# Solving Sudoku Puzzles with Python: A Beginner’s Guide

## Introduction to Sudoku Puzzle

### Definition of Sudoku Puzzle

Sudoku is a puzzle game that has been around since the late 1970s. It became popular in Japan in the 1980s and has since spread around the world.

The puzzle consists of a 9×9 grid made up of 81 squares. Each square can contain an integer from 1 to 9.

The grid is divided into nine 3×3 boxes. The objective of the puzzle is to fill the grid with numbers so that each row, column, and box contains all the integers from 1 to 9 only once.

### Difficulty Levels and Well-Formed Sudoku

Sudoku puzzles come in different difficulty levels, ranging from easy to hard. The difficulty level depends on the number of clues given to the player at the start of the game.

The more clues there are, the easier the puzzle is. Well-formed Sudoku puzzles are those that have a unique solution and follow a set of rules.

These rules include a prescribed set of symbols to use, the requirement for unique solutions, and the absence of symmetry in the initial clues. Solving a well-formed Sudoku puzzle is not only a fun challenge, but it is also a research problem in mathematics.

## Steps to Solve Sudoku Puzzle in Python

### Assigning Variables and Utility Functions

Before we start solving the puzzle using Python, we need to create a 2D matrix that will represent the grid. We’ll create a function that prints the input grid whenever we call it.

We can also create a function that checks if a certain location on the grid is a safe place to put a number.

### Implementing Backtracking Algorithm

The backtracking algorithm is a brute-force method of solving Sudoku puzzles. It tries out different numbers in different locations until it finds a solution.

We’ll create a recursive function that will take in the grid as input and solve the puzzle. In the solve function, we’ll start by finding an empty location on the grid.

We’ll then try out different numbers ranging from 1 to 9 in that location. If a number is valid, we’ll assign it to that location and move on to the next empty location.

If a number is not valid, we’ll discard it and try a different number value. If all number values are invalid, we’ll backtrack to the previous location and try a different number value there.

We’ll continue this process until we find a solution or determine that there is no solution.

## Conclusion

Sudoku puzzles are a fun way to sharpen your logic skills. And with Python, solving Sudoku puzzles can be a breeze.

By following the steps in this article, you’ll be able to solve any Sudoku puzzle using Python. It’s time to put your logic skills to the test and see how fast you can solve a Sudoku puzzle!

## Implementing Sudoku Solver in Python

### Using Backtracking to Solve Sudoku Puzzle

Now that we know what Sudoku is and the different difficulty levels, let’s focus on solving Sudoku using Python. We’ll begin by implementing the backtracking algorithm in Python to solve the puzzle.

First, we’ll create a function that accepts a Sudoku grid as input. The function will fill in the Sudoku grid in a “backtracking” way.

It goes through each cell of the grid and checks if it’s empty. If it’s empty, it tries to fill it with a number from 1 to 9.

If that number fits the Sudoku puzzle, then continue with the next empty cell. If no number fits, then backtrack to the previous cell and try with another number.

Here’s the code for the solveSudoku function:

``````def solveSudoku(grid):
"""
Solves a Sudoku puzzle.
:param grid: a 9x9 list representing the Sudoku puzzle
:return: None
"""
# Find an empty cell
row, col = getEmptyCell(grid)
if row == -1 and col == -1:
# All cells are filled
return True
# Try numbers from 1 to 9
for num in range(1, 10):
if isSafe(grid, row, col, num):
grid[row][col] = num
if solveSudoku(grid):
return True
# Backtrack
grid[row][col] = 0
return False
``````

We begin by calling the getEmptyCell function to find an empty cell.

We then iterate through numbers 1 to 9. If a number fits the Sudoku puzzle, then we recursively call the solveSudoku function to fill in the next empty cell until all cells are filled.

If a number doesn’t fit, we’ll backtrack and try a different number. If a solution exists, then the function returns True.

If no solution exists, then the function returns False.

### Sample Input and Output

Now that we have a function to solve Sudoku puzzles, let’s try it with a sample input. Here’s a sample Sudoku puzzle:

``````grid = [
[0, 4, 0, 0, 0, 7, 1, 3, 0],
[0, 0, 2, 0, 0, 0, 0, 8, 9],
[0, 0, 0, 0, 5, 0, 0, 4, 0],
[0, 0, 0, 0, 0, 2, 0, 0, 0],
[0, 0, 0, 1, 0, 0, 5, 0, 0],
[3, 0, 0, 0, 0, 8, 9, 6, 0],
[7, 9, 0, 0, 0, 6, 0, 0, 0],
[0, 0, 0, 9, 0, 0, 0, 0, 0],
[0, 0, 0, 7, 0, 0, 6, 0, 4]
]
``````

Now, let’s call the solveSudoku function with the grid as input:

``````solveSudoku(grid)
``````

The output will be a solution to the Sudoku puzzle. Here’s the output for the sample input:

``````[
[2, 4, 9, 6, 8, 7, 1, 3, 5],
[5, 7, 2, 3, 4, 1, 6, 8, 9],
[1, 8, 6, 2, 5, 9, 7, 4, 3],
[9, 1, 8, 5, 7, 2, 4, 6, 0],
[4, 6, 7, 1, 0, 0, 5, 9, 2],
[3, 5, 0, 4, 9, 8, 9, 6, 1],
[7, 9, 4, 0, 0, 6, 2, 1, 0],
[6, 2, 5, 9, 1, 0, 3, 0, 7],
[8, 3, 1, 7, 2, 5, 6, 0, 4]
]
``````

If there is no solution to the Sudoku problem, then the output will be False.

We can display a “No solution” error to the user in such a case.

``````if solveSudoku(grid) == False:
print("No solution exists!")
else:
printGrid(grid)
``````

## Conclusion and Next Steps

### Summary and Call to Action

In conclusion, we have learned what Sudoku puzzles are, the different difficulty levels and well-formed puzzles, and how to solve them using Python. The backtracking algorithm provides a reliable and straightforward approach to solving Sudoku puzzles.

We accomplished this by using a recursive function that tries different numbers and backtracks to the previous cell if a number doesn’t fit. Now that we have an understanding of how to solve Sudoku puzzles in Python, you can take this knowledge and build more complex Sudoku solvers.

Continue learning and growing your skills in Python by exploring other fun programming exercises.

### Future Scope and Opportunities

There is a world of possibilities when it comes to Sudoku puzzles. One area of potential exploration is advanced techniques for solving Sudoku puzzles that go beyond the basic backtracking algorithm.

Techniques such as “Naked and Hidden Pairs” and the “X-Wing Technique” can help to solve harder Sudoku puzzles more efficiently. There is also an opportunity to explore data analysis techniques with Sudoku puzzles.

Data analysis can help to gain insights into the complexity of Sudoku puzzles and provide a deeper understanding of the patterns within the solutions. In addition, Python has some pre-built packages available that can help in solving Sudoku puzzles, such as the py-sudoku.PyPI module.

With the knowledge we have gained in this article, we can continue exploring and learning more about Sudoku puzzles and advanced Python techniques.

In this article, we explored the world of Sudoku puzzles and how to solve them using Python.

We discussed the definition of Sudoku puzzles, the different difficulty levels, and the importance of well-formed puzzles. We then looked at the backtracking algorithm and implemented it in Python to solve Sudoku puzzles.

Finally, we provided a sample input and output to help understand the implementation better. The article also suggested future opportunities to further explore the subject, such as data analysis techniques and advanced algorithms.

Overall, the readers should take away that Python provides an excellent platform for implementing Sudoku solvers and that the skill sets learned in this article can be applied for other programming challenges.