How do I play Sudoku?

I haven't been sucked up in the Sudoku craze yet either, but a colleague of mine, Mark Danburg-Wyld, has been busy building Sudoku Place and was kind enough to send along the following introduction:

You have probably heard people talking about SuDoku, but what exactly is it? SuDoku is a puzzle -- an exceptionally pure and addictive puzzle! This article will help you tackle SuDoku. Solving SuDoku requires no special knowledge of trivia, no particular mathematical ability; merely an ability to think logically and a temperament not given to despair.

The Object of SuDoku

Each SuDoku is made of 81 cells, divided into 9 rows, 9 columns, and 9 boxes. The boxes are 3 cells by 3 cells, and are marked off by the thicker lines. An example is shown just below. Notice that some of the cells contain a digit, 1 through 9. The object is to fill in the empty cells so that the digits 1 through 9 appear:

  •   exactly once in each row
  •   exactly once in each column
  •   exactly once in each 3 x 3 box

That is all there is to it. Sounds simple, right? Here's a sample puzzle for you to try:

Tactics for solving SuDoku

Look for places where there is only one number possible for a cell. How can you tell? Here is an example. Consider the following (partial) puzzle:

The red cell has to be a "4". Why? There are several ways to think of it. Consider the 4s which are already showing -- each eliminates the possibility of another 4 in a particular row, column, and box.

So there is only that one cell left in that box in which a 4 is possible. Since the rightmost box must have a 4 in it somewhere, the one possible cell is where it goes.

The reasoning can be more complicated than that. Consider this example:

For the middle box, we do not know exactly where the 4 is, but we know it must be in one of the two red cells. That means the third row is "spoken for" and we can remove it from consideration, leaving just the one cell in the rightmost box.

Believe it or not, there are even more convoluted lines of reasoning that can come into play, but this much will let you get started on any SuDoku puzzle.

More SuDoku Strategy

Generally speaking, the more information you have, the easier it is to fill in the remaining unknowns. So look for places where you have a lot of information.

Does one box have 5 cells filled in, where the others have at most 3? Start there. Does one row have 6 cells filled in, where the others have at most 4? Start there. Does one column have 8 cells filled in? You know what the last one must be!

One less obvious example of this -- count how many of each digit you have. If you have a lot of "5", the remaining 5's should be comparatively easy to find.

Final Thoughts

Like anything, solving SuDoku puzzles gets easier with practice, so keep at it! One thing to keep in mind, though: with SuDoku, if you find that you have made a mistake somewhere (for instance, if you have 2 "3"s in the same box), it is generally easiest to start over entirely. Be sure of each step before you take it! Once you start guessing, the puzzle is likely to fall apart. Finally, always remember: it is just a puzzle. SuDoku is meant to be enjoyed. So make sure to have fun!

Oh, and here's the solution to the sample puzzle. Hope you didn't peek!

Thanks, Mark! This makes my head swim, but maybe the rest of you find this sort of puzzle relaxing. :-)

A couple of starting points:

  •   Sudoku place
  •   Sudoku puzzles online