Just a Sudoku… how hard do you find it?

Following yesterday’s theme of scaling back to simpler puzzles, here’s a plain and simple Sudoku.

It might not be an unusual type of puzzle, but it’s interesting to me because I’m trying to calibrate the difficulty of tougher puzzles whilst avoiding solving strategies that casual solvers won’t know – I don’t want to make difficult puzzles simply by requiring obscure logic! (There’s nothing wrong with that in itself, but it’s not what I’m trying to do here). I’ve never put much effort into making really hard puzzles, mainly because it’s a minority interest and it’s not one I personally share too much (I like to get stuck, but not for too long! ). In general I’d rather my puzzles be tricky because they’re unfamiliar or are novel in some way.

Anyway, after a few easy placements this puzzle gets quite hard to make progress on – or so I think. I’d really, really like to hear how difficult (or not!) you find this puzzle, and how long it takes you to solve it. (I’ve also added it to puzzlemix.com if you’d like to play it online).

Thanks, and good luck!

Tags: Sudoku

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#1 written by Spittledung
about 15 years ago

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SPOILER! Please do not read until you have tried the puzzle!
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OW! This was a good hard one. I would recommend this one only for Sudoku veterans. It took me 3.5 hours but I did overlook something:

I ran the usual steamroller over it doing the “basic” things. Then I went to some advanced techniques. I found a swordfish and a 3-link bivalent chain. After that there were numerous squares with only 2 digits. When I see this in the paper it screams: “Do a Y-wing! You know it is in there!” So I practiced looking for one (this is one area I wish to improve). Doing that cost me a bunch of time.

Didn’t find one, tried some other advanced techniques, and then I found a Hidden Triple I missed earlier. DOH! I think this is the key to the puzzle because once I cleared that everything exploded from the advanced work I did earlier.

I don’t like to do Sudoku on a computer because I use a special notation to give me more information about the candidates and no computer program does it the way I like.

So, if this was punishment for saying your Wrap-around Consecutive 3-grid 6×6 Samurai Skyscraper with extra cheese Sudoku was very easy, I’m sorry.
#2 written by Gareth Moore
about 15 years ago

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SPOILER!
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Thanks very much for the feedback – I’m pleased you found it as tricky as I hoped!
You’re absolutely right – the secret is a pretty well concealed hidden triple straight after you’ve placed the first few ‘obvious’ digits, and then that’s followed immediately by an x-wing which is also reasonably difficult to spot (although at least the candidates are fairly close together).

There are some other tricky parts too, involving naked triples and hidden pairs, but that’s the hardest bit right at the start – the tail end of the puzzle once you’ve cracked these is mostly straightforward.

I added it to puzzlemix as I mentioned, and it currently has an average solve time of 33:17 for the “without aids” option – the very best time on it without help is 18:50. It’s also the hardest-rated Sudoku (by public vote) on the site, although the large number of solvers who use the aids does deflate these ratings quite a lot.
#3 written by Andrew
about 15 years ago

OK, maybe this is bad, but I don’t know all these techniques you guys use. So, when I got to (probably) the part where you started doing all that stuff that I don’t know what it means, I guessed (in an organized and useful manner), and managed to solve this puzzle in under twenty minutes. Probably closer to ten or fifteen.
#4 written by Andrew
about 15 years ago

OK, I sort of take back what I said about not knowing the techniques.

I use all that logic, I just never knew it had names.
#5 written by Gareth Moore
about 15 years ago

Guessing is an okay strategy, albeit a somewhat risky one! Actually, if there’s a powerful strategy that you don’t know, and you happen to guess in the place where that strategy could be used, then you could reach a contradiction very quickly as you butt up against the implications of the bit of logic that could alternatively have been used.

I solved the puzzle twice online at puzzlemix myself – the first time I forgot to fill in some candidates and so found it very easy, purely by a lucky accident! The second time I found it a lot harder…

And yes, that’s the thing about the techniques I think it’s fair to use – it shouldn’t matter whether you’ve read up on tactics that can be used (and therefore know the common names); a “fair” Sudoku should only use tactics you’ll probably already have worked out/be able to workout for yourself.
#6 written by Andrew
about 15 years ago

Yeah, I should clarify about the guessing. When I “guess”, I make a special notification in that square about my arbitrary guess in that square, and then solve the puzzle while keeping all my previous notifications in tact. When I reach a contradiction, I erase all “guessed” numbers and switch my guess to the correct number (I only guess on squares with two possible numbers so if I guess wrong I then know the correct number).
#7 written by Christine
about 15 years ago

Good for you Andrew!

It took me just over an hour to solve this puzzle and it sounds as though my strategies were very similar to your own! I placed all the easy digits, and then, there being nothing obvious, I just listed all the possibilities in each cell (a bit painstaking and I might not have bothered if I hadn’t felt the need to rise to the challenge). I haven’t a clue what those advance solving techniques are either (maybe I ought to look into it sometime) but I eventually solved it fairly quickly in the end by looking at one of the cells which had only two possible digits and seeing what would happen to the rest of my possibles if I chose one. The first number I tried came to a dead end, so I knew it had to be the other and this led to the solution.

