// Scala Lecture 3+
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//=================+
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+
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// Pattern Matching+
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//==================+
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+
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// A powerful tool which is supposed to come to Java in a few years+
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// time (https://www.youtube.com/watch?v=oGll155-vuQ)...Scala already+
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// has it for many years. Other functional languages have it already for+
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// decades. I think I would be really upset if a programming language +
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// I have to use does not have pattern matching....its is just so +
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// useful. ;o)+
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+
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// The general schema:+
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//+
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//    expression match {+
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//       case pattern1 => expression1+
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//       case pattern2 => expression2+
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//       ...+
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//       case patternN => expressionN+
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//    }+
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+
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+
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// remember+
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val lst = List(None, Some(1), Some(2), None, Some(3)).flatten+
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+
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+
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def my_flatten(xs: List[Option[Int]]): List[Int] = {+
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  if (xs == Nil) Nil+
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  else if (xs.head == None) my_flatten(xs.tail)+
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  else xs.head.get :: my_flatten(xs.tail)+
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}+
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+
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+
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+
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val lst = List(None, Some(1), Some(2), None, Some(3))+
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+
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def my_flatten(lst: List[Option[Int]]): List[Int] = lst match {+
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  case Nil => Nil+
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  case None::xs => my_flatten(xs)+
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  case Some(n)::xs => n::my_flatten(xs)+
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}+
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+
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my_flatten(lst)+
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+
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Nil == List()+
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+
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+
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// another example including a catch-all pattern+
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def get_me_a_string(n: Int): String = n match {+
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  case 0 => "zero"+
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  case 1 => "one"+
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  case 2 => "two"+
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  case _ => "many"+
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}+
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+
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get_me_a_string(10)+
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+
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// you can also have cases combined+
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def season(month: String) = month match {+
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  case "March" | "April" | "May" => "It's spring"+
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  case "June" | "July" | "August" => "It's summer"+
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  case "September" | "October" | "November" => "It's autumn"+
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  case "December" | "January" | "February" => "It's winter"+
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}+
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 +
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println(season("November"))+
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+
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// What happens if no case matches?+
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+
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println(season("foobar"))+
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+
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+
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// we can also match more complicated pattern+
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//+
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// let's look at the Collatz function on binary strings+
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+
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// adding two binary strings in a very, very lazy manner+
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+
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def badd(s1: String, s2: String) : String = +
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  (BigInt(s1, 2) + BigInt(s2, 2)).toString(2)+
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+
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+
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"111".dropRight(1)+
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"111".last+
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+
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def bcollatz(s: String) : Long = (s.dropRight(1), s.last) match {+
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  case ("", '1') => 1                               // we reached 1+
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  case (rest, '0') => 1 + bcollatz(rest)            +
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                                  // even number => divide by two+
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  case (rest, '1') => 1 + bcollatz(badd(s + '1', s))+
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                                  // odd number => s + '1' is 2 * s + 1+
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                                  // add another s gives 3 * s + 1  +
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} +
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+
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bcollatz(6.toBinaryString)+
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bcollatz(837799.toBinaryString)+
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bcollatz(100000000000000000L.toBinaryString)+
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bcollatz(BigInt("1000000000000000000000000000000000000000000000000000000000000000000000000000").toString(2))+
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+
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+
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+
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+
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// User-defined Datatypes+
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//========================+
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+
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abstract class Colour+
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case object Red extends Colour +
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case object Green extends Colour +
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case object Blue extends Colour+
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+
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def fav_colour(c: Colour) : Boolean = c match {+
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  case Red   => false+
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  case Green => true+
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  case Blue  => false +
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}+
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+
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fav_colour(Green)+
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+
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+
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// actually colors can be written with "object",+
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// because they do not take any arguments+
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+
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abstract class Day+
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case object Monday extends Day +
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case object Tuesday extends Day +
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case object Wednesday extends Day+
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case object Thursday extends Day +
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case object Friday extends Day +
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case object Saturday extends Day+
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case object Sunday extends Day +
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+
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+
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+
