How to reverse a Map in Kotlin?

Question

I am trying to reverse a Map in Kotlin. So far, I have come up with:

mapOf("foo" to 42)   .toList()   .map { (k, v) -> v to k }   .toMap() 

Is there any better way of doing this without using a middleman(middlelist)?

Answer 1

Since the Map consists of Entrys and it is not Iterable you can use Map#entries instead. It will be mapped to Map#entrySet to create a backed view of Set<Entry>, for example:

val reversed = map.entries.associateBy({ it.value }) { it.key } 

OR use Iterable#associate, which will create additional Pairs.

val reversed = map.entries.associate{(k,v)-> v to k} 

OR using Map#forEach:

val reversed = mutableMapOf<Int, String>().also {     //     v-- use `forEach` here          map.forEach { (k, v) -> it.put(v, k) }  }.toMap() // ^--- you can add `toMap()` to create an immutable Map.