Merge maps by key

Question

Say I have two maps:

val a = Map(1 -> \"one\", 2 -> \"two\", 3 -> \"three\")
val b = Map(1 -> \"un\", 2 -> \"deux\", 3 -> \"trois\")
         


        

Answer 1

Here is my first approach before looking for the other solutions:

for (x <- a) yield 
  x._1 -> Seq (a.get (x._1), b.get (x._1)).flatten

To avoid elements which happen to exist only in a or b, a filter is handy:

(for (x <- a) yield 
  x._1 -> Seq (a.get (x._1), b.get (x._1)).flatten).filter (_._2.size == 2)

Flatten is needed, because b.get (x._1) returns an Option. To make flatten work, the first element has to be an option too, so we can't just use x._2 here.

For sequences, it works too:

scala> val b = Map (1 -> Seq(1, 11, 111), 2 -> Seq(2, 22), 3 -> Seq(33, 333), 5 -> Seq(55, 5, 5555))
b: scala.collection.immutable.Map[Int,Seq[Int]] = Map(1 -> List(1, 11, 111), 2 -> List(2, 22), 3 -> List(33, 333), 5 -> List(55, 5, 5555))

scala> val a = Map (1 -> Seq(1, 101), 2 -> Seq(2, 212, 222), 3 -> Seq (3, 3443), 4 -> (44, 4, 41214))
a: scala.collection.immutable.Map[Int,ScalaObject with Equals] = Map(1 -> List(1, 101), 2 -> List(2, 212, 222), 3 -> List(3, 3443), 4 -> (44,4,41214))

scala> (for (x <- a) yield x._1 -> Seq (a.get (x._1), b.get (x._1)).flatten).filter (_._2.size == 2) 
res85: scala.collection.immutable.Map[Int,Seq[ScalaObject with Equals]] = Map(1 -> List(List(1, 101), List(1, 11, 111)), 2 -> List(List(2, 212, 222), List(2, 22)), 3 -> List(List(3, 3443), List(33, 333)))