Merge two LinkedHashMap in Scala - scala

Question

Having this code
def mergeWith[K, X, Y, Z](xs: mutable.LinkedHashMap[K, X], ys: mutable.LinkedHashMap[K, Y])(f: (X, Y) => Z): mutable.LinkedHashMap[K, Z] =
xs.flatMap {
case (k, x) => ys.get(k).map(k -> f(x, _))
}
it gives me this:
val map1 = LinkedHashMap(4 -> (4), 7 -> (4,7))
val map2 = LinkedHashMap(3 -> (3), 6 -> (3,6), 7 -> (3,7))
val merged = mergeWith(map1,map2){ (x, y) => (x, y) }
merged: scala.collection.mutable.LinkedHashMap[Int,(Any, Any)] = Map(7 -> ((4,7),(3,7)))
But what i want is this:
merged: scala.collection.mutable.LinkedHashMap[Int,(Any, Any)] = Map(3 -> (3), 4 -> (4), 6 -> (3,6), 7 -> ((4,7),(3,7)))
How to modify my code to obtain it?

Answer 1

It can't be done with the current mergeWith() signature. In particular, you're trying to create a LinkedHashMap[K,Z] but there is no Z input. The only way to get a Z is to invoke f() which requires both X and Y as passed parameters.
So if xs is type LinkedHashMap[Int,Char] and has element (2 -> 'w'), and ys is type LinkedHashMap[Int,Long] and has element (8 -> 4L), how are you going to invoke f(c:Char, l:Long) so that you have a [K,Z] entry for both keys 2 and 8? Not possible.
If the mergeWith() signature can be simplified you might do something like this.
def mergeWith[K,V](xs: collection.mutable.LinkedHashMap[K, V]
,ys: collection.mutable.LinkedHashMap[K, V]
)(f: (V, V) => V): collection.mutable.LinkedHashMap[K,V] = {
val ns = collection.mutable.LinkedHashMap[K,V]()
(xs.keySet ++ ys.keySet).foreach{ k =>
if (!xs.isDefinedAt(k)) ns.update(k, ys(k))
else if (!ys.isDefinedAt(k)) ns.update(k, xs(k))
else ns.update(k, f(xs(k), ys(k)))
}
ns
}
This produces the desired result for the example you've given, but it has a number of undesirable qualities, not the least of which is the mutable data structures.
BTW, there is no such thing as a Tuple1 so (4) is the same thing as 4. And whenever you see type Any, it's a pretty good sign that your design needs a re-think.