Reverse a map in Kotlin

This article explores different ways to reverse a map in Kotlin.

1. Using `associate()` function

A simple solution is to use the associate() function, which returns a map containing key-value pairs provided by the transform function applied to the given sequence elements.

fun main() {

val mutableMap: Map<Int, String> = mutableMapOf(1 to "one", 2 to "two", 3 to "three")

val reverseMap: Map<String, Int> = mutableMap.entries.associate { (key, value) -> value to key }

print(reverseMap) // {one=1, two=2, three=3}

}

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2. Using `associateBy()` function

Alternatively, you can use the associateBy() function that returns a map containing the elements from the given sequence indexed by the key returned from the keySelector function applied to each element.

fun main() {

val mutableMap: Map<Int, String> = mutableMapOf(1 to "one", 2 to "two", 3 to "three")

val reverseMap: Map<String, Int> = mutableMap.entries.associateBy({ it.value }) { it.key }

print(reverseMap) // {one=1, two=2, three=3}

}

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3. Using `map()` function

Another simple approach is to use the map() function to invert the mapping of a map. Here’s an example of its usage:

fun main() {

val mutableMap: Map<Int, String> = mutableMapOf(1 to "one", 2 to "two", 3 to "three")

val reverseMap: Map<String, Int> = mutableMap.map { (key, value) -> value to key }.toMap()

print(reverseMap) // {one=1, two=2, three=3}

}

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4. Using `also()` function

Finally, you can write your own code to reverse a map. This can be easily done with combination of also() and forEach() function.

fun main() {

val mutableMap: Map<Int, String> = mutableMapOf(1 to "one", 2 to "two", 3 to "three")

val reverseMap: Map<String, Int> = mutableMapOf<String, Int>().also {

mutableMap.forEach { (k, v) -> it[v] = k }

}

print(reverseMap) // {one=1, two=2, three=3}

}

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That’s all about reversing a map in Kotlin.

1. Using associate() function

2. Using associateBy() function

3. Using map() function

4. Using also() function

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1. Using `associate()` function

2. Using `associateBy()` function

3. Using `map()` function

4. Using `also()` function