Appending an element to the end of a list in Scala

I can't add an element of type T into a list List[T]. I tried with myList ::= myElement but it seems it creates a strange object and accessing to myList.last always returns the first element that was put inside the list. How can I solve this problem?

6 Answers

List(1,2,3) :+ 4 Results in List[Int] = List(1, 2, 3, 4) 

Note that this operation has a complexity of O(n). If you need this operation frequently, or for long lists, consider using another data type (e.g. a ListBuffer).

That's because you shouldn't do it (at least with an immutable list). If you really really need to append an element to the end of a data structure and this data structure really really needs to be a list and this list really really has to be immutable then do eiher this:

(4 :: List(1,2,3).reverse).reverse 

or that:

List(1,2,3) ::: List(4) 

Lists in Scala are not designed to be modified. In fact, you can't add elements to a Scala List; it's an immutable data structure, like a Java String. What you actually do when you "add an element to a list" in Scala is to create a new List from an existing List. (Source)

Instead of using lists for such use cases, I suggest to either use an ArrayBuffer or a ListBuffer. Those datastructures are designed to have new elements added.

Finally, after all your operations are done, the buffer then can be converted into a list. See the following REPL example:

scala> import scala.collection.mutable.ListBuffer import scala.collection.mutable.ListBuffer scala> var fruits = new ListBuffer[String]() fruits: scala.collection.mutable.ListBuffer[String] = ListBuffer() scala> fruits += "Apple" res0: scala.collection.mutable.ListBuffer[String] = ListBuffer(Apple) scala> fruits += "Banana" res1: scala.collection.mutable.ListBuffer[String] = ListBuffer(Apple, Banana) scala> fruits += "Orange" res2: scala.collection.mutable.ListBuffer[String] = ListBuffer(Apple, Banana, Orange) scala> val fruitsList = fruits.toList fruitsList: List[String] = List(Apple, Banana, Orange) 

This is similar to one of the answers but in different way :

scala> val x = List(1,2,3) x: List[Int] = List(1, 2, 3) scala> val y = x ::: 4 :: Nil y: List[Int] = List(1, 2, 3, 4) 

We can append or prepend two lists or list&array
Append:

var l = List(1,2,3) l = l :+ 4 Result : 1 2 3 4 var ar = Array(4, 5, 6) for(x <- ar) { l = l :+ x } l.foreach(println) Result:1 2 3 4 5 6 

Prepending:

var l = List[Int]() for(x <- ar) { l= x :: l } //prepending l.foreach(println) Result:6 5 4 1 2 3 

List is immutable in Scala

You can use MutableList

var l = scala.collection.mutable.MutableList(1,2,3) l += 4