Scala Cookbook

11.1. Different Ways to Create and Populate a List

Problem

You want to create and populate a List.

Solution

There are many ways to create and initially populate a List:

// 1
scala> val list = 1 :: 2 :: 3 :: Nil
list: List[Int] = List(1, 2, 3)

// 2
scala> val list = List(1, 2, 3)
x: List[Int] = List(1, 2, 3)

// 3a
scala> val x = List(1, 2.0, 33D, 4000L)
x: List[Double] = List(1.0, 2.0, 33.0, 4000.0)

// 3b
scala> val x = List[Number](1, 2.0, 33D, 4000L)
x: List[java.lang.Number] = List(1, 2.0, 33.0, 4000)

// 4
scala> val x = List.range(1, 10)
x: List[Int] = List(1, 2, 3, 4, 5, 6, 7, 8, 9)

scala> val x = List.range(0, 10, 2)
x: List[Int] = List(0, 2, 4, 6, 8)

// 5
scala> val x = List.fill(3)("foo")
x: List[String] = List(foo, foo, foo)

// 6
scala> val x = List.tabulate(5)(n => n * n)
x: List[Int] = List(0, 1, 4, 9, 16)

// 7
scala> val x = collection.mutable.ListBuffer(1, 2, 3).toList
x: List[Int] = List(1, 2, 3)

// 8
scala> "foo".toList
res0: List[Char] = List(f, o, o)

The first two approaches shown are the most common and straightforward ways to create a List. Examples 3a and 3b show how you can manually control the List type when your collection has mixed types. When the type isn’t manually set in Example 3a, it ends up as a List[Double], and in 3b it’s manually set to be a List[Number].

Examples 4 through 6 show different ways to create and populate a List with data. Examples 7 and 8 show that many collection types also have a toList method that converts their data to a List.

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