Timewaster Tuesday: Basic Sudoku

I love puzzles. Therefore for Christmas, the Mothership bought me a Sudoku page-a-day calendar. Since January 1st, I have done a sudoku puzzle a day. I figured, since I’m getting quite good (not that I wasn’t decent before) that I would start a sudoku series here to explain them to anyone who might want to give them a try and to give away my best tips to those who want to improve their sudoku-fu skills.

What is Sudoku?
Sudoku is a logic puzzle. It is not, contrary to popular belief, a number puzzle. The numbers are simply acceptable placeholders, but the puzzles can be solved with letters or symbols just as effectively. The idea of the puzzle is that there are 9 separate objects, and there are 9 sets of these objects arranged in a 3×3 set of 3×3 boxes. In the whole 9×9 box, there cannot be more than one of each symbol in any row or column, and within each 3×3 box there cannot be more than one symbol of each symbol. A completed set looks like this:

Then, a certain amount of symbols are removed and your job is to use logic to solve for the rest. The symbols are always removed in a symmetrical pattern, by the way. Have you sudoku players every noticed that? Seriously, go check it out. Here’s an example of a ready to solve puzzle:

If you want to know more about the history of sudoku, check out the wiki page. However, it is a fairly recent puzzle with numerous possible modifications. I originally found them before they were even named in logic puzzle magazines in the 90s and liked doing them because I could complete them in pen when I couldn’t find a pencil. I still do them in pen. They hit it huge in about 2005 and since then they are everywhere. I consider them a more mindless puzzle most of the time because they are solved in a system, and don’t require out-of-the-box thinking or complex logic. That makes them a great timewaster, until I get bored. If I bring sudoku on an airplane, I also bring a book and a crossword puzzle.

Solving Sudoku: Process of Elimination
This is the first step to any sudoku puzzle, simply going through and filling in the most obvious spaces using process of elimination. I like to do this within boxes. So, I start with number 1 and look in a box without a 1. I look at all the empty spaces, and see which ones are in a row or column with another 1 in another box. These are eliminated. If there is only a single box left, the 1 goes in it. I then go to the next box without a 1. I do this with every box for every number. Here are my stepwise examples of an easy puzzle.

Solving Sudoku: One Box Left
This is a relatively obvious one, but worth keeping an eye out for because you have to learn to see it. Essentially, when you have filled 8 of 9 boxes in a row, column, or box, you count up and fill in the missing number. If you look at the last picture above, you’ll see that happened in the third row up from the bottom, and it was missing a 9 in the middle box.

Solving Sudoku: Only One Number Fits
This one takes a little work to find, but once you start to see them they pop out at you. These are the boxes that every number but one is eliminated from. Pick a box in a square that seems to have a lot of numbers and start counting from 1. Can a 1 go in the box or is it eliminated by another 1 in the box, row, or column. Then 2, 3 etc. If by the end only one number fits, write it in! This is a good technique for when nothing else seems obvious, just pick boxes and look for the one that works.

Solving Sudoku: Inferred Elimination
This might be hard to describe. Essentially, it’s when you have narrowed a particular number within a box down to just one row or column so that you can use that to eliminate space in another box. Okay, let me show you this one.
In the bottom row of boxes there is a 2 in the middle box. In the left box where the red arrow is there could be a 2 in either the top or bottom boxes, but you can’t tell which one. However, in the right box the 2 can be in one of two boxes in the top row, BUT it HAS to be in the top row. That eliminates the top row for the box on the left, leaving only the bottom row far left corner box for the 2.

So, with these four methods, you can get through the vast majority of easy and medium sudoku puzzles, and maybe even a few hard ones. Maybe sometime in the future I’ll show you some of my more advanced tricks. My goals with sudoku are to never make little notes in the boxes, and I’m doing pretty well. However, I’m still doing a puzzle a day, and I’ll let you know if that eventually makes me crazy or just awesome. Now, go out and waste some time on Sudoku.

1 Comment (+add yours?)

  1. ang
    Jan 25, 2011 @ 12:01:21

    Free online sudoku– this site stole a lot of my study hours in college.


    Easy, medium, hard, and evil. And yes the evil ones really are evil.


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