Sample run of chapter's interpreter examples
// In file working-with-lists/Misc.scala val fruit = List("apples", "oranges", "pears") val nums = List(1, 2, 3, 4) val diag3 = List( List(1, 0, 0), List(0, 1, 0), List(0, 0, 1) ) val empty = List()
// In file working-with-lists/Misc.scala val fruit: List[String] = List("apples", "oranges", "pears") val nums: List[Int] = List(1, 2, 3, 4) val diag3: List[List[Int]] = List( List(1, 0, 0), List(0, 1, 0), List(0, 0, 1) ) val empty: List[Nothing] = List()
// In file working-with-lists/Misc.scala // List() is also of type List[String]! val xs: List[String] = List()
// In file working-with-lists/Misc.scala val fruit = "apples" :: ("oranges" :: ("pears" :: Nil)) val nums = 1 :: (2 :: (3 :: (4 :: Nil))) val diag3 = (1 :: (0 :: (0 :: Nil))) :: (0 :: (1 :: (0 :: Nil))) :: (0 :: (0 :: (1 :: Nil))) :: Nil val empty = Nil
// In file working-with-lists/Misc.scala val nums = 1 :: 2 :: 3 :: 4 :: Nil
scala> Nil.head java.util.NoSuchElementException: head of empty list
// In file working-with-lists/InsertionSort1.scala def isort(xs: List[Int]): List[Int] = if (xs.isEmpty) Nil else insert(xs.head, isort(xs.tail)) def insert(x: Int, xs: List[Int]): List[Int] = if (xs.isEmpty || x <= xs.head) x :: xs else xs.head :: insert(x, xs.tail)
scala> val List(a, b, c) = fruit a: String = apples b: String = oranges c: String = pears
scala> val a :: b :: rest = fruit a: String = apples b: String = oranges rest: List[String] = List(pears)
// In file working-with-lists/InsertionSort2.scala def isort(xs: List[Int]): List[Int] = xs match { case List() => List() case x :: xs1 => insert(x, isort(xs1)) } def insert(x: Int, xs: List[Int]): List[Int] = xs match { case List() => List(x) case y :: ys => if (x <= y) x :: xs else y :: insert(x, ys) }
scala> List(1, 2) ::: List(3, 4, 5) res0: List[Int] = List(1, 2, 3, 4, 5) scala> List() ::: List(1, 2, 3) res1: List[Int] = List(1, 2, 3) scala> List(1, 2, 3) ::: List(4) res2: List[Int] = List(1, 2, 3, 4)
// In file working-with-lists/Misc.scala xs ::: ys ::: zs
// In file working-with-lists/Misc.scala xs ::: (ys ::: zs)
def append[T](xs: List[T], ys: List[T]): List[T]
def append[T](xs: List[T], ys: List[T]): List[T] = xs match { case List() => ??? case x :: xs1 => ??? }
case List() => ys
// In file working-with-lists/Misc.scala def append[T](xs: List[T], ys: List[T]): List[T] = xs match { case List() => ys case x :: xs1 => x :: append(xs1, ys) }
scala> List(1, 2, 3).length res3: Int = 3
scala> val abcde = List('a', 'b', 'c', 'd', 'e') abcde: List[Char] = List(a, b, c, d, e) scala> abcde.last res4: Char = e scala> abcde.init res5: List[Char] = List(a, b, c, d)
scala> List().init java.lang.UnsupportedOperationException: Nil.init at scala.List.init(List.scala:544) at ... scala> List().last java.util.NoSuchElementException: Nil.last at scala.List.last(List.scala:563) at ...
