Scala example source code file (sudoku.scala)

This example Scala source code file (sudoku.scala) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Java - Scala tags/keywords

app, app, array, array, boolean, boolean, char, int, int, string, sudokusolver

The Scala sudoku.scala source code

object SudokuSolver extends App {
  // The board is represented by an array of strings (arrays of chars),
  // held in a global variable m. The program begins by reading 9 lines
  // of input to fill the board
  var m: Array[Array[Char]] = Array.tabulate(9)((x: Int) => readLine.toArray)
 
  // For printing m, a method print is defined
  def print = { println(""); m map (carr => println(new String(carr))) }
 
  // The test for validity is performed by looping over i=0..8 and
  // testing the row, column and 3x3 square containing the given
  // coordinate
  def invalid(i: Int, x: Int, y: Int, n: Char): Boolean =
    i<9 && (m(y)(i) == n || m(i)(x) == n ||
      m(y/3*3 + i/3)(x/3*3 + i % 3) == n || invalid(i+1, x, y, n))
 
  // Looping over a half-closed range of consecutive integers [l..u)
  // is factored out into a higher-order function
  def fold(f: (Int, Int) => Int, accu: Int, l: Int, u: Int): Int =
    if(l==u) accu else fold(f, f(accu, l), l+1, u)
 
  // The search function examines each position on the board in turn,
  // trying the numbers 1..9 in each unfilled position
  // The function is itself a higher-order fold, accumulating the value
  // accu by applying the given function f to it whenever a solution m
  // is found
  def search(x:Int, y:Int, f: (Int) => Int, accu: Int): Int = (x, y) match {
    case (9, y) => search(0, y+1, f, accu) // next row
    case (0, 9) => f(accu)                 // found a solution
    case (x, y) => if (m(y)(x) != '0') search(x+1, y, f, accu) else
      fold((accu: Int, n: Int) =>
           if (invalid(0, x, y, (n + 48).toChar)) accu else {
             m(y)(x) = (n + 48).toChar;
             val newaccu = search(x+1, y, f, accu);
             m(y)(x) = '0';
             newaccu}, accu, 1, 10)}
 
  // The main part of the program uses the search function to accumulate
  // the total number of solutions
  println("\n"+search(0,0,i => {print; i+1},0)+" solution(s)")
}