The Ruby Programming Language

A Sudoku Solver in Ruby

This chapter concludes with a nontrivial Ruby application to give you a better idea of what Ruby programs actually look like. We’ve chosen a Sudoku^[*] solver as a good short to medium-length program that demonstrates a number of features of Ruby. Don’t expect to understand every detail of Example 1-1, but do read through the code; it is very thoroughly commented, and you should have little difficulty following along.

Example 1-1. A Sudoku solver in Ruby

# # This module defines a Sudoku::Puzzle class to represent a 9x9 # Sudoku puzzle and also defines exception classes raised for # invalid input and over-constrained puzzles. This module also defines # the method Sudoku.solve to solve a puzzle. The solve method uses # the Sudoku.scan method, which is also defined here. # # Use this module to solve Sudoku puzzles with code like this: # # require 'sudoku' # puts Sudoku.solve(Sudoku::Puzzle.new(ARGF.readlines)) # module Sudoku # # The Sudoku::Puzzle class represents the state of a 9x9 Sudoku puzzle. # # Some definitions and terminology used in this implementation: # # - Each element of a puzzle is called a "cell". # - Rows and columns are numbered from 0 to 8, and the coordinates [0,0] # refer to the cell in the upper-left corner of the puzzle. # - The nine 3x3 subgrids are known as "boxes" and are also numbered from # 0 to 8, ordered from left to right and top to bottom. The box in # the upper-left is box 0. The box in the upper-right is box 2. The # box in the ...