scala> abcde.reverse res6: List[Char] = List(e, d, c, b, a)
scala> abcde res7: List[Char] = List(a, b, c, d, e)
xs.reverse.reverse equals xs
xs.reverse.init equals xs.tail.reverse xs.reverse.tail equals xs.init.reverse xs.reverse.head equals xs.last xs.reverse.last equals xs.head
// In file working-with-lists/Reverse1.scala def rev[T](xs: List[T]): List[T] = xs match { case List() => xs case x :: xs1 => rev(xs1) ::: List(x) }
scala> abcde take 2 res8: List[Char] = List(a, b) scala> abcde drop 2 res9: List[Char] = List(c, d, e) scala> abcde splitAt 2 res10: (List[Char], List[Char]) = (List(a, b),List(c, d, e))
scala> abcde apply 2 // rare in Scala res11: Char = c
scala> abcde(2) // rare in Scala res12: Char = c
scala> abcde.indices res13: scala.collection.immutable.Range = Range(0, 1, 2, 3, 4)
scala> List(List(1, 2), List(3), List(), List(4, 5)).flatten res14: List[Int] = List(1, 2, 3, 4, 5) scala> fruit.map(_.toCharArray).flatten res15: List[Char] = List(a, p, p, l, e, s, o, r, a, n, g, e, s, p, e, a, r, s)
scala> List(1, 2, 3).flatten <console>:8: error: No implicit view available from Int => scala.collection.IterableOnce[B]. List(1, 2, 3).flatten ^
scala> abcde.indices zip abcde res17: scala.collection.immutable.IndexedSeq[(Int, Char)] = Vector((0,a), (1,b), (2,c), (3,d), (4,e))
scala> val zipped = abcde zip List(1, 2, 3) zipped: List[(Char, Int)] = List((a,1), (b,2), (c,3))
scala> abcde.zipWithIndex res18: List[(Char, Int)] = List((a,0), (b,1), (c,2), (d,3), (e,4))
scala> zipped.unzip res19: (List[Char], List[Int]) = (List(a, b, c),List(1, 2, 3))
scala> abcde.toString res20: String = List(a, b, c, d, e)
scala> abcde.mkString("[", ",", "]") res21: String = [a,b,c,d,e] scala> abcde mkString "" res22: String = abcde scala> abcde.mkString res23: String = abcde scala> abcde.mkString("List(", ", ", ")") res24: String = List(a, b, c, d, e)
scala> val buf = new StringBuilder buf: StringBuilder = scala> abcde.addString(buf, "(", ";", ")") res25: StringBuilder = (a;b;c;d;e)
scala> val arr = abcde.toArray arr: Array[Char] = Array(a, b, c, d, e) scala> arr.toList res26: List[Char] = List(a, b, c, d, e)
xs.copyToArray(arr, start)
scala> val arr2 = new Array[Int](10) arr2: Array[Int] = Array(0, 0, 0, 0, 0, 0, 0, 0, 0, 0) scala> List(1, 2, 3).copyToArray(arr2, 3) scala> arr2 res28: Array[Int] = Array(0, 0, 0, 1, 2, 3, 0, 0, 0, 0)
scala> val it = abcde.iterator it: Iterator[Char] = <iterator>
scala> it.next res29: Char = a scala> it.next res30: Char = b
// In file working-with-lists/Misc.scala def msort[T](less: (T, T) => Boolean) (xs: List[T]): List[T] = { def merge(xs: List[T], ys: List[T]): List[T] = (xs, ys) match { case (Nil, _) => ys case (_, Nil) => xs case (x :: xs1, y :: ys1) => if (less(x, y)) x :: merge(xs1, ys) else y :: merge(xs, ys1) } val n = xs.length / 2 if (n == 0) xs else { val (ys, zs) = xs splitAt n merge(msort(less)(ys), msort(less)(zs)) } }
scala> msort((x: Int, y: Int) => x < y)(List(5, 7, 1, 3)) res31: List[Int] = List(1, 3, 5, 7)
scala> val intSort = msort((x: Int, y: Int) => x < y) _ intSort: List[Int] => List[Int] = <function1>
scala> val reverseIntSort = msort((x: Int, y: Int) => x > y) _ reverseIntSort: (List[Int]) => List[Int] = <function>
scala> val mixedInts = List(4, 1, 9, 0, 5, 8, 3, 6, 2, 7) mixedInts: List[Int] = List(4, 1, 9, 0, 5, 8, 3, 6, 2, 7) scala> intSort(mixedInts) res0: List[Int] = List(0, 1, 2, 3, 4, 5, 6, 7, 8, 9) scala> reverseIntSort(mixedInts) res1: List[Int] = List(9, 8, 7, 6, 5, 4, 3, 2, 1, 0)
scala> List(1, 2, 3) map (_ + 1) res32: List[Int] = List(2, 3, 4) scala> val words = List("the", "quick", "brown", "fox") words: List[String] = List(the, quick, brown, fox) scala> words map (_.length) res33: List[Int] = List(3, 5, 5, 3) scala> words map (_.toList.reverse.mkString) res34: List[String] = List(eht, kciuq, nworb, xof)
scala> words map (_.toList) res35: List[List[Char]] = List(List(t, h, e), List(q, u, i, c, k), List(b, r, o, w, n), List(f, o, x)) scala> words flatMap (_.toList) res36: List[Char] = List(t, h, e, q, u, i, c, k, b, r, o, w, n, f, o, x)
scala> List.range(1, 5) flatMap ( | i => List.range(1, i) map (j => (i, j)) | ) res37: List[(Int, Int)] = List((2,1), (3,1), (3,2), (4,1), (4,2), (4,3))
