sudoku

Sudoku (1979- ) is a number game that you commonly see in newspapers. The rules are simple but winning is not. It is one of those sorts of puzzles that are hard to put down till you have the answer. And once you have the answer, you want to do another one!

Sudoku is made up of nine rows and nine columns with some numbers filled in. In addition, the puzzle is divided into nine 3×3 boxes, shown by heavy, dark lines.

The rule is this:

Fill in the missing numbers so that each row and each column and each 3×3 box has each number only once.

You cannot, for example, have two 6′s in the same row.

That is all you need to know. The rest is just thinking through which numbers must go where. At each step you will know enough to fill in at least one more number. What more do you need? Well, yes, some care and patience: one false move – one number in the wrong place – and you are done for.

My first sudoku puzzle took me an hour. It was hard! And even now it takes me about 15 minutes. You learn some tricks along the way.

Here are some of the things I do:

1. Look at all the ones and see if any are forced. Then all the twos and so on. By the time you get the nines you will have a fair amount of the puzzle filled in.
2. Look for rows, columns or squares with only a few missing numbers.
3. If you can narrow a number to one of two places, make a note of it. That way if one of the two places is filled with some other number, your number must go in the other one.
4. When you complete a row, column or box check to make sure that it has each number only once.

Despite the name, the game is American, not Japanese.

Even though it was an American, Howard Garns, who first thought up sudoku in 1979, it caught on in Japan in 1986 and then in Britain in 2004 before it caught on in America.

And even then it might not have made it to the English-speaking world if a retired New Zealand judge, Wayne Gould, had not walked into a Japanese bookshop in Hong Kong and saw a book of sodoku. He spent the next six years writing a computer program that could create sudoku puzzles quickly. In 2004 when he was done he gave free puzzles away, first to The Times in London and then to newspapers throughout Britain and America.

Sudoku is a kind of Latin square, a much older puzzle. A Latin square is, in effect, sudoku without the boxes. In 1979 Garns added the boxes and called it “Number Place”. We know it by its Japanese name, sudoku, because that is where it caught on first.

See also:

• The daily sudoku puzzle in the Guardian
• newspaper