I have a MutableList
and I want to remove an element from it but I cannot find the appropriate method. There is a method to remove element from ListBuffer
like this:
val x = ListBuffer(1, 2, 3, 4, 5, 6, 7, 8, 9)
x -= 5
I am unable to find an equivalent method on MutableList
.
Scala provides a data structure, the ListBuffer, which is more efficient than List while adding/removing elements in a list. It provides methods to prepend, append elements to a list.
Because a List is immutable, if you need to create a list that is constantly changing, the preferred approach is to use a ListBuffer while the list is being modified, then convert it to a List when a List is needed. The ListBuffer Scaladoc states that a ListBuffer is “a Buffer implementation backed by a list.
MutableList
lacks -=
and --=
because it does not extend the Shrinkable
trait. Various motivations for this can be found here.
MutableList
does have diff
, filter
, and other methods which can help you in case you are in a situation where reassigning a variable (or instantiating a new variable) might be an option, and performance concerns aren't paramount:
var mylist = MutableList(1, 2, 3)
mylist = mylist diff Seq(1)
val myNewList = mylist.filter(_ != 2)
val indexFiltered = mylist.zipWithIndex.collect { case (el, ind) if ind != 1 => el }
You can often use ListBuffer
instead of MutableList
, which will unlock the desired -=
and --=
methods:
val mylist = ListBuffer(1, 2, 3)
mylist -= 1 //mylist is now ListBuffer(2, 3)
mylist --= Seq(2, 3) //mylist is now empty
It's not the answer, just to warn you about problems (at least in 2.11.x):
//street magic
scala> val a = mutable.MutableList(1,2,3)
a: scala.collection.mutable.MutableList[Int] = MutableList(1, 2, 3)
scala> a += 4
res7: a.type = MutableList(1, 2, 3, 4)
scala> a
res8: scala.collection.mutable.MutableList[Int] = MutableList(1, 2, 3, 4)
scala> a ++= List(8,9,10)
res9: a.type = MutableList(1, 2, 3, 4, 8, 9, 10)
scala> val b = a.tail
b: scala.collection.mutable.MutableList[Int] = MutableList(2, 3, 4, 8, 9, 10)
scala> b.length
res10: Int = 6
scala> a.length
res11: Int = 7
scala> a ++= List(8,9,10)
res12: a.type = MutableList(1, 2, 3, 4, 8, 9, 10, 8, 9, 10)
scala> b += 7
res13: b.type = MutableList(2, 3, 4, 8, 9, 10, 7)
scala> a
res14: scala.collection.mutable.MutableList[Int] = MutableList(1, 2, 3, 4, 8, 9, 10, 7)
scala> b
res15: scala.collection.mutable.MutableList[Int] = MutableList(2, 3, 4, 8, 9, 10, 7)
scala> a ++= List(8,9,10)
res16: a.type = MutableList(1, 2, 3, 4, 8, 9, 10, 7)
This example is taken from some gist - I've posted it on facebook with #devid_blein #street_magic tags, but can't find original link on the internet.
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