GroupBy is used to partition specific collections into a map of other Traversable collections according to a specific partitioning function. It can be defined formally as follows:

def groupBy[K](f: ((A, B)) ⇒ K): Map[K, Map[A, B]]  

Let's see an example as follows:

scala> // Given a list of numbers
scala> // Group them as positive and negative numbers.
scala> List(1,-2,3,-4) groupBy (x => if (x >= 0) "positive" else "negative")
res6: scala.collection.immutable.Map[String,List[Int]] = Map(negative -> List(-2, -4), positive -> List(1, 3))

In Scala, if you want to select all the elements in a Traversable collection but the last one, you can use init. In the next subsection, we will see examples of it.