Appending an element to the end of a list in Scala

244

votes

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?

arraysscala

419

votes

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).

68

votes

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)

36

votes

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)

5

votes

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)

2

votes

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

Appending an element to the end of a list in Scala

5 Answers