Sudoku Basics: Scanning

Is it your first time playing a Sudoku puzzle after learning the basic rules and terms? Then there are many things to learn first to improve your chances of solving your puzzle within the day! Before we continue, we’d like to recommend that you have a basic and easy Sudoku puzzle ready at hand so that after reading the Sudoku tips in this article, you can go straight into practice.

To become a successful Sudoku player, learning this primary skill called “scanning” is essential. From its name, you can already tell it’s meant for the numbers in the grid—but how does it work?

What Is Scanning?

Scanning is a basic Sudoku skill used by a majority of Sudoku players to start analyzing the puzzle. After all, at the end of the day, Sudoku is a game completely moored in logic. And while lucky guesses can help you fill in a row or a column through trial and error, learning how the grid and the numbers work will provide more consistent performance and results.

Scanning is noting patterns, similarities and differences within the game. It may be repeating numbers on the same box, row or column, and other relationships between the digits. By learning this skill, it becomes a lot easier to place numbers within the Sudoku puzzle. And technically, it is a basic number placement technique that created other variation tricks, too!

How Does It Work?

Let’s start by giving this example.

Now, let’s focus on the top boxes—specifically, boxes 1, 2 and 3. Why do we choose to start scanning in this section of the grid? It’s because there’s a good amount of givens around this site, and having more givens makes it easier to solve the Sudoku puzzle.

Speaking of the givens, can you see something notable among them?

If you answer that there are numbers that repeat twice, then you get a big, green check! Multiple numbers repeat within these three boxes, and by using scanning, you were able to identify them. With that in mind, let’s now use scanning to press further and try putting in a digit.

As you noted, the number 1 repeats in these three boxes, and there’s a 1 in cells r3c1 and r1c6, and they are in box 1 and 2 respectively. But box 3 has no cell with the number 1, so we’re gonna put one in there.

Scanning Used for Placing Digits in Free Sudoku Puzzles

Since the other two boxes have their 1s in place, where do you put the 1 in box 3? Scan these boxes, and check the rows and columns of those existing 1s. Remember that in Sudoku, no number can repeat in any row, column or box. So with that in mind, it means that you can no longer put a 1 in row 1 and 3, so the 1 for box 3 should be in row 2.

Congratulations! You just used scanning to place your first number!

Another Example

Let’s do another one. You can see that the number 9 repeats on these three boxes, and they are in cells r3c4 and r2c9. These givens are in boxes 2 and 3, so the digit you’ll place should be in box 1. Like before, scan the respective rows of the given 9s, and you’ll be able to see that the 9 you’ll need to place should be in row 1.

And just like that, you’ve put another number into the puzzle!

You can use this scanning technique all around the grid. But for beginners, it’s ideal to try scanning for three boxes that are adjacent to each other horizontally or vertically. Now that you’ve grasped the basics of scanning, try it out on three vertical boxes like box 1, 4 and 7!

The Groundwork for Further Techniques

Now that you’ve seen how effective scanning is, it wouldn’t be a stretch to say that it is a prerequisite skill to learn before moving on to other basic skills, like elimination, pattern reading, pencil marking and many more. Placement techniques like this are extremely useful to corner the final result you're solving for, and many beginners choose scanning as their first big step into seriously completing a Sudoku puzzle.

Try It Out On Your First Puzzle

Don't be shy to copy the notes above while you try out your first Sudoku game. While most online Sudoku platforms don't have this feature, you can try using your finger to track the columns and rows of your Sudoku grid to be more confident in your scanning. Personally, we've used a pen lying around our table to determine our own scanning too!

Going Beyond Scanning

As you continue playing the game, you'll be able to choose and fill in more cells with confidence. But for now, it's best to master scanning by practicing on easy Sudoku puzzles first. Then you can start learning about elimination, pattern recognition and pencil marking in easy to medium Sudoku puzzles and their related techniques.