How to reverse a Map in Kotlin?

user_2229438 0
Viewed 13646

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)?

bimap kotlin dictionary
6 Answers

If you need to reverse a multimap like m: Map<K, List<V>> to a Map<V, List<K>> you can do

m
  .flatMap { it.value.map { oneValue -> oneValue to it.key } }
  .groupBy({ it.first }, { it.second })
  .toMap()

In sequence,

  • mapOf('a' to listOf('b', 'c'), 'd' to listOf('b')) gets flat mapped to a sequence like
  • listOf('b' to 'a', 'c' to 'a', 'b' to 'd') which gets grouped to
  • listOf('b' to listOf('a', 'd'), 'c' to listOf('a')) which then gets converted to a map.

This probably creates intermediate objects.

user_203940

In case your map is not a 1-1 mapping and you want the inversion to be a list of values:

mapOf(1 to "AAA", 2 to "BBB", 3 to "BBB").toList()
        .groupBy { pair -> pair.second } // Pair<Int, String>
        .mapValues { entry -> 
          entry.value.map { it.first } // Entry<String, List<Pair<Int, String>>
        }
user_13492630

I'm still learning the ins and outs of Kotlin, but I had the same requirement and as of Kotlin 1.2 it appears that you can iterate over a Map and so map() it directly like this:

@Test
fun testThatReverseIsInverseOfMap() {
    val intMap = mapOf(1 to "one", 2 to "two", 3 to "three")
    val revMap = intMap.map{(k,v) -> v to k}.toMap()
    assertTrue(intMap.keys.toTypedArray() contentEquals revMap.values.toTypedArray())
    assertTrue(intMap.values.toTypedArray() contentEquals revMap.keys.toTypedArray())
}
user_19784960

This is my take on a 1:1 map

    private fun <K, V> Map<K, V>.reverseOneToOneMap(): Map<V, K> {
        val result = this.entries.associateBy({ it.value }) { it.key }
        if (result.size != this.size) {
            throw RuntimeException("Map must be 1:1")
        }
        return result
    }
user_27153840
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