Add element to a list In Scala

Question

I m running Scala 2 10 2  I want to create a list  then add some elements to the list and expect to see all the elements in the lists when I call the list s name  But I observed something quite weird  At least weird for me since I m a newbie   Below is the what I tried to do in my sbt console  scala gt  val l   1 0    5 5    Nil l  List Double    List 1 0  5 5   scala gt  l res0  List Double    List 1 0  5 5   scala gt  l     List 2 2  3 7  res1  List Double    List 1 0  5 5  2 2  3 7   scala gt  List l     2 2 res2  List Any    List List 1 0  5 5   2 2   scala gt  l res3  List Double    List 1 0  5 5   scala gt     First  I created the list l with 2 elements  1 0 and 5 5   I call l and get what I expect  the two elements  Now I tried to add another element to the list using     which returned a new list with a new list of elements I added  2 2 and 3 7  Sweet  I even checked someone else s code for help  Appending an element to the end of a list in Scala to use a new construct     So at this stage I m all happy  but I call l and I get the unexpected   res3  List Double    List 1 0  5 5      Where are the elements I added  And how do I add these elements correctly so that when I call l I get a new list with all the stuff I added

User · Answer

I will try to explain the results of all the commands you tried.

scala> val l = 1.0 :: 5.5 :: Nil
l: List[Double] = List(1.0, 5.5)

First of all, List is a type alias to scala.collection.immutable.List (defined in Predef.scala).

Using the List companion object is more straightforward way to instantiate a List. Ex: List(1.0,5.5)

scala> l
res0: List[Double] = List(1.0, 5.5)

scala> l ::: List(2.2, 3.7)
res1: List[Double] = List(1.0, 5.5, 2.2, 3.7)

::: returns a list resulting from the concatenation of the given list prefix and this list

The original List is NOT modified

scala> List(l) :+ 2.2
res2: List[Any] = List(List(1.0, 5.5), 2.2)

List(l) is a List[List[Double]] Definitely not what you want.

:+ returns a new list consisting of all elements of this list followed by elem.

The type is List[Any] because it is the common superclass between List[Double] and Double

scala> l
res3: List[Double] = List(1.0, 5.5)

l is left unmodified because no method on immutable.List modified the List.

User · Answer

You are using an immutable list  The operations on the List return a new List  The old List remains unchanged  This can be very useful if another class   method holds a reference to the original collection and is relying on it remaining unchanged  You can either use different named vals as in  val myList1   1 0    5 5    Nil  val myList2   2 2    3 7    mylist1   or use a var as in  var myList   1 0    5 5    Nil  myList      List 2 2  3 7    This is equivalent syntax for   myList   myList     List 2 2  3 7     Or you could use one of the mutable collections such as  val myList   scala collection mutable MutableList 1 0  5 5  myList     List 2 2  3 7     Not to be confused with the following that does not modify the original mutable List  but returns a new value   myList     List 2 2  3 7     However you should only use mutable collections in performance critical code  Immutable collections are much easier to reason about and use  One big advantage is that immutable List and scala collection immutable Vector are Covariant  Don t worry if that doesn t mean anything to you yet  The advantage of it is you can use it without fully understanding it  Hence the collection you were using by default is actually scala collection immutable List its just imported for you automatically   I tend to use List as my default collection  From 2 12 6 Seq defaults to immutable Seq prior to this it defaulted to immutable

User · Answer

Since you want to append elements to existing list  you can use var List Int  and then keep on adding elements to the same list   Note -  You have to make sure that you insert an element into existing list as follows -  var l  List int    List        creates an empty list  l   3    l                    adds 3 to the head of the list  l   4    l                   makes int 4 as the head of the list     Now when you will print l  you will see two elements in the list   4  3

User · Answer

Use import scala collection mutable MutableList or similar if you really need mutation   import scala collection mutable MutableList val x   MutableList 1  2  3  4  5  x    6    MutableList 1  2  3  4  5  6  x     MutableList 7  8  9     MutableList 1  2  3  4  5  6  7  8  9

[list] Add element to a list In Scala

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