I'm really new to kotlin and here I'm trying to reverse a list by defining a function without any returns. My logic is simply swapping indexes up to the middle.

However, I am getting an error message which I have attached below. I would appreciate it if anyone could help me understand the mistake. I have attached my code and error message below.

Reverse function

fun reverse (list:  List<Int>){

    var j = list.size-1
    for (i in 0..(list.size-1)/2){
        var t = list[i]
        list[i] = list[j]
        list[j] = t
        j--
    }

}

Main function

fun main() {
     
    var list =  listOf(1,2,3,4,5,6,7,8,9,10)
    reverse(list)
    println(list)
}

Error message

Kotlin: Unresolved reference. None of the following candidates is applicable because of receiver type mismatch: (This is for the swapping line list[i] = list[j])

CodePudding user response：

You need a MutableList, you cannot change elements in a List.

fun reverse(list: MutableList<Int>) {
  var j = list.size - 1
  for (i in 0..(list.size - 1) / 2) {
    val t = list[i]
    list[i] = list[j]
    list[j] = t
    j--
  }
}

var list = mutableListOf(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

reverse(list)

println(list)   // Output [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

Also: there is a built-in function reversed(), see: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/reversed.html, which will return a List. And there is a built-in function asReversed(), see: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/as-reversed.html, which will return a MutableList.