How to write in-place extension-function

Question

fun String.reversed():String{
// return another String..no 
prob
}

fun String.reverse(){
//in-place change this
}

How to write the String.reverse()?

Strings are immutable: https://kotlinlang.org/docs/reference/native/immutability.html — , Aug 21 '19 at 21:14
ok change example to MutableList.myfunc(). The point is it has to be an in-place function like reverse() instead of reversed() — user9507356, Aug 21 '19 at 21:26
fun String.reversed():String{ return this.toMutableList().reversed().joinToString(separator = "") } — user9507356, Aug 21 '19 at 21:34
fun main(args: Array) { var s=mutableListOf('a','b','c') s.myReverse() println(s) } fun MutableList.myReverse(){ with(this){ reverse() } } — user9507356, Aug 22 '19 at 09:49

score 0 · Accepted Answer · answered Aug 22 '19 at 00:45

You can check how the in-place extension function reverse is implemented for MutableList in the Kotlin codebase: https://github.com/JetBrains/kotlin/blob/1.3.50/libraries/stdlib/js/src/generated/_CollectionsJs.kt#L20

fun <T> MutableList<T>.reverse(): Unit {
    val midPoint = (size / 2) - 1
    if (midPoint < 0) return
    var reverseIndex = lastIndex
    for (index in 0..midPoint) {
        val tmp = this[index]
        this[index] = this[reverseIndex]
        this[reverseIndex] = tmp
        reverseIndex--
    }
}

In general, you take this instance and call mutating methods on it to modify its content in place.

However, String class is immutable: it doesn't provide methods to modify its content in place, and, therefore, you cannot write an extension that modifies a string in place either.

How to write in-place extension-function

1 Answers1