How to build a multimap from a list of tuples in Scala?

Question

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 ?

score 22 · Accepted Answer · answered Aug 26 '11 at 20:23

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)}

Derek Wyatt · Answer 2 · 2011-08-26T20:15:42.320

19

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))

edited Aug 26 '11 at 20:15

answered Aug 26 '11 at 20:07

Derek Wyatt

2,697
15
23

6

You can also use `mapValues`, which won't actually produce a new collection, but will act as a mapped `view` on values. – Daniel C. Sobral Aug 26 '11 at 20:12
1

`mapValues` - nice. I've never used that one before. Updated to use that instead. – Derek Wyatt Aug 26 '11 at 20:18
7

cleaner still: `val m = l groupBy (_._1) mapValues (_ map {_._2} toSet)` – Kevin Wright Aug 26 '11 at 23:14
5

That's not cleaner, it's just less characters. – myyk Sep 27 '12 at 01:24
1

mapValues means an overhead every time a lookup is performed. A workaround is to do .map(identity). Credit tpolecat and referer on #scala – malaverdiere Mar 21 '14 at 02:31
2

Use map directly: `list.groupBy(_._1).map{case(x, xs) => (x, xs.map(_._2))}` – r0estir0bbe Jul 07 '15 at 20:34

How to build a multimap from a list of tuples in Scala?

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