Convert a List of Pairs to a Map in Kotlin

This article explores different ways to convert a List<Pair<K,V>> to a Map<K,V> in Kotlin.

1. Using `associate()` function

The associate() function returns a map containing key-value pairs provided by the transform function applied to elements of the given map. It can be used as follows to convert a List<Pair<K,V>> to a Map<K,V> in Kotlin:

class Point(var x: Int, var y: Int)

fun main() {

val points = listOf(Point(0, 1), Point(1, 2), Point(1, 1), Point(2, 3))

val map = points.associate { Pair(it.x, it.y) }

println(map) // {0=1, 1=1, 2=3}

}

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Alternatively, we can use the following shorthand form:

class Point(var x: Int, var y: Int)

fun main() {

val points = listOf(Point(0, 1), Point(1, 2), Point(1, 1), Point(2, 3))

val map = points.associate { it.x to it.y }

println(map) // {0=1, 1=1, 2=3}

}

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2. Using `associateBy()` function

The associateBy() function returns a map containing the values provided by valueTransform and indexed by keySelector functions applied to elements of the given map. It can be used as follows to convert a List<Pair<K,V>> to a Map<K,V> in Kotlin:

class Point(var x: Int, var y: Int)

fun main() {

val points = listOf(Point(0, 1), Point(1, 2), Point(1, 1), Point(2, 3))

val map = points.associateBy({ it.x }, { it.y })

println(map) // {0=1, 1=1, 2=3}

}

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The above code is short for the following:

class Point(var x: Int, var y: Int)

fun main() {

val points = listOf(Point(0, 1), Point(1, 2), Point(1, 1), Point(2, 3))

val map = points.associateBy(keySelector = { point -> point.x },

valueTransform = { point -> point.y })

println(map) // {0=1, 1=1, 2=3}

}

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3. Using `map()` function

Another option is to use the map() library function to extract keys and values from the List of Pairs and uses the toMap() function to build a map:

class Point(var x: Int, var y: Int)

fun main() {

val points = listOf(Point(0, 1), Point(1, 2), Point(1, 1), Point(2, 3))

val map = points.map { it.x to it.y }.toMap()

println(map) // {0=1, 1=1, 2=3}

}

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4. Using Loop

Finally, we can write custom logic for converting a list of pairs into a map. This can be implemented as follows in Kotlin. Note that the code throws IllegalStateException on encountering a duplicate key. The code can be easily modified to handle duplicate keys.

class Point(var x: Int, var y: Int)

fun main() {

val points = listOf(Point(0, 1), Point(1, 2), Point(2, 3), Point(3, 4))

val map = mutableMapOf<Int, Int>()

for (point in points) {

check(map.put(point.x, point.y) == null) { "Duplicate key" }

}

println(map) // {0=1, 1=2, 2=3, 3=4}

}

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That’s all about converting a List<Pair<K,V>> to a Map<K,V> in Kotlin.

Convert a List of Pairs to a Map in Kotlin

1. Using associate() function

2. Using associateBy() function

3. Using map() function

4. Using Loop

1. Using `associate()` function

2. Using `associateBy()` function

3. Using `map()` function