Rotate a List in Kotlin

This article explores different ways to rotate a list in Kotlin.

1. Using `Collections.rotate()` function

To rotate a list by some position, use the built-in static function Collections.rotate(). The following code example shows invocation for this function, where the code left-rotated a list by 2 positions.

import java.util.*

fun main() {

val values = (1.. 5).toMutableList()

val leftShift = 2

Collections.rotate(values, leftShift)

println(values) // [4, 5, 1, 2, 3]

}

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To right-shift the list, pass the negative position to the Collections.rotate() function. For example, passing -2 will right-rotate a list by 2 positions, as shown below:

import java.util.*

fun main() {

val values = (1.. 5).toMutableList()

val rightShift = 2

Collections.rotate(values, -rightShift)

println(values) // [3, 4, 5, 1, 2]

}

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2. Using `drop()` + `take()` function

Alternatively, you can write your own extension function on MutableList<T>. To left-rotate a list by specified positions, you can use drop() with take() library function. And to right-rotate a list by specified positions, use takeLast() with dropLast() library function.

fun <T> MutableList<T>.rotateLeft(n: Int) = drop(n) + take(n)

fun <T> MutableList<T>.rotateRight(n: Int) = takeLast(n) + dropLast(n)

fun main() {

val values = (1.. 5).toMutableList()

val shift = 2

println(values.rotateLeft(shift)) // [3, 4, 5, 1, 2]

println(values.rotateRight(shift)) // [4, 5, 1, 2, 3]

}

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3. Using Loop

You can write your custom logic for rotating a list. For example, the following code left-shifts elements in the list by specific positions. The code can be easily modified to right-shift a list.

fun <T> rotateLeft(values: MutableList<T>, shift: Int): List<T> {

if (values.isEmpty()) {

return values

}

for (i in 0 until shift) {

values.add(0, values.removeAt(values.lastIndex))

}

return values

}

fun main() {

val values = (1.. 9).toMutableList()

val shift = 3

val rotatedList = rotateLeft(values, shift)

println(rotatedList) // [7, 8, 9, 1, 2, 3, 4, 5, 6]

}

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That’s all about rotating a list in Kotlin.

1. Using Collections.rotate() function

2. Using drop() + take() function

3. Using Loop

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1. Using `Collections.rotate()` function

2. Using `drop()` + `take()` function