kotlin - How to reverse a map and group values in a list?

Question

I have the following map

val mapToTransform = mapOf(1 to 4, 2 to 5, 4 to 4, 6 to 3, 9 to 3)    

I want to transform it to the map

val transformedMap = mapOf(3 to listOf(6,9), 4 to listOf(1,4), 5 to listOf(2))

My attempt is the following:

val attempt = mapToTransform.map{ it.value to it.key}.groupBy{it.first}

that gives {4=[(4, 1), (4, 4)], 5=[(5, 2)], 3=[(3, 6), (3, 9)]}.

Any help?

question from:https://stackoverflow.com/questions/66050404/how-to-reverse-a-map-and-group-values-in-a-list

Answer 1

There is an overload of groupBy that takes a valueTransform in addition to the keySelector:

mapToTransform.map { it.value to it.key }.groupBy({ it.first }, { it.second })

But then you can simplify your first part, as you don't need to swap things up front:

mapToTransform.entries.groupBy({ it.value }, { it.key })