# Sudoku Puzzles Primer

Sudoku is a puzzle game with few and very simple rules. Though it has a Japanese name, it was actually invented by an American architect in 1979. To play this game you will need a Sudoku puzzle, a pencil, and an eraser - Yes! an eraser.

The Pattern of a Sudoku Puzzle:
In Sudoku we put numbers in little cells called 'squares.' Squares are sectioned off in groups of 9 to create 'boxes.' In each box, the squares are arranged in 3 rows by 3 columns. The layout of a Sudoku puzzle is 3 boxes by 3 boxes. Hence, typical Sudoku puzzle has a total of 81 squares in 9 boxes. Usually, Sudoku puzzles have dark lines separating the nine boxes for easy identification. Solving a Sudoku Puzzle:
Each row of 9 squares must contain the numbers 1 through 9. Each column must also contain the numbers 1 through 9, and each box must contain the numbers 1 through 9. No row, column or box may repeat any number.

When you start a new Sudoku puzzle, many squares are already filled in. A typical Sudoku puzzle has at least 17 squares already filled in. Some of the more difficult puzzles have fewer than 17 squares already filled in, and some have more than one possible solution.

Based on the filled-in squares, using logic and the process of elimination, we can begin deducing which numbers fit in the empty squares. Cross-hatching is the process of figuring out where a number fits by eliminating possibilities based on numbers in the other squares in the same row, column, and box.

For example, the top row of our puzzle is missing the numbers 2, 4, and 7. Let's start with the number 2. We find a 2 in the top right box (right column, middle row), so that eliminates the possibility of placing the 2 in that box. The only possibility left is in the top left box in the empty square in the top row.

All that is left now in the top row are two squares in the top right box. Which one gets the number 4? Could it go in the middle square? If you look at the whole column (column 8 from the left), you will see a 4 in that column in the bottom box (between 3 and 5, above 9). So we cannot place another 4 in that column. That leaves only the top left square in that box (above 1, to the right of 5). At this point the only missing number in the top row is 7. Just to make sure - and to avoid having to use the eraser later on - we cross-hatch with the column. There is no other 7 in the column (just 8, 5, 4, and 9), and there is no other 7 in the box (just 4, 9, 1, and 2). So it is safe to deduce that 7 goes in the empty square in the top row. At this point we have solved one row by cross-hatching. We can continue to solve the puzzle row by row, or column by column, or box by box. You will probably use a combination of all three.

A common strategy is to find a row, or a column, or a box with few blank squares. When we began, the top row had only 3 missing numbers. Now the third column from the left has only two missing numbers. And the middle box on the left side is missing only three numbers. It can all be solved by cross-hatching,

The one rule you need to know for suduku is this: Before you write a number in a square, make sure the same number does Not appear in the same row, or the same column, or the same box. That is all there is to Sudoku.  