README.md

Sudoku

Summary

Figure 1. Animation solving a sudoku puzzle and partially verifying the solution.

Sudoku is a logic-based, combinatorial number-placement puzzle. The objective is to fill a 9×9 grid with digits so that each column, each row, and each of the nine 3×3 sub-grids that compose the grid (also called "boxes") contains the digits from 1 to 9.

Generally, a puzzle provides a partial solution so that some squares already have numbers. To solve the puzzle, we need to fill in the unsolved squares, as demonstrated in Figure 1.

By the end of this challenge we'll have a fully functioning Sudoku solver that runs from the command line. We'll be presented with 15 unsolved Sudoku puzzles. The puzzles can be found in the file sudoku_puzzles.txt.

• Five puzzles can be solved with basic logic.
• Five require slightly more advanced logic.
• Five require educated guessing.

A complete solution to this challenge will solve all 15 puzzles.

Restrictions

Failing to follow the following restrictions will result in an invalid submission:

• Do not create classes, you will only be creating (many) methods.
• No instance variables and no globals, you will only use local variables.

To accomplish this, you will be writing methods that accept arguments as inputs and return useful values represnting their work. You should be writing many methods, and using them together to build up your solver.

Testing

Good news: you will be creating many small methods, and small methods are easier to test.

The number of tests you write is completely up to you. Testing is for you, and will help you develop code. You must balance testing with the need to write code and make progress on the solution, but remember that being able to easily run tests that prove your code is working correctly can help you move faster.

Releases

Pre-release: Understanding and Modeling Sudoku

A computer program that solves Sudoku is simulating a person playing Sudoku. Understanding how a person plays the game is key. What does someone do when solving a Sudoku puzzle?

Let's play an actual Sudoku puzzle, paying attention to how we approach the problem.

• What strategies are we adopting and why?
• How do we choose where to start?
• How do we know when to really put a number in a square?
• What do we do when we don't definitively know how to fill in any more squares?

Reflecting on our human strategy, it's important to see that sometimes the strategies that work for humans would be difficult to implement on a computer. However, the reverse is also true: strategies humans avoid because we'd have to write too much, use too many sheets of paper, or remember too much information are possible for a computer.

Before we begin coding, let's develop and write a pseudocode solution for your solver.

Release 0: Basic Logic

The first five puzzles in the file sudoku_puzzles.txt can be solved with basic logic: identifying when a square has only one possible value. For this release, we want to develop our solver to the point of solving these first five puzzles.

• We'll build a collection of methods that work together to solve puzzles. A few methods are already there for you in sudoku.rb.

• To run our solver we'll use runner.rb, which uses the methods in sudoku.rb. Make sure you read the comments and code in runner.rb and sudoku.rb.

• The solve method in sudoku.rb will take a String representing an unsolved Sudoku board. Unsolved squares are marked with a "-". Solved squares have a character from "1" to "9". For example... "1-58-2----9--764-52--4--819-19--73-6762-83-9-----61-5---76---3-43--2-5-16--3-89--"

• For this release, your code should give up if it gets stuck

• Be sure to write the pretty_board method, which takes your board and returns a string that is well formatted for display to the screen. For example:

1 - 5 8 - 2 - - -
- 9 - - 7 6 4 - 5
2 - - 4 - - 8 1 9
- 1 9 - - 7 3 - 6
7 6 2 - 8 3 - 9 -
- - - - 6 1 - 5 -
- - 7 6 - - - 3 -
4 3 - - 2 - 5 - 1
6 - - 3 - 8 9 - -

Release 1: More Advanced Logic

Puzzles 6 - 10 can be solved using logic alone but require more than just identifying when a square has only one possible value. Enhance the behavior your solve these new puzzles.

Note: Your code should still give up if it gets stuck.

Release 2: Educated Guessing

Puzzles 11 - 15 can be solved by making informed guesses about the values of unsolved squares and then trying to solve the puzzles based on those guessed values. Guessing should be started once your solving code gets stuck. In other words, solve as many squares as possible using logic and then start guessing.

Conclusion

Writing a Sudoku solver presents a lot of problems: identifying the logic involved in the game itself, translating that logic to code, manipulating data structures, etc. It provides a nice comprehensive challenge, summing the skills that we've developed to this point at Dev Bootcamp.