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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]:
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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))
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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 parenthesis 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
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CHAPTER 3 COLLECTIONS AND THE JOY OF IMMUTABILITY
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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:
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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)}
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You can create Tuples using a variety of syntax:
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scala> Tuple2(1,2) == Pair(1,2) scala> Pair(1,2) == (1,2) scala> (1,2) == 1 -> 2
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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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Map[K, V]
A Map is a collection of key/value pairs. Any value can be retrieved based on its key. Keys are unique in the Map, but values need not be unique. In Java, Hashtable and HashMap are common Map classes. The default Scala Map class is immutable. This means that you can pass an instance of Map to another thread, and that thread can access the Map without synchronizing. The performance of Scala s immutable Map is indistinguishable from the performance of Java s HashMap. We can create a Map:
scala> var p = Map(1 -> "David", 9 -> "Elwood")
p: Map[Int,String] = Map(1 -> David, 9 -> Elwood)
We create a new Map by passing a set of Pair[Int, String] to the Map object s apply method. Note that we created a var p rather than a val p. This is because the Map is immutable, so when we alter the contents on the Map, we have to assign the new Map back to p. We can add an element to the Map:
scala> p + 8 -> "Archer"
res4: Map[Int,String] = Map(1 -> David, 9 -> Elwood, 8 -> Archer)
CHAPTER 3 COLLECTIONS AND THE JOY OF IMMUTABILITY
But we haven t changed the immutable Map:
scala> p
res5: Map[Int,String] = Map(1 -> David, 9 -> Elwood)
In order to update p, we have to assign the new Map back to p:
scala> p = p + 8 -> "Archer"
scala> p += 8 -> "Archer"
And we can see that p is updated:
scala> p
res7: Map[Int,String] = Map(1 -> David, 9 -> Elwood, 8 -> Archer)
We can get elements out of the Map:
scala> p(9)
res12: java.lang.String = Elwood
What happens when we ask for an element that doesn t exist
scala> p(88)
java.util.NoSuchElementException: key not found: 88
This is mighty inconvenient. If you try to get an element that s not in the Map, you get an exception. That s kind of jarring. So far, we haven t seen much in Scala that results in exceptions being thrown, but it makes logical sense. If you request something that doesn t
CHAPTER 3 COLLECTIONS AND THE JOY OF IMMUTABILITY
exist, that s an exceptional situation. Java s Map classes handle this situation by returning null, which has two drawbacks. First, you have to null-test the result of every Map access. Second, it means you can t store a null in a Map. Scala has a kinder and gentler mechanism for dealing with this situation. The get() method on Map returns an Option (Some or None) that contains the result:
scala> p.get(88)
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