// In file working-with-lists/Misc.scala for (i <- List.range(1, 5); j <- List.range(1, i)) yield (i, j)
scala> var sum = 0 sum: Int = 0 scala> List(1, 2, 3, 4, 5) foreach (sum += _) scala> sum res39: Int = 15
scala> List(1, 2, 3, 4, 5) filter (_ % 2 == 0) res40: List[Int] = List(2, 4) scala> words filter (_.length == 3) res41: List[String] = List(the, fox)
scala> List(1, 2, 3, 4, 5) partition (_ % 2 == 0) res42: (List[Int], List[Int]) = (List(2, 4),List(1, 3, 5))
scala> List(1, 2, 3, 4, 5) find (_ % 2 == 0) res43: Option[Int] = Some(2) scala> List(1, 2, 3, 4, 5) find (_ <= 0) res44: Option[Int] = None
scala> List(1, 2, 3, -4, 5) takeWhile (_ > 0) res45: List[Int] = List(1, 2, 3) scala> words dropWhile (_ startsWith "t") res46: List[String] = List(quick, brown, fox)
scala> List(1, 2, 3, -4, 5) span (_ > 0) res47: (List[Int], List[Int]) = (List(1, 2, 3),List(-4, 5))
scala> def hasZeroRow(m: List[List[Int]]) = | m exists (row => row forall (_ == 0)) hasZeroRow: (m: List[List[Int]])Boolean scala> hasZeroRow(diag3) res48: Boolean = false
scala> def sum(xs: List[Int]): Int = xs.foldLeft(0)(_ + _) sum: (xs: List[Int])Int
scala> def product(xs: List[Int]): Int = xs.foldLeft(1)(_ * _) product: (xs: List[Int])Int
scala> words.foldLeft("")(_ + " " + _) res49: String = " the quick brown fox"
scala> words.tail.foldLeft(words.head)(_ + " " + _) res50: String = the quick brown fox
def flattenLeft[T](xss: List[List[T]]) = xss.foldLeft(List[T]())(_ ::: _) def flattenRight[T](xss: List[List[T]]) = xss.foldRight(List[T]())(_ ::: _)
scala> def flattenRight[T](xss: List[List[T]]) = | xss.foldRight(List())(_ ::: _) <console>:8: error: type mismatch; found : List[T] required: List[Nothing] xss.foldRight(List())(_ ::: _) ^
def reverseLeft[T](xs: List[T]) = xs.foldLeft(\startValue)(\operation)
// In file working-with-lists/Reverse2.scala def reverseLeft[T](xs: List[T]) = xs.foldLeft(List[T]()) { (ys, y) => y :: ys }
scala> List(1, -3, 4, 2, 6) sortWith (_ < _) res51: List[Int] = List(-3, 1, 2, 4, 6) scala> words sortWith (_.length > _.length) res52: List[String] = List(quick, brown, the, fox)
scala> List.apply(1, 2, 3) res53: List[Int] = List(1, 2, 3)
scala> List.range(1, 5) res54: List[Int] = List(1, 2, 3, 4) scala> List.range(1, 9, 2) res55: List[Int] = List(1, 3, 5, 7) scala> List.range(9, 1, -3) res56: List[Int] = List(9, 6, 3)
scala> List.fill(5)('a') res57: List[Char] = List(a, a, a, a, a) scala> List.fill(3)("hello") res58: List[String] = List(hello, hello, hello)
scala> List.fill(2, 3)('b') res59: List[List[Char]] = List(List(b, b, b), List(b, b, b))
scala> val squares = List.tabulate(5)(n => n * n) squares: List[Int] = List(0, 1, 4, 9, 16) scala> val multiplication = List.tabulate(5,5)(_ * _) multiplication: List[List[Int]] = List(List(0, 0, 0, 0, 0), List(0, 1, 2, 3, 4), List(0, 2, 4, 6, 8), List(0, 3, 6, 9, 12), List(0, 4, 8, 12, 16))
scala> List.concat(List('a', 'b'), List('c')) res60: List[Char] = List(a, b, c) scala> List.concat(List(), List('b'), List('c')) res61: List[Char] = List(b, c) scala> List.concat() res62: List[Nothing] = List()
scala> (List(10, 20) zip List(3, 4, 5)). map { case (x, y) => x * y } res63: List[Int] = List(30, 80)
scala> (List(10, 20) lazyZip List(3, 4, 5)).map(_ * _) res63: List[Int] = List(30, 80)
scala> (List("abc", "de") lazyZip List(3, 2)). | forall(_.length == _) res64: Boolean = true scala> (List("abc", "de") lazyZip List(3, 2)). | exists(_.length != _) res65: Boolean = false
scala> msort((x: Char, y: Char) => x > y)(abcde) res66: List[Char] = List(e, d, c, b, a)
scala> abcde sortWith (_ > _) res67: List[Char] = List(e, d, c, b, a)
scala> msort(_ > _)(abcde) <console>:12: error: missing parameter type for expanded function ((x$1, x$2) => x$1.$greater(x$2)) msort(_ > _)(abcde) ^
scala> msort[Char](_ > _)(abcde) res68: List[Char] = List(e, d, c, b, a)
def msortSwapped[T](xs: List[T])(less: (T, T) => Boolean): List[T] = { // same implementation as msort, // but with arguments swapped }
scala> msortSwapped(abcde)(_ > _) res69: List[Char] = List(e, d, c, b, a)
xss.foldRight(List[T]())(_ ::: _)
xs.foldRight(z)(op)
xss.foldRight(List())(_ ::: _) // this won't compile
(List[T], List[Nothing]) => List[Nothing]
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