Scala Cookbook

11.3. Adding Elements to a List

Problem

You want to add elements to a List that you’re working with.

Solution

“How do I add elements to a List?” is a bit of a trick question, because a List is immutable, so you can’t actually add elements to it. If you want a List that is constantly changing, use a ListBuffer (as described in Recipe 11.2), and then convert it to a List when necessary.

To work with a List, the general approach is to prepend items to the list with the :: method while assigning the results to a new List:

scala> val x = List(2)
x: List[Int] = List(2)

scala> val y = 1 :: x
y: List[Int] = List(1, 2)

scala> val z = 0 :: y
z: List[Int] = List(0, 1, 2)

Rather than continually reassigning the result of this operation to a new variable, you can declare your variable as a var, and reassign the result to it:

scala> var x = List(2)
x: List[Int] = List(2)

scala> x = 1 :: x
x: List[Int] = List(1, 2)

scala> x = 0 :: x
x: List[Int] = List(0, 1, 2)

As these examples illustrate, the :: method is right-associative; lists are constructed from right to left, which you can see in this example:

scala> val list1 = 3 :: Nil
list1: List[Int] = List(3)

scala> val list2 = 2 :: list1
list2: List[Int] = List(2, 3)

scala> val list3 = 1 :: list2
list3: List[Int] = List(1, 2, 3)

Note

Any Scala method that ends with a : character is evaluated from right to left. This means that the method is invoked on the right operand. You can see how this works by analyzing the following code, where both methods print the ...