Suppose I have a list of tuples List[(A, B)]
. What is the best way to convert it to a multimap
, which maps A
to Set[B]
? Can I build an immutable multimap
?
Can I build an immutable multimap ?
Not with the MultiMap
in Scala standard library. Of course, you can write your own.
What is the best way to convert it to a multimap?
import scala.collection.mutable.{HashMap, Set, MultiMap}
def list2multimap[A, B](list: List[(A, B)]) =
list.foldLeft(new HashMap[A, Set[B]] with MultiMap[A, B]){(acc, pair) => acc.addBinding(pair._1, pair._2)}
I'm a bit confused, Multimap
doesn't map A
to Set[B]
, it maps A
to B
where B
can have many values. Since you want something immutable, I'm going to change this to Map[A, Set[B]]
which isn't a Multimap
but does one of the things you said you wanted.
// This is your list of (A, B)
val l = List((1, "hi"),
(2, "there"),
(1, "what's"),
(3, "up?"))
// Group it and snip out the duplicate 'A'
// i.e. it initially is Map[A, List[(A, B)]] and we're going to convert it
// to Map[A, Set[B]]
val m = l.groupBy(e => e._1).mapValues(e => e.map(x => x._2).toSet)
println(m)
// Prints: Map(3 -> Set(up?), 1 -> Set(hi, what's), 2 -> Set(there))
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