nietzsche