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// ... a bit more useful: Roman Numerals+
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+
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abstract class RomanDigit +
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case object I extends RomanDigit +
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case object V extends RomanDigit +
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case object X extends RomanDigit +
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case object L extends RomanDigit +
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case object C extends RomanDigit +
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case object D extends RomanDigit +
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case object M extends RomanDigit +
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+
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type RomanNumeral = List[RomanDigit] +
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+
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def RomanNumeral2Int(rs: RomanNumeral): Int = rs match { +
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  case Nil => 0+
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  case M::r    => 1000 + RomanNumeral2Int(r)  +
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  case C::M::r => 900 + RomanNumeral2Int(r)+
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  case D::r    => 500 + RomanNumeral2Int(r)+
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  case C::D::r => 400 + RomanNumeral2Int(r)+
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  case C::r    => 100 + RomanNumeral2Int(r)+
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  case X::C::r => 90 + RomanNumeral2Int(r)+
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  case L::r    => 50 + RomanNumeral2Int(r)+
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  case X::L::r => 40 + RomanNumeral2Int(r)+
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  case X::r    => 10 + RomanNumeral2Int(r)+
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  case I::X::r => 9 + RomanNumeral2Int(r)+
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  case V::r    => 5 + RomanNumeral2Int(r)+
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  case I::V::r => 4 + RomanNumeral2Int(r)+
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  case I::r    => 1 + RomanNumeral2Int(r)+
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}+
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+
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RomanNumeral2Int(List(I,V))             // 4+
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RomanNumeral2Int(List(I,I,I,I))         // 4 (invalid Roman number)+
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RomanNumeral2Int(List(V,I))             // 6+
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RomanNumeral2Int(List(I,X))             // 9+
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RomanNumeral2Int(List(M,C,M,L,X,X,I,X)) // 1979+
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RomanNumeral2Int(List(M,M,X,V,I,I))     // 2017+
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+
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+
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+
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// another example+
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//=================+
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+
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// Once upon a time, in a complete fictional country there were Persons...+
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+
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abstract class Person+
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case object King extends Person+
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case class Peer(deg: String, terr: String, succ: Int) extends Person+
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case class Knight(name: String) extends Person+
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case class Peasant(name: String) extends Person+
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case object Clown extends Person+
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+
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def title(p: Person): String = p match {+
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  case King => "His Majesty the King"+
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  case Peer(deg, terr, _) => s"The ${deg} of ${terr}"+
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  case Knight(name) => s"Sir ${name}"+
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  case Peasant(name) => name+
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  case Clown => "My name is Boris Johnson"+
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+
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}+
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+
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title(Clown)+
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+
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+
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+
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def superior(p1: Person, p2: Person): Boolean = (p1, p2) match {+
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  case (King, _) => true+
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  case (Peer(_,_,_), Knight(_)) => true+
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  case (Peer(_,_,_), Peasant(_)) => true+
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  case (Peer(_,_,_), Clown) => true+
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  case (Knight(_), Peasant(_)) => true+
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  case (Knight(_), Clown) => true+
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  case (Clown, Peasant(_)) => true+
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  case _ => false+
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}+
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+
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val people = List(Knight("David"), +
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                  Peer("Duke", "Norfolk", 84), +
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                  Peasant("Christian"), +
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                  King, +
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                  Clown)+
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+
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println(people.sortWith(superior(_, _)).mkString(", "))+
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+
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+
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+
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+
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// Tail recursion+
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//================+
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+
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+
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def fact(n: Long): Long = +
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  if (n == 0) 1 else n * fact(n - 1)+
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+
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fact(10)              //ok+
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fact(10000)           // produces a stackoverflow+
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+
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def factT(n: BigInt, acc: BigInt): BigInt =+
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  if (n == 0) acc else factT(n - 1, n * acc)+
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+
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factT(10, 1)+
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factT(100000, 1)+
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+
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// there is a flag for ensuring a function is tail recursive+
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import scala.annotation.tailrec+
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+
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@tailrec+
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def factT(n: BigInt, acc: BigInt): BigInt =+
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  if (n == 0) acc else factT(n - 1, n * acc)+
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+
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+
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+
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// for tail-recursive functions the Scala compiler+
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// generates loop-like code, which does not need+
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// to allocate stack-space in each recursive+
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// call; Scala can do this only for tail-recursive+
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// functions+
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+
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+
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+
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// sudoku again+
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+
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val game0 = """.14.6.3..+
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              |62...4..9+
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              |.8..5.6..+
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              |.6.2....3+
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              |.7..1..5.+
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              |5....9.6.+
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              |..6.2..3.+
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              |1..5...92+
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              |..7.9.41.""".stripMargin.replaceAll("\\n", "")+
