I have recently started doing Sudoku puzzles, and (if I do say so myself) I have gotten to the point where I can usually solve them, even the “hard” ones that I’ve encountered. I think I’ve figured out most of the tricks for narrowing down correct options, but today’s USA Today Sudoku (Jan 27) has me stumped.
Using all of my wiles and skills I can fill in exactly one missing number.
The 2 in the top center 3x3 box
Without giving away too much, can anyone give me a hint on how to get off of dead center on this puzzle? I’m sure there’s some trick I must be missing.
There are 11 squares on the grid that I can reduce down to 2 possibilities, but none that I can say for certain.
Thanks, Pochacco, after applying your trick to all the other rows and columns, I now have exactly 2 numbers filled in, including the one I did before, and the one you pointed me to.
Is it ever a good idea to “guess” on some of the squares that you have narrowed down? It seems to me that would make it hard to keep track of which squares you were sure of and which were based on guesses.
This one is a tough one. I did get Pochacco’s 5, and it didn’t get me very far. Often what I’ll do is, where I can narrow the possibilities down to two for a given box, just take a guess and go from there – but in pencil. That way, if it doesn’t pan out, you erase back to your initial guess, and at least at that point, you know the number for the box you *didn’t *pick for your original guess. Often this is the key to unlocking the puzzle, even if you didn’t get many numbers to start with.
There was another sudoku site that I tried that would let you narrow down the possible numbers, and put them in small text in the upper left of that square. And from that you could make further deductions.
For example, if there are two squares in a column (or row, or box) that have the same two possibilites, then those two numbers must go in those two squares (even if you don’t know which is which, yet), which means you can eliminate them as possibilities everywhere else in the column.
I looked at the puzzle you linked to, just briefly, and I didn’t see any way to keep notes on that sort of thing.
I’m almost done with it. I have to go to a lunch meeting (free lunch!), but I wrote down some of my initial steps. Here’s another hint: in the upper right set of 9, ther eis only one place a 7 can go. Bigger hint is it’s in the top row
When I guess (which seems to represent a bug in the Sudoku creation software) I circle the guessed number (one of two) and underline all the numbers that proceed from that guess. If it leads to a contradiction, I erase all the underlined numbers, erase the circled number, and put a box around the other choice, which now must be correct (assuming I didn’t screw up somewhere.)
In making the guess, I try to make the choice that will lead to the most other boxes being filled in, which should lead to a solution or a quick contradiction. I’m getting better at this, but don’t have a good heuristic yet.
The USA Today site doesn’t seem to allow pencil marks (I don’t understand why some online sudoku sites are like that - solving hard puzzles requires it!) I tried importing into a program that permits them, and it said the puzzle isn’t verified, which might mean it’s impossible, has more than one solution, or requires guessing.
Sorry, this isn’t the one of the first ones that I came across, and, it’s one line lower than I typed (I scribbled some notes down and was not cognizant of quite how many I wrote; I am secretly trying to make it look like I’m working today). :smack:
Got it. Had to guess one number though, but the puzzle solved so I guessed right
I wrote all the possible numbers for the middle three rows. Two boxes came down to only 2 numbers (5or9 and 3or5) so i guessed 5 in second of those. 5 should give 4 boxes (2 5s and 2 9s) and you can solve from there.
I’ve been doing these for about 3/4 months and I started by going through these hints and tips
Not everyone wants to do this though, but I found it helps by giving you a procedure to follow on the sticky ones.
If you do read the hints I would ignore the end of the procedure titled “for the addict” - it’s rarely that you need to go that deep.
I have often heard that in a properly designed puzzle you should never have to guess a number.
Here’s a general Sudoku question - at what level do you have to guess, or can all puzzles be solved by “logic”?
I refuse to ever guess at a number - the hell if I’m going to fill in another 10 or 20 numbers only to find I’ve guessed wrong and backtrack. Instead I’ll just go to another puzzle.
I know the difficulties are ranked - I have a book called “Brown Belt Sudoku” or something like that, which I gather is the next-to-last most difficult, and I can solve about 90% of them purely by logic and process of elimination.
For what it’s worth, it seems Brown Belt puzzles give you 27 to 29 filled-in numbers.
So…can all puzzles be solved by logic? Or do the more difficult brown belts and (I’d guess) black belts (like the USA today one here) require guessing?
I finally got it, but it required a chained “what-if”. That is, I assumed a paticular space was something (a 2, I think), and followed where that led. Eventually, I discovered that given that 2, some other space was either 8 or 9. I tried 8 in that space, and it eventually led to a contradiction, and I tried 9, and that led to a contradiction, too. Therefore, I knew that my original guess was wrong, and that spot wasn’t a 2. That turned out to be a good place to make a guess; the rest of the puzzle followed smoothly (but tediously) from there.
Here is another good Sudoku hints site. It explains some advanced logic tools for narrowing down possibilities. Usually though, I end up needing to use a “cheat sheet” I made in Word that allows me to check off eliminated possibilities until a clue emerges (or you can download freeware programs that automate that).
Even then, sometimes I get stuck. I consider it ok to guess if I’ve gotten enough clues that the entire remaining puzzle reduces to a single chain of 50/50 guesses. That is, I can start on any square that has just two possibilites, with one guess leading to a contradiction and the other solving the puzzle. I’m not sure but I think some of the harder sudokus have no “local” solutions- you eventually end up with that web of 50/50 guesses I mentioned.
Finally got it. After solving what I could, I found a single square that had two possibilities, one which contradicted and the other finished the puzzle. However this was an extraordinarily difficult one- I’ve never had to leave so much open before guessing.