I wouldn’t normally do this (I might possibly have given in) but the beauty of sites like this is that being able to discuss and give feedback gives you more of an incentive to try!
#8 written by Spittledung
about 15 years ago

On Guessing:

I have been in many “to guess or not to guess” debates on the puzzle front. I came away victorious in knowing myself and the reason why I do puzzles…

I am of the “no guessing” belief. I know that I can guess, do an OOPS, and erase back (and probably complete the puzzle faster), but for me the puzzle is like a ticking bomb and cutting the wrong wire means BOOM and I lose.

The fun for me is coming to a unique solution using tools and logic. It is also finding new ways to solve the same problem so I can learn for the future. I use sudoku to practice methods, and many of those allow me to finish simple puzzles faster when I recognize the patterns. I can get bored with most sudoku publications, and if I don’t have to do at least an X-wing on a sudoku I’d rather not waste my time.

I also use sudoku to relax and practice focusing my attention. And this is where I need the most improvement.

Each person does sudoku for different reasons, and one reason is not better than any other, as long as you enjoy doing them.

Personally, I believe the Sudoku craze happened because for decades people have been fed mindless drek in the media and people have been starving for something thoughtful to do.
#9 written by Christine
about 15 years ago

Hi Spittledung, I completely agree with you about guessing. I do think the whole point of a puzzle is to solve it by logic. Had I come across this puzzle in a magazine I would have probably left it half done, because I didn’t have the necessary tools to solve it. It’s just that my stubborn streak set in here because it was posted on the site, and I was determined to solve it, by any means I could!

I’m not particularly mathematical (just got an ‘O’ level years ago), and had never done a number puzzle in my life before until sudoku came out, but then I became hooked (as you suggest because they are something to keep the brain occupied). I’m not interested in doing the simple ones either because if you can just sit and fill in number after number then there is no point. I’m wanting to improve too. My ideal puzzle is one where I just keep chipping away at it bit by bit, until eventually you get the one number which opens up the whole puzzle. Like Andrew, I probably have worked out some of the strategies for myself but just haven’t learnt the techncal names. I just think it would be a shame if some people got switched off the puzzles because they thought you had to be some sort of mathematical whizz to solve them, which using all the technical terms suggests.

Having said that, this has been a good talking point and I think I WILL go and read up on some of these strategies now, to see if I recognise any of them, and to see if there are new ones I have still to learn!

I totally agree, we all do our puzzles for our own reasons, and thanks to Gareth for providing us with some new challenges!
#10 written by Andrew
about 15 years ago

I agree 100% with your point about guessing. If I could solve a puzzle without guessing, I would do it. This is one thing I really need to practice. Like Christine, I plan to go and learn some of those techniques and hopefully wont need to guess anymore.

And Spittledung, I really like the bomb analogy. That’s a really good way to look at it. I’m starting more and more to want to solve puzzles that way, and I think after this one I will put more effort into the logic aspect of puzzles rather than the solving aspect of puzzles.

I like that you guys bothered commenting on the guessing aspect, as it will surely help me and my sudoku experience.
#11 written by Gareth Moore
about 15 years ago

As a puzzle creator I’m probably even more against guessing than most people, for multiple reasons. Firstly, and perhaps most mundanely, when I make a puzzle I need to be absolutely certain that there is only one solution, and if at any point I need to guess then there is a risk I’ll miss something. If instead I can break it down into a series of clear logical deductions then I can be sure there is only one solution. (That said, for Sudoku it’s easy enough to run a brute-force search on a computer to check all solutions, but in general this is true).

Secondly, whilst guessing is a logic of sorts it is not to my mind a fair logic to require of a solver – it simply rewards reckless solving whilst penalising a more intelligent approach, and at the end of the day if you’re going to do that it’s no longer a logic puzzle but a game of luck. Essentially you’re misleading your audience by turning a game of skill into a game of chance.

In a similar way I absolutely don’t consider an assumption of uniqueness an appropriate method to require the use of when solving a puzzle (NB you absolutely can’t use this whilst creating a puzzle, but you could in principle use it when rating a known valid puzzle). Nonetheless it’s an effective strategy, sometimes for Sudoku but more often in Kakuro, Hitori, Nurikabe, Slitherlink and many other Japanese puzzles – simply observe which squares must contain certain values/be in certain states in order for there to be a unique solution. You’re trusting the puzzle creator to have got it right (which is fair enough), but the puzzle creator should never require this of a solver because they themselves should (must!) have a logical solution which does not rely on this. Similarly when I solve other people’s puzzles, I like to try and beat the puzzle the way they intended, not via an accidental shortcut.

So for both guessing and assumptions of uniqueness, it’s entirely up to a solver how they decide to approach a puzzle, but as a creator I like to make sure that none of my puzzles ever “require” them!

April 1st Sudoku

about 9 years ago - Comments Off

Wow, it is a long time since I posted here. So much to do, so little time!
I might as well keep it brief now, then. Here’s a puzzle for April 1st (it’s actually a normal puzzle without tricks – it just spells ‘FOOL’ in the given numbers). Place 1 to 9 in each row, column More >