Avoid having to convert to an intermediate data-structure before calling .toMap in Scala

Question

Let's consider the following code:

Set(1, 2, 3, 4, 5)
  .map(k => (k, if (k % 2 == 0) "even" else "odd"))
  .toMap

Is there a way to simplify this in Scala, avoiding the creation of the intermediate set? I'll be doing something performance-sensitive on big collections and it wouldn't hurt to get the Map created on-the-spot instead.

If nothing better comes along, I was thinking of ending up implementing something like this:

Map.from(S, k => if (k % 2 == 0) "even" else "odd"))

I'm still using Scala 2.12.

Thanks

Can use `scala.collection.breakOut` – cchantep Aug 14 '19 at 08:13 — cchantep, Aug 14 '19 at 08:13

score 3 · Accepted Answer · edited Jun 20 '20 at 09:12

3

You can use foldleft :

(1 to 5).foldLeft(Map.empty[Int, String])(
  (r, k) => r + (k -> (if (k % 2 == 0) "even" else "odd"))
)

Edit

Or, to avoid intermediate collection in toMap approach, you can use view :

(1 to 5).view
  .map(k => (k, if (k % 2 == 0) "even" else "odd"))
  .toMap

Scala: How to create a Map[K,V] from a Set[K] and a function from K to V?

edited Jun 20 '20 at 09:12

Community

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answered Aug 14 '19 at 06:39

Rafaël

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1

I agree so much, I really prefer the toMap approach ! It's just an alternative, not sure if the best... – Rafaël Aug 14 '19 at 06:42
I've add the view option to my answer. It may answer your question on the 'intermediate set' issue. – Rafaël Aug 14 '19 at 06:52
Edited without type ascription – Rafaël Aug 14 '19 at 07:19
1

Since performance is a requirement in this case, and the `view` is not reused. It would be better to use `iterator` instead _(also that way the code is correct in 2.11 2.12 & 2.13)_. – Luis Miguel Mejía Suárez Aug 14 '19 at 11:56

Avoid having to convert to an intermediate data-structure before calling .toMap in Scala

1 Answers1

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