Sudoku
Sudoku seems very popular these days, so I took the chance to try out a puzzle in the Examiner a couple days ago. The difficulty rating was one star out of five and it surprised me how difficult the puzzle was.
As I got stumped on the last few squares I glanced back at the instructions and realized that there was a third aspect. I’d neglected to realize that each 3×3 box contained each digit as well as each row and each column. With this information, the last squares were filled easily.
Today’s puzzle was rated three stars out of five. Since I started out understanding the parameters, this puzzle got finished much more easily than Tuesday’s.
I can see why the game is so popular. It’s not very challenging, yet it does engage the mind and it passes time. I’m waiting for the next couple of days to see if the more difficult levels provide any real challenge, but I’m skeptical. It seems to be a simple process of elimination. The hardest it seems it could be made is to make fewer starting points.
OK, so I went to websudoku.com and tried it out on the “evil” level. It’s still a 9×9 puzzle, but I was able to solve it with a little perseverance. I don’t know how the 16×16 version works – hexadecimal numbers perhaps?
websudoku.com’s kung fu is
websudoku.com’s kung fu is weak. Try sudoku.com.au on “Tough”. And you’re not allowed to use guessing and backtracking, only logic.
I don’t get what’s wrong
I don’t get what’s wrong with guessing and backtracking. Isn’t saying “if this were X then that could not possibly be Y therefore this is not X” perfectly good logic? I call that testing a hypothesis, not guessing. Perfectly logical.
I must be missing something.
Arrrgh.
OK, I hated the
Arrrgh.
OK, I hated the interface. It kept on not taking my clicks, so I had to be very careful so as not to screw up. After three tries, each time ending up with multiples in the same row or column, I knew I had to have screwed it up and missed a click somewhere along the way. I gave up and entered the problem into my Sudoku program on the Palm. When I was done entering it, the program told me that it was not a solveable puzzle, and therefore won’t let me play it.
No fair! (Yes, I could print it out and play it on paper, but if it doesn’t even have a solution, grrrrr.…)
Your Palm’s kung fu is
Your Palm’s kung fu is weak!
As for guessing and backtracking, my personal rule is that it’s okay if I can keep the chain of logic in my head. Filling in squares at random and doing the puzzle until it fails, then filling in another random number, is right out.
By coincidence, I tried my
By coincidence, I tried my first sudoku puzzle today from sudoku.com. I picked a medium difficulty puzzle that they said took from 10 to 30 minutes. I copied it to paper and did it with a pencil in three minutes. I didn’t think that was too bad for my first puzzle. Unless the hardest ones are a lot tougher, I think I’ll stick to chess.
There was no need for guessing. I suspect if they only filled in three or four numbers as a start it would be more of a challenge. Like creating the puzzle yourself.
Dad
Wouldn’t it take quite a bit
Wouldn’t it take quite a bit longer to do it by trial-and-error? Like hours? My knowledge is based on a single puzzle, but it only required thinking a move or two ahead.
Dad
OK. I tried sudoku.com.au
OK. I tried sudoku.com.au and selected their tough one for the day. It took me nearly a half hour, but it still felt tedious instead of challenging. And, no, I didn’t have to guess at anything. BTW, two of the 3×3 sections started out completely blank. Interesting, but I guess it isn’t my cup of tea.
Dad
I still don’t like the word
I still don’t like the word “guess.” Maybe I’m missing some logical method, but I’ve come across puzzles that I could solve to a certain point and there was a set of numbers that could be transposed without making any (immediately obvious) difference. After working a few steps through I could see that one way it would lead to having the same number twice on the same line. It’s not arbitrarily picking a number for a blank spot, but it sounds like that’s what Paul is calling a “guess”. I suppose I need to improve my logic skills. You’re definitely faster at it than I am.
Well the thing is that trial
Well the thing is that trial and error always succeeds eventually on even the toughest puzzle, and therefore is no test of skill. So if you’re h4rdc0re, doing the puzzle that way isn’t worth doing. You have to really solve it.
Eh, I still don’t get it. I
Eh, I still don’t get it. I have pencil notation that this square might be a 1 or a 4, but if it’s a 4 then the one on the other side has to be a 1 and there’s a 1 already in that row. OH NO I’VE JUST USED TRIAL AND ERROR. How is that different from looking ahead at the possibilities three or four or five iterations out?
I’ll tell you what I do though. I always backtrack on success and retry until I have an error if I’m venturing enough moves beyond my ability to memorize. If my notes solve the game I backtrack to the beginning of the hypothetical and work it out to find that the option I’m not choosing *does not* work. A proper Sudoku board only has one solution, but I don’t use that as an axiom. I find the proof that a number does NOT work in order to rule it out.
What you described I would
What you described I would call guessing. If you have two open squares and can’t prove which number goes in which spot, solve some other section. You should be able to rigorously solve these puzzles as per the directions. (I don’t have nearly enough experience to know whether or not that is actually true.) The difference between guessing and your X‑Y-Z example is that you can actually see the logic in picking one or the other, instead of simply picking one and working through until the puzzle fails or succeeds. Sure, the results are the same, but one way you can know you SOLVED it. But, heck, it’s only a game. Do it however you like and I’ll do how I like.
Dad