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+
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type Pos = (Int, Int)+
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val EmptyValue = '.'+
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val MaxValue = 9+
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+
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val allValues = "123456789".toList+
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val indexes = (0 to 8).toList+
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+
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+
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def empty(game: String) = game.indexOf(EmptyValue)+
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def isDone(game: String) = empty(game) == -1 +
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def emptyPosition(game: String) = +
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  (empty(game) % MaxValue, empty(game) / MaxValue)+
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+
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+
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def get_row(game: String, y: Int) = +
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  indexes.map(col => game(y * MaxValue + col))+
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def get_col(game: String, x: Int) = +
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  indexes.map(row => game(x + row * MaxValue))+
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+
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def get_box(game: String, pos: Pos): List[Char] = {+
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    def base(p: Int): Int = (p / 3) * 3+
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    val x0 = base(pos._1)+
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    val y0 = base(pos._2)+
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    val ys = (y0 until y0 + 3).toList+
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    (x0 until x0 + 3).toList.flatMap(x => ys.map(y => game(x + y * MaxValue)))+
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}+
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+
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// this is not mutable!!+
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def update(game: String, pos: Int, value: Char): String = +
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  game.updated(pos, value)+
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+
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def toAvoid(game: String, pos: Pos): List[Char] = +
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  (get_col(game, pos._1) ++ get_row(game, pos._2) ++ get_box(game, pos))+
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+
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def candidates(game: String, pos: Pos): List[Char] = +
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  allValues.diff(toAvoid(game,pos))+
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+
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//candidates(game0, (0,0))+
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+
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def pretty(game: String): String = +
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  "\n" + (game sliding (MaxValue, MaxValue) mkString "\n")+
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+
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/////////////////////+
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// not tail recursive +
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def search(game: String): List[String] = {+
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  if (isDone(game)) List(game)+
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  else {+
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    val cs = candidates(game, emptyPosition(game))+
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    cs.map(c => search(update(game, empty(game), c))).toList.flatten+
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  }+
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}+
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+
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// tail recursive version that searches +
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// for all solutions+
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+
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def searchT(games: List[String], sols: List[String]): List[String] = games match {+
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  case Nil => sols+
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  case game::rest => {+
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    if (isDone(game)) searchT(rest, game::sols)+
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    else {+
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      val cs = candidates(game, emptyPosition(game))+
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      searchT(cs.map(c => update(game, empty(game), c)) ::: rest, sols)+
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    }+
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  }+
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}+
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+
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searchT(List(game3), List()).map(pretty)+
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+
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+
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// tail recursive version that searches +
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// for a single solution+
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+
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def search1T(games: List[String]): Option[String] = games match {+
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  case Nil => None+
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  case game::rest => {+
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    if (isDone(game)) Some(game)+
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    else {+
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      val cs = candidates(game, emptyPosition(game))+
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      search1T(cs.map(c => update(game, empty(game), c)) ::: rest)+
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    }+
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  }+
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}+
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+
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search1T(List(game3)).map(pretty)+
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+
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// game with multiple solutions+
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val game3 = """.8...9743+
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              |.5...8.1.+
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              |.1.......+
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              |8....5...+
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              |...8.4...+
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              |...3....6+
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              |.......7.+
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              |.3.5...8.+
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              |9724...5.""".stripMargin.replaceAll("\\n", "")+
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+
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searchT(List(game3), Nil).map(pretty)+
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search1T(List(game3)).map(pretty)+
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+
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// Moral: Whenever a recursive function is resource-critical+
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// (i.e. works with large recursion depth), then you need to+
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// write it in tail-recursive fashion.+
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// +
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// Unfortuantely, Scala because of current limitations in +
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// the JVM is not as clever as other functional languages. It can +
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// only optimise "self-tail calls". This excludes the cases of +
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// multiple functions making tail calls to each other. Well,+
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// nothing is perfect. +
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+
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+
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+
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+
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// Polymorphic Types+
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//===================+
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+
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// You do not want to write functions like contains, first +
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// and so on for every type of lists.+
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+
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+
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def length_string_list(lst: List[String]): Int = lst match {+
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  case Nil => 0+
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  case x::xs => 1 + length_string_list(xs)+
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}+
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+
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def length_int_list(lst: List[Int]): Int = lst match {+
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  case Nil => 0+
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  case x::xs => 1 + length_int_list(xs)+
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}+
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+
