Flatten a List of Lists in Kotlin

This article explores different ways to flatten a List of Lists in Kotlin.

1. Using `flatten()` function

The standard solution to flatten a list in Kotlin is using the flatten() function. It returns a single list of all elements from all lists in the given list. The following code example shows invocation for this function:

fun main() {

val listOfLists = (1.. 3).map { i -> (1..4).map { i } }

println("Original List: $listOfLists")

val flattenedList = listOfLists.flatten()

println("Flattened List: $flattenedList")

}

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Output:

Original List: [[1, 1, 1, 1], [2, 2, 2, 2], [3, 3, 3, 3]]
Flattened List: [1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3]

2. Using `flatMap()` function

Another option is to use the flatMap() function that returns a single list of all elements yielded from the results of the transform function being invoked on each element of the original collection. The following code demonstrates its usage:

fun main() {

val listOfLists = (1.. 3).map { i -> (1..4).map { i } }

println("Original List: $listOfLists")

val flattenedList = listOfLists.flatMap { it }

println("Flattened List: $flattenedList")

}

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Output:

Original List: [[1, 1, 1, 1], [2, 2, 2, 2], [3, 3, 3, 3]]
Flattened List: [1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3]

3. Using `forEach()` function

Another alternative is to use a foreach-construct or the forEach() function to add elements of each list into a new list. This logic would translate to the following code:

fun <T> flatten(listOfLists: List<List<T>>): List<T> {

val result: MutableList<T> = mutableListOf()

listOfLists.forEach { result.addAll(it) }

return result

}

fun main() {

val listOfLists = (1.. 3).map { i -> (1..4).map { i } }

println("Original List: $listOfLists")

val flattenedList = flatten(listOfLists)

println("Flattened List: $flattenedList")

}

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Output:

Original List: [[1, 1, 1, 1], [2, 2, 2, 2], [3, 3, 3, 3]]
Flattened List: [1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3]

4. Using `reduce()` function

Finally, we can perform a reduction operation on the elements of the list using an associative accumulation function. This is demonstrated below:

fun main() {

val listOfLists = (1.. 3).map { i -> (1..4).map { i } }

println("Original List: $listOfLists")

val flattenedList = listOfLists.reduce { x, y -> x + y }

println("Flattened List: $flattenedList")

}

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Output:

Original List: [[1, 1, 1, 1], [2, 2, 2, 2], [3, 3, 3, 3]]
Flattened List: [1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3]

That’s all about flattening a List of Lists in Kotlin.

Flatten a List of Lists in Kotlin

1. Using flatten() function

2. Using flatMap() function

3. Using forEach() function

4. Using reduce() function

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1. Using `flatten()` function

2. Using `flatMap()` function

3. Using `forEach()` function

4. Using `reduce()` function