CODEWITHSUNDEEP
scala
user9715375
user9715375

Reputation: 61

Difference between ++ and ++= in scala

object Dcoder extends App {
  var c = scala.collection.immutable.Map(12 -> "jd", 13 -> "ff")
  c = c ++ Map(16 -> "hh", 17 -> "℅")
  println(c)

  c ++= Map(18 -> "|||")
  println(c)
}

a = a ++ Map() as well as a ++= Map() performs concatenation. Does a=a++b and a++=b mean the same?

Upvotes: 0

Views: 2136

Answers (2)

Andrey Tyukin
Andrey Tyukin

Reputation: 44918

Sometimes they are the same, sometimes they are not.

If the collection is immutable (e.g. List), then it has no special method called ++=, so the statement

collection ++= someOtherCollection

is syntactic sugar, and it is desugared into

collection = collection ++ someOtherCollection

On the other hand, most mutable collections (e.g. ListBuffer) have a special ++= method, so that

collection ++= someOtherCollection

mutates collection in-place by adding all elements someOtherCollection.

To see that ++= and = ... ++ really behave differently, consider the following two examples:

var x = collection.mutable.ListBuffer(1, 2, 3)
val y = collection.mutable.ListBuffer(4, 5)
val z = x

x ++= y

println(x)
println(z)

This prints

ListBuffer(1, 2, 3, 4, 5)
ListBuffer(1, 2, 3, 4, 5)

But 

var x = collection.mutable.ListBuffer(1, 2, 3)
val y = collection.mutable.ListBuffer(4, 5)
val z = x

x = x ++ y

println(x)
println(z)

prints

ListBuffer(1, 2, 3, 4, 5)
ListBuffer(1, 2, 3)

because ++ forces the creation of a completely new collection, that is not referenced by z.

Upvotes: 4

pramesh
pramesh

Reputation: 1954

Its the same as a = a+2 and a+=2 in case of Integer

For second question, a=a++b and a++=b depends on the type of a and b. Depends if they support ++ and ++= operator or not

Upvotes: 0

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