Solve my Sudoku

August 25, 2008 10:17 PM

Is this a medium difficulty Sudoku puzzle? Am I a Sudoku idiot?

I've just started playing Sudoku (on my iPhone) and this puzzle comes up under medium difficulty, and apparently is going to keep coming up until I solve it. I'll get through it eventually, I'm sure, but it's requiring a lot more thought then I was expecting. Is the difficulty of this puzzle off, or have I not given Sudoku puzzles (and their solvers) enough credit?

posted by cosmonaught to sports, hobbies, & recreation (18 answers total)

The difficulty for sudoku puzzles is relative - meaning someone at some point solved it, then compared the difficulty to other puzzles they've solved, how much time it took, how many dead ends / wrong turns / diabolical twists they made.

This puzzle looks like most other mediums I've worked - no obvious areas to start or yarns to pull. Usually mediums require you to try pulling out one yarn and see if it unravels or gets you into a knot - one reason the professionals use a pencil or really tiny numbers.

Good luck :)
posted by chrisinseoul at 10:33 PM on August 25, 2008

Doesn't look hard to me, either. While I don't know your solving method, there's some pretty obvious clues (9s on the right column, 6s on the left) to start with.
posted by sfkiddo at 10:52 PM on August 25, 2008

Agreed, it doesn't look hard, and I am a novice... But even marking up all the squares, and using all the techniques I can find on the web, short of "x-wings" and other things that scare me, doesn't seem to do the trick... Not to say I'm doing those things well, and there's a good chance I'm missing something obvious, but I'm starting to resort to trial-and-error, which doesn't really feel like solving, more like, well... trial-and-error.
posted by cosmonaught at 11:04 PM on August 25, 2008

I don't know what "x-wings" are (but am now also scared of them) and don't play Sudoku on an iPhone, but you basically need to come up with hypotheses (will 5 work here?) and prove or disprove them through the process of... mechanical trial-and-error (try 5 here, follow the results, etc.). To be honest, I like Sudoku because it is rather mindless and perfect for the early morning bus commute, but I LOVE logic puzzles. Completely different past-time which may interest you more.
posted by sfkiddo at 11:21 PM on August 25, 2008

This puzzle does look pretty easy at first, but I've worked on it and it's takes longer than it first appears to get to the point where the cascade of solutions can be teased out (to get to the point where you say, ok, so I guess this one needs to be a three, so that has to be the seven, etc.) So I think you might be justified in your assessment. The thread always has to start somewhere, though.
posted by tula at 11:22 PM on August 25, 2008

My automated sudoku solver says the answer is:

4 5 8 2 3 9 1 7 6
3 7 9 1 6 8 2 5 4
6 2 1 7 4 5 3 9 8
1 4 6 9 7 3 5 8 2
7 9 5 4 8 2 6 3 1
8 3 2 6 5 1 7 4 9
2 1 3 8 9 7 4 6 5
5 8 4 3 2 6 9 1 7
9 6 7 5 1 4 8 2 3

It rated the difficulty as 4 out of 4, meaning it employed its cleverest solving techniques. Many 'hard' sudoku puzzles are rated 2 or 3 out of 4, so I agree with your assessment.
posted by beniamino at 11:41 PM on August 25, 2008

I just did it, and I thought it was reasonably hard...harder than most mediums. Here's the solution, if you want it:

http://tinyurl.com/6l2uxj

I'm not really a guess-and-check person, and there wasn't much to go on, deduction-wise, until pretty late in the game. Looking for paired boxes was key. Good luck!
posted by LittleMissCranky at 11:44 PM on August 25, 2008

The step-by-step solution from my solver is here as an ordered series of PNGs.
posted by beniamino at 12:01 AM on August 26, 2008

If you want something short of the entire solution, try putting a '1' in the position indicated:

0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 1 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0

Should make things easier for you.
posted by Mike1024 at 12:55 AM on August 26, 2008

This is how I would start. Label the columns A-I, and the rows 1-9, 1 being the top. Similarly, the 3x3 boxes are labeled A1, A2, A3 (top row), B1, B2 B3 (middle row), etc.

Start with what I call the 'gimmes'. Cell I6 must be 9, and C3 a 6. Those are the only 2 obvious ones.

Notice the 1 and 3 in box A2. This means that row 3 has it's 1 & 3 in C3 and G3. (I write little 13 down the side of those cells.) This means that the middle 3 numbers in row 3 are 4,5 and 7. Looking at columns D, E and F shows that the order is 7, 4, 5. This now gives cell E4 as a 7.

That should be enough to get you started.
posted by salmacis at 2:49 AM on August 26, 2008

Followup - having finished it, it's not quite as simple as it looks. I'd rate it a medium.
posted by salmacis at 3:07 AM on August 26, 2008

My software rates it as a 3 out of 5; so medium. It bases this (I think) on how many squares are given.

