How do I create a List in Scala?

Posted 2020-08-03 尧字节

Scala List class FAQ: How do I create a List in Scala?

You can create a Scala List in several different ways, including these approaches:

• Lisp style
• Java style
• Using the List class rangemethod
• Using the List class fillmethod
• Using the List classtabulate 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.

1) Create a Scala List in the Lisp style

First, if you prefer the Lisp-style of programming, you can create a ScalaList 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.)

2) 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 theNumber type.

 

3) Create a Scala List with the range method

Another convenient way to create a List is with the 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 creates 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)

4) Create a Scala List with the List class fill method

Another convenient way to create a Scala List is with the 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.

5) Create a Scala List with the List class tabulate method

Finally, you can create a Scala List with the tabulate method of the Listclass.

The tabulate method creates a new List whose elements are created according to the function you supply. The book Programming in Scalashows how to create a List using a simple "squares" function with thetabulate 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.

6) 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.