Question

I have two lists in Scala, how to merge them such that the tuples are grouped together?

Is there an existing Scala list API which can do this or need I do it by myself?

Input:

 List((a,4), (b,1), (c,1), (d,1))
 List((a,1), (b,1), (c,1))

Expected output:

List((a,5),(b,2),(c,2),(d,1))

Answer 1

You can try the following one-line:

scala> ( l1 ++ l2 ).groupBy( _._1 ).map( kv => (kv._1, kv._2.map( _._2).sum ) ).toList
res6: List[(Symbol, Int)] = List(('a,5), ('c,2), ('b,2), ('d,1))

Where l1 and l2 are the lists of tuples you want merge.

Now, the breakdown:

• (l1 ++ l2) you just concatenate both lists
• .groupBy( _._1) you group all tuples by their first element. You will receive a Map with
  the first element as key and lists of tuples starting with this element as values.
• .map( kv => (kv._1, kv._2.map( _._2).sum ) ) you make a new map, with similar keys, but the values are the sum of all second elements.
• .toList you convert the result back to a list.

Alternatively, you can use pattern matching to access the tuple elements.

( l1 ++ l2 ).groupBy( _._1 ).map{
  case (key,tuples) => (key, tuples.map( _._2).sum ) 
}.toList