Sudoku takes time and patience to develop your own strategies. Some 'easy' puzzles take time. Some hard puzzles take less.

You haven't figured out some 'more advanced' strategies. Are you marking possibilities?

I threw 'Sudoku' strategies into a search and came back with this link. I suggest looking at only ONE OR TWO of these stratagies at a time. Many you have intuitively figured out. Some you might need pointed out to you. Only look at one or two, until you feel like you've integrated them (or understand them) in your problem solving
posted by filmgeek at 4:37 AM on August 26, 2008

This is a neat solver tool.

It was able to solve it with logic alone, no guessing and checking.
posted by Perplexity at 4:50 AM on August 26, 2008 [1 favorite]

Are you playing SudokuFree on the iPhone? I used to be pretty decent at Sudoku, haven't really done them in a year or so, and the medium ones on that program consistently kick my ass. While my skill has probably dulled somewhat, these medium puzzles really are quite hard.
posted by yellowbinder at 8:50 AM on August 26, 2008

Are you playing SudokuFree on the iPhone? I used to be pretty decent at Sudoku, haven't really done them in a year or so, and the medium ones on that program consistently kick my ass. While my skill has probably dulled somewhat, these medium puzzles really are quite hard.

Yep, SodokuFree, and it seems inconsistent. It's almost like I've run out of medium puzzles, and rather then telling me, it's pulling from a different pool.

My software rates it as a 3 out of 5; so medium. It bases this (I think) on how many squares are given.

I'm guessing that's what this one does, too, but certainly the numbers available matter just as much as how many of them there are.

I threw 'Sudoku' strategies into a search and came back with this link.

Thanks for the link, those help. I am doing a lot of those already, at least in the basic section (again, scared of X-Wings, and Jelly-Fish).


I don't know what "x-wings" are (but am now also scared of them) and don't play Sudoku on an iPhone, but you basically need to come up with hypotheses (will 5 work here?) and prove or disprove them through the process of... mechanical trial-and-error (try 5 here, follow the results, etc.). To be honest, I like Sudoku because it is rather mindless and perfect for the early morning bus commute, but I LOVE logic puzzles. Completely

That's exactly what I'm trying to avoid. I feel like these should be solvable logically, even if I'm having a hard time seeing what the logic is. I'm not afraid to mark up my squares with possibilities, but I don't want to start filling in numbers just see where they lead...
posted by cosmonaught at 9:37 AM on August 26, 2008

Late to the party here. However, I would say that it was a pretty challenging puzzle, but then I put any puzzle which requires me to start looking for naked pairs and triples into that category. After locating a couple of naked pairs everything fell into place easily, but for me finding these things takes too much work. Crossword puzzles at least don't punish you like Sudoku for wild ass guesses. Anyway, take my "pretty challenging" rating for what it is worth as I am no great Sudoku solver.
posted by caddis at 9:50 AM on August 26, 2008

Assigning a rating to a puzzle is a completely subjective endeavor... in my experience every 'solver' uses its own logarithms to determine difficulty, and many of them don't have any knowledge of the very advanced solving techniques like XYZ-Wings, Forcing Chains, BUGs, etc. so the 'hardest' puzzle some of these programs can come up with are really relatively simple in the grand scheme of sudoku puzzles.

The 'difficulty' of some of these techniques themselves is also subjective. There are some techniques (Unique Rectangles for one) in the analyzer that I use that get lumped into the higher difficulty buckets, but for me are really very easy to spot, especially if you are pencil-marking possible values.

Here is a link to the analyzer I use, it's called Sudoku Explainer... it's just a .jar file, nothing to install, and free. It rates this puzzle a 2.6 out of 10. It can recognize most, but not all, of the techniques described on the link posted by filmgeek, so it's a good example of how deep a program is willing to go. It says the only techniques required for this particular puzzle are 4 hidden pairs and 2 applications of 'pointing'... both pretty basic solving techniques as far as this app is concerned, but to other (simpler) programs these may easily constitute medium or hard techniques.

That's why these puzzles are so much fun, they're always a challenge.... I've stared at puzzles with less difficulty than this one for a half hour before giving up and asking it for a hint, only to realize it was starting me in the face the whole time. If they were always easy we would stop playing them.
posted by SquidLips at 10:50 PM on August 27, 2008

Here, by the way, is my favorite Sudoku page, Sudoku Slam, which comes to us via MeFi Projects. I tried to weave it into my answer yesterday, but it was down at that time. It automates the process of finding candidates for cells leaving you with the more interesting task of eliminating candidates to find solutions.
posted by caddis at 8:21 AM on August 28, 2008