CODEWITHSUNDEEP
arrayslistscala
Vadim Samokhin
Vadim Samokhin

Reputation: 3446

scala: are lists really immutable?

In "Programming in scala" it is stated that lists are immutable and arrays are mutable. But I can't change the array length -- neither can I with list, so in this way they are both immutable. I can change array's element value -- just setting a new one, and I can modify a list's element value with, say, map method. So in this way they are both mutable.

So why arrays are considered to be mutable and lists not?

Upvotes: 1

Views: 975

Answers (3)

nonVirtualThunk
nonVirtualThunk

Reputation: 2852

First, it's important to know that Scala actually has both mutable and immutable collections. If you use scala.collection.mutable.ListBuffer, it will (as the name indicates) be mutable. If you use scala.collection.immutable.List it will not be.

Second, the map function does not alter the elements of the list, rather, it creates an entirely new list containing the result of the map function applied to each element of the start list, for example:

var l1 = List(1,3,5)
var l2 = l1.map( _ + 2 )
println(l1) // List(1, 3, 5)
println(l2) // List(2, 4, 6)

l2 now contains a new list, entirely separate from l1, and l1 has not been changed.

Upvotes: 10

kiritsuku
kiritsuku

Reputation: 53348

You can't transform the elements of a List - if you use a function which transforms the elements a new List is created:

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

scala> xs.map(_+1)
res0: List[Int] = List(2, 3, 4)

scala> xs
res1: List[Int] = List(1, 2, 3)

In contrast, the elements of an Array are mutable:

scala> val xs = Array(1,2,3)
xs: Array[Int] = Array(1, 2, 3)

scala> xs(0) = 5

scala> xs
res3: Array[Int] = Array(5, 2, 3)

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

scala> xs(0) = 5
<console>:9: error: value update is not a member of List[Int]
              xs(0) = 5
              ^

Nevertheless you can't change the size of an array. If you want to do this you have to use scala.collection.mutable.Buffer:

scala> val xs = Array(1,2,3)
xs: Array[Int] = Array(1, 2, 3)

scala> xs += 4
<console>:9: error: reassignment to val
              xs += 4
                 ^

scala> val xs = collection.mutable.Buffer(1,2,3)
xs: scala.collection.mutable.Buffer[Int] = ArrayBuffer(1, 2, 3)

scala> xs += 4
res6: xs.type = ArrayBuffer(1, 2, 3, 4)

Upvotes: 3

slayer_b
slayer_b

Reputation: 551

List's map method will create a new list.

If you want to use a mutable collections see * scala.collection.mutable package.

Upvotes: 1

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