Beginning Scala by Layka Vishal & Pollak David

Author:Layka, Vishal & Pollak, David
Language: eng
Format: epub
ISBN: 978-1-4842-0233-3
Publisher: Apress
Published: 2015-02-25T05:00:00+00:00

stream: scala.collection.immutable.Stream[Int] = Stream(1, ?)

you can attempt to access the head and tail of the stream. The head is returned immediately:

scala> stream.head

res0: Int = 1

but the tail isn't evaluated yet:

scala> stream.tail

res1: scala.collection.immutable.Stream[Int] = Stream(2, ?)

The ? symbol is the way a lazy collection shows that the end of the collection hasn’t been evaluated yet.

Tuples

Have you ever written a method that returns two or three values? Let’s write a method that takes a List[Double] and returns the count, the sum, and the sum of squares returned in a three-element Tuple, a Tuple3[Int, Double, Double]:

def sumSq(in: List[Double]): (Int, Double, Double) =

in.foldLeft((0, 0d, 0d))((t, v) => (t._1 + 1, t._2 + v, t._3 + v * v))

The sumSq method takes a List[Double] as input and returns a Tuple3[Int, Double, Double]. The compiler desugars (Int, Double, Double) into Tuple3[Int, Double, Double]. The compiler will treat a collection of elements in parentheses as a Tuple. We seed the foldLeft with (0, 0d, 0d), which the compiler translates to a Tuple3[Int, Double, Double]. The function takes two parameters: t and v. t is a Tuple3, and v is a Double. The function returns a new Tuple3 by adding 1 to the first element of the Tuple, adding v to the second element of the Tuple, and adding the square of v to the third element of the Tuple. Using Scala’s pattern matching, we can make the code a little more readable:

def sumSq(in: List[Double]) : (Int, Double, Double) =

in.foldLeft((0, 0d, 0d)){

case ((cnt, sum, sq), v) => (cnt + 1, sum + v, sq + v * v)}

You can create Tuples using a variety of syntax:

scala> Tuple2(1,2) == Pair(1,2)

scala> Pair(1,2) == (1,2)

scala> (1,2) == 1 -> 2

The last example, 1 -> 2, is a particularly helpful and syntactically pleasing way for passing pairs around. Pairs appear in code very frequently, including name/value pairs for creating Maps.

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