Scala Map from tuple iterable

Question

Constructing scala.collection.Map from other collections, I constantly find myself writing:

val map = Map(foo.map(x=>(x, f(x)))

However, this doesn't really work since Map.apply takes variable arguments only - so I have to write:

val map = Map(foo.map(x=>(x, f(x)) toSeq :_*)

to get what I want, but that seems painful. Is there a prettier way to construct a Map from an Iterable of tuples?

Debilski · Accepted Answer · 2011-06-29T14:54:05.437

34

Use TraversableOnce.toMap which is defined if the elements of a Traversable/Iterable are of type Tuple2. (API)

val map = foo.map(x=>(x, f(x)).toMap

edited Jun 29 '11 at 14:54

answered Jun 29 '11 at 14:38

Debilski

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Or, `foo.mapValues(f).toMap`. – missingfaktor Feb 04 '12 at 09:06
@missingfaktor is mapValues a method to classes other than Map? – juanchito May 27 '16 at 01:18
@mayonesa, no, but I guess I didn't know any better at the time! :) – missingfaktor May 28 '16 at 10:24

score 5 · Answer 2 · answered Jun 30 '11 at 06:13

Alternatively you can use use collection.breakOut as the implicit CanBuildFrom argument to the map call; this will pick a result builder based on the expected type.

scala> val x: Map[Int, String] = (1 to 5).map(x => (x, "-" * x))(collection.breakOut)
x: Map[Int,String] = Map(5 -> -----, 1 -> -, 2 -> --, 3 -> ---, 4 -> ----)

It will perform better than the .toMap version, as it only iterates the collection once.

It's not so obvious, but this also works with a for-comprehension.

scala> val x: Map[Int, String] = (for (i <- (1 to 5)) yield (i, "-" * i))(collection.breakOut)
x: Map[Int,String] = Map(5 -> -----, 1 -> -, 2 -> --, 3 -> ---, 4 -> ----)

Thanks for this - while I'm not going to use it in code that other people have to read without pasting in links to the numerous explanations to the `CanBuildFrom` craziness, it *is* useful for the case where I want to convert an immutable range to a `mutable.Map`, where the answer I accepted before just gives me a type error. — themel, Jul 01 '11 at 07:50

score 2 · Answer 3 · answered Jun 30 '11 at 07:39

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val map = foo zip (foo map f) toMap

answered Jun 30 '11 at 07:39

Landei

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Scala Map from tuple iterable

3 Answers3