Assignment_05/src/main/scala/Main.scala

   1
   2 object Main {
   3   // Question 1: HOF on lists.
   4   def lengthStrings(l: List[String]) = l map (_ length)
   5   def dup[T](e: T, n: Int) = (1 to n) map (_ => e)
   6   def dot(l1: List[Int], l2: List[Int]) = (l1 zip l2) map { case (a, b) => a * b }
   7
   8   // Question 2: Folding functions on lists.
   9   def areTrue(l: List[Boolean]) = l reduceLeft (_ && _)
  10   def lString(l: List[String]): Int = l.foldLeft(0)((acc, s) => acc + s.length())
  11   def longest(l: List[String]): (Int, String) = l.foldLeft(0, ""){ case (curr@(len, longestStr), str) => if (str.length() > len) (str.length(), str) else curr }
  12   def isPresent[T](l: List[T], e: T): Boolean = l.foldLeft(false)((acc, e2) => acc || e == e2)
  13   def flattenList(l: List[Any]): List[Any] = l.foldRight (List.empty[Any]) {
  14     case (e: List[Any], acc) => flattenList(e) ++ acc
  15     case (e, acc) => e:: acc
  16   }
  17
  18   // Question 3: Tuples.
  19   def primeSum(max: Int): List[(Int, Int)] =
  20     for {
  21       i <- (1 to max).toList
  22       j <- (1 to max).toList
  23       if isPrime(i + j)
  24     } yield (i, j)
  25
  26   def isPrime(i: Int): Boolean =
  27     if (i <= 1 || i % 2 == 0)
  28       false
  29     else
  30       !(3 to Math.sqrt(i).toInt).exists(i % _ == 0)
  31
  32   // a)
  33   def primeSumUnique1(max: Int): List[(Int, Int)] = {
  34     def loop(l: List[(Int, Int)]): List[(Int, Int)] = l match {
  35       case Nil => Nil
  36       case (t@(a, b)) :: rest => if (a > b) t :: loop(rest) else loop(rest)
  37     }
  38     loop(primeSum(max))
  39   }
  40
  41   // b)
  42   def primeSumUnique2(max: Int): List[(Int, Int)] =
  43     primeSum(max).foldLeft(List[(Int, Int)]())((acc, t) => if (t._1 > t._2) t :: acc else acc)
  44
  45   // Question 4.
  46   val cities = List("Paris", "London", "Berlin", "Lausanne")
  47   val relatives = List("Grandma", "Grandpa", "Aunt Lottie", "Dad")
  48   val travellers = List("Pierre-Andre", "Rachel")
  49
  50   def postcards : List[String] =
  51     for {
  52       city <- cities
  53       relative <- relatives
  54       traveller <- travellers
  55       if relative.charAt(0).toLower == 'g'
  56     } yield s"Dear ${relative}, Wish you were here in ${city}! Love, ${traveller}"
  57
  58   // Question 5.
  59   def queens(n: Int): List[List[(Int, Int)]] = {
  60     def placeQueens(k: Int): List[List[(Int, Int)]] =
  61       if (k == 0)
  62         List(List())
  63       else
  64         for {
  65           queens <- placeQueens(k - 1)
  66           column <- 1 to n
  67           queen = (k, column)
  68           if isSafe(queen, queens)
  69         } yield queen :: queens
  70     placeQueens(n)
  71   }
  72
  73   def isSafe(queen: (Int, Int), queens: List[(Int, Int)]) =
  74     queens forall (q => !inCheck(queen, q))
  75
  76   def inCheck(q1: (Int, Int), q2: (Int, Int)) =
  77     q1._1 == q2._1 || // Same row.
  78     q1._2 == q2._2 || // Same column.
  79     (q1._1 - q2._1).abs == (q1._2 - q2._2).abs // On diagonal.
  80
  81   def printChessBoard(boards: List[List[(Int, Int)]]) =
  82     (boards zipWithIndex).foldLeft("") { case (acc, (board, index)) =>
  83       acc + s"Solution ${index + 1}:\n" + {
  84         val l = board.length
  85         (for {r <- 1 to l; c <- 1 to l} yield (if (c == 1) "|" else "") + (if (board contains(r, c)) "\u265b" else "_") + "|" + (if (c == l) "\n" else "")) reduce (_ + _)
  86       }
  87     }
  88
  89   def main(args: Array[String]): Unit = {
  90     println("Question 1")
  91     println(s"a) ${lengthStrings(List("How", "long", "are", "we?"))}")
  92     println(s"b) ${dup("foo", 5)}")
  93     println(s"c) ${dot(List(1, 2, 3), List(2, 4, 3))}")
  94
  95     println("\nQuestion 2")
  96     println("a)")
  97     println(s"${areTrue(List(true, true, false))}")
  98     println(s"${areTrue(List(true, true, true))}")
  99     println("b)")
 100     println(s"${lString(List("Folding", "is", "fun"))}")
 101     println("c)")
 102     println(s"${longest(List("What", "is", "the", "longest?"))}")
 103     println("d)")
 104     println(s"${isPresent(List(1, 2, 3, 4), 5)}")
 105     println(s"${isPresent(List(1, 2, 3, 4), 3)}")
 106     println("e)")
 107     val l = List(List(1, 1), 2, List(3, List(5, 8)));
 108     println(s"flattenList($l): ${flattenList(l)}")
 109
 110     println("\nQuestion 3")
 111     println(s"a) ${primeSumUnique1(7)}")
 112     println(s"b) ${primeSumUnique2(7)}")
 113
 114     println("\nQuestion 4")
 115     postcards.foreach(p => println(p))
 116
 117     println("\nQuestion 5")
 118     println(printChessBoard(queens(4)))
 119   }
 120 }