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length_string_list(List("1", "2", "3", "4"))+
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length_int_list(List(1, 2, 3, 4))+
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+
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//-----+
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def length[A](lst: List[A]): Int = lst match {+
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  case Nil => 0+
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  case x::xs => 1 + length(xs)+
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}+
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length(List("1", "2", "3", "4"))+
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length(List(King, Knight("foo"), Clown))+
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length(List(1, 2, 3, 4))+
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+
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def map[A, B](lst: List[A], f: A => B): List[B] = lst match {+
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  case Nil => Nil+
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  case x::xs => f(x)::map_int_list(xs, f) +
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}+
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+
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map_int_list(List(1, 2, 3, 4), square)+
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+
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+
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// Remember?+
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def first[A, B](xs: List[A], f: A => Option[B]): Option[B] = ...+
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+
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+
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+
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+
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+
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// Cool Stuff+
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//============+
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+
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+
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// Implicits +
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//===========+
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//+
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// For example adding your own methods to Strings:+
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// Imagine you want to increment strings, like+
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//+
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//     "HAL".increment+
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//+
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// you can avoid ugly fudges, like a MyString, by+
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// using implicit conversions.+
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+
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+
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implicit class MyString(s: String) {+
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  def increment = for (c <- s) yield (c + 1).toChar +
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}+
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+
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"HAL".increment+
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+
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+
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+
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+
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// Regular expressions - the power of DSLs in Scala+
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//==================================================+
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+
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abstract class Rexp+
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case object ZERO extends Rexp                       // nothing+
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case object ONE extends Rexp                        // the empty string+
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case class CHAR(c: Char) extends Rexp               // a character c+
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case class ALT(r1: Rexp, r2: Rexp) extends Rexp     // alternative  r1 + r2+
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case class SEQ(r1: Rexp, r2: Rexp) extends Rexp     // sequence     r1 o r2  +
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case class STAR(r: Rexp) extends Rexp               // star         r*+
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+
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+
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+
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// (ab)*+
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val r0 = STAR(SEQ(CHAR('a'), CHAR('b')))+
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+
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+
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// some convenience for typing in regular expressions+
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import scala.language.implicitConversions    +
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import scala.language.reflectiveCalls +
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+
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def charlist2rexp(s: List[Char]): Rexp = s match {+
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  case Nil => ONE+
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  case c::Nil => CHAR(c)+
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  case c::s => SEQ(CHAR(c), charlist2rexp(s))+
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}+
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implicit def string2rexp(s: String): Rexp = charlist2rexp(s.toList)+
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+
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+
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val r1 = STAR("ab")+
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val r2 = STAR(ALT("ab"))+
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val r3 = STAR(ALT("ab", "baa baa black sheep"))+
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+
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implicit def RexpOps (r: Rexp) = new {+
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  def | (s: Rexp) = ALT(r, s)+
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  def % = STAR(r)+
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  def ~ (s: Rexp) = SEQ(r, s)+
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}+
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+
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implicit def stringOps (s: String) = new {+
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  def | (r: Rexp) = ALT(s, r)+
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  def | (r: String) = ALT(s, r)+
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  def % = STAR(s)+
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  def ~ (r: Rexp) = SEQ(s, r)+
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  def ~ (r: String) = SEQ(s, r)+
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}+
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+
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//example regular expressions+
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val digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"+
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val sign = "+" | "-" | ""+
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val number = sign ~ digit ~ digit.% +
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+
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+
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+
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+
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+
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// The End+
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//=========+
−
+
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// A function should do one thing, and only one thing.+
−
+
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// Make your variables immutable, unless there's a good +
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// reason not to.+
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+
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// You can be productive on Day 1, but the language is deep.+
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//+
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// http://scalapuzzlers.com+
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//+
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// http://www.latkin.org/blog/2017/05/02/when-the-scala-compiler-doesnt-help/+
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+
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List(1, 2, 3) contains "your mom"+
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+
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// I like best about Scala that it lets me often write+
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// concise, readable code.+
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+
−
+
−
+
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// You can define your own while loop+
−
+
−
+
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def my_while(condition: => Boolean)(block: => Unit): Unit = +
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  if (condition) { block ; my_while(condition) { block } } else { }+
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+
−
+
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var x = 10+
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my_while (x > 0) { +
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  println(s"$x") ; x = x - 1 +
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}+
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+
−
+
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`symbol+
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`symbol`+
−