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Scala List, Tuple, Option, and Map Classes
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Scala has a wide variety of collections classes. Collections are containers of things. Those containers can be sequenced, linear sets of items (e.g., List):
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scala> val x = List(1,2,3,4)
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x: List[Int] = List(1, 2, 3, 4)
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scala> x.filter(a => a % 2 == 0)
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res14: List[Int] = List(2, 4)
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scala> x
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res15: List[Int] = List(1, 2, 3, 4)
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CHAPTER 3 COLLECTIONS AND THE JOY OF IMMUTABILITY
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They may be indexed items where the index is a zero-based Int (e.g., Array) or any other type (e.g., Map).
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scala> val a = Array(1,2,3)
a: Array[Int] = Array(1, 2, 3)
scala> a(1)
res16: Int = 2
scala> val m = Map("one" -> 1, "two" -> 2, "three" -> 3)
m: Map[java.lang.String,Int] = Map(one -> 1, two -> 2, three -> 3)
scala> m("two")
res17: Int = 2
The collections may have an arbitrary number of elements or be bounded to zero or one element (e.g., Option). Collections may be strict or lazy. Lazy collections have elements that may not consume memory until they are accessed (e.g., Range). Let s create a Range:
scala> 0 to 10
res0: Range.Inclusive = Range(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
CHAPTER 3 COLLECTIONS AND THE JOY OF IMMUTABILITY
The nifty thing about Ranges is that the actual elements in the Range are not instantiated until they are accessed. So we can create a Range for all positive Integers but take only the first five elements. This code runs without consuming many gigabytes of RAM because only the elements that are needed are created.
scala> (1 to Integer.MAX_VALUE - 1).take(5)
res18: RandomAccessSeq[Int] = RandomAccessSeq(1, 2, 3, 4, 5)
Collections may be mutable (the contents of the reference can change) or immutable (the thing that a reference refers to is never changed). Note that immutable collections may contain mutable items. In this chapter, we ll be focusing on List, Option, and Map. These immutable data structures form the backbone of most of the programs I write.
List[T]
Scala s List[T] is a linked list of type T. That means it s a sequential list of any type, including Java s primitives (Int, Float, Double, Boolean, Char) because Scala takes care of boxing (turning primitives into objects) for you. Internally, List is made up of a cons cell (the scala.:: class [yes, that s two colons]) with a tail that refers to another cons cell or the Nil object. It s easy to create a List:
scala> 1 :: 2 :: 3 :: Nil
res20: List[Int] = List(1, 2, 3)
The previous code creates three cons cells, each with an Int in it. Anything that looks like an operator with a : (colon) as the first character is evaluated right to left. Thus, the previous code is evaluated just like the following:
scala> new ::(1, new ::(2, new ::(3, Nil)))
res21: ::[Int] = List(1, 2, 3)
CHAPTER 3 COLLECTIONS AND THE JOY OF IMMUTABILITY
:: takes a head which is a single element and a tail which is another List. The expression on the left of the :: is the head, and the expression on the right is the tail. To
create a List using ::, we must always put a List on the right side. That means that the right-most element has to be a List, and in this case, we re using an empty List, Nil. We can also create a List using the List object s apply method (which is defined as def apply[T](param: T*): List[T], which translates to the apply method of type T takes zero or more parameters of type T and returns a List of type T ):
scala> List(1,2,3)
res22: List[Int] = List(1, 2, 3)
The type inferencer is pretty good at figuring out the type of the List, but sometimes you need to help it along:
scala> List(1, 44.5, 8d)
res27: List[AnyVal] = List(1, 44.5, 8.0)
scala> List[Number](1, 44.5, 8d)
res28: List[java.lang.Number] = List(1, 44.5, 8.0)
If you want to prepend an item to the head of the List, you can use ::, which actually creates a new cons cell with the old list as the tail:
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