Scala Cookbook

10.22. Merging Sequential Collections

Problem

You want to join two sequences into one sequence, either keeping all of the original elements, finding the elements that are common to both collections, or finding the difference between the two sequences.

Solution

There are a variety of solutions to this problem, depending on your needs:

• Use the ++= method to merge a sequence into a mutable sequence.

• Use the ++ method to merge two mutable or immutable sequences.

• Use collection methods like union, diff, and intersect.

Use the ++= method to merge a sequence (any TraversableOnce) into a mutable collection like an ArrayBuffer:

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

scala> a ++= Seq(4,5,6)
res0: a.type = ArrayBuffer(1, 2, 3, 4, 5, 6)

Use the ++ method to merge two mutable or immutable collections while assigning the result to a new variable:

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

scala> val b = Array(4,5,6)
b: Array[Int] = Array(4, 5, 6)

scala> val c = a ++ b
c: Array[Int] = Array(1, 2, 3, 4, 5, 6)

You can also use methods like union and intersect to combine sequences to create a resulting sequence:

scala> val a = Array(1,2,3,4,5)
a: Array[Int] = Array(1, 2, 3, 4, 5)

scala> val b = Array(4,5,6,7,8)
b: Array[Int] = Array(4, 5, 6, 7, 8)

// elements that are in both collections
scala> val c = a.intersect(b)
c: Array[Int] = Array(4, 5)

// all elements from both collections
scala> val c = a.union(b) c: Array[Int] ...