Scala List class FAQ: How do I create a List inScala?
You can create a Scala List in several different ways, including these approaches:
- Lisp style
- Java style
- Using the List class range method
- Using the List class fill method
- Using the List class tabulate method
In this Scala List tutorial, I'll demonstrate each of these approaches. I'll execute each command in the Scala command-line interpreter so you can see the results of each approach.
Create a Scala List in the Lisp style
First, if you prefer the Lisp-style of programming, you can create a Scala List using the "cons" syntax, like this:
scala> val list = 1 :: 2 :: 3 :: Nil list: List[Int] = List(1, 2, 3)
As you can see, this creates a List that contains the Ints 1, 2, and 3. With this approach, you need to end the list with the Nil object.
In this "cons" style, the :: method takes two arguments, a "head", which is a single element, and a "tail", which is a List. (And yes, "::" is a function/method.)
Create a Scala List in the Java style
My guess is that the most popular way to create a List is with what I call the "Java style":
scala> val list = List(1,2,3) x: List[Int] = List(1, 2, 3)
This syntax looks a lot like the Java way to create an object, except (a) you don't need the "new" keyword before the List, and (b) you don't have to declare the type of elements in the List.
Note that if you're going to mix types in a List constructor, you may need to manually specify the type of the List. This example demonstrates the syntax to specify the List type:
scala> val x = List[Number](1, 2.0, 33d, 0x1) x: List[java.lang.Number] = List(1, 2.0, 33.0, 1)
In this example I've explicitly stated that the values in the List are of the Number type.
Create a Scala List with the range method
Another convenient way to create a List is with the List class range method:
scala> val x = List.range(1, 10) x: List[Int] = List(1, 2, 3, 4, 5, 6, 7, 8, 9)
As you can see, this example created a List of Int values, beginning at 1, and ending at 9.
In addition to this simple approach, the range function can also take a third argument, which serves as a "step" value when creating the List:
scala> val x = List.range(0, 10, 2) x: List[Int] = List(0, 2, 4, 6, 8)
Create a Scala List with the List class fill method
Another convenient way to create a Scala List is with the List class fill method:
scala> val x = List.fill(3)("foo")
x: List[java.lang.String] = List(foo, foo, foo)
As you can see, you just specify how many items you want, and the object value you want to fill each List element with.
Create a Scala List with the List class tabulate method
Finally, you can create a Scala List with the tabulate method of the List class.
The tabulate method creates a new List whose elements are created according to the function you supply. The excellent book "Programming in Scala" shows how to create a List using a simple "squares" function with the tabulate method:
scala> val x = List.tabulate(5)(n => n * n) x: List[Int] = List(0, 1, 4, 9, 16)
As you can see, that example creates a List of five elements, where the element values are the square of the index of each element, so 0 becomes 0, 1 becomes 1, 2 becomes 4, 3 becomes 9, and 4 becomes 16.
Creating Scala Lists - Summary
In summary, as you have seen, you can create Scala lists in several different ways, including these approaches:
- Lisp style
- Java style
- Using the List class range method
- Using the List class fill method
- Using the List class tabulate method
I hope this Scala List class tutorial has been helpful.
