Kotlin prepend element

Question

I am searching for Kotlin alternative to:
(cons 1 '(2 3)) in lisp or
1 : [2, 3] in haskell or
1 :: List(2, 3) in scala,
(which all result in sth like [1, 2, 3])
so I can prepend an element to a List<T> (or any other list you can offer).

It will also be fine if one could provide O(1) head and tail Kotlin alternatives (I've found just first())

Answer 1

I think the easiest would be to write:

var list = listOf(2,3)
println(list) // [2, 3]
list = listOf(1) + list
println(list) // [1, 2, 3]

There is no specific tail implementation, but you can call .drop(1) to get the same. You can make this head\tail more generic by writing these extension properties:

val <T> List<T>.tail: List<T>
  get() = drop(1)

val <T> List<T>.head: T
  get() = first()

Then:

val list = listOf(1, 2, 3)
val head = list.head
val tail = list.tail

Some more info: Kotlin List tail function

Answer 2

Any class which implements Deque will suitable for you, for example LinkedList:

val linkedList = LinkedList(listOf(2, 3))
linkedList.push(1)
println(linkedList) // [1, 2, 3]

Creating lists throught constructor LinkedList(listOf(2, 3)) in many places can be annoying, so feel free to write factory method:

fun <T> linkedListOf(vararg elements: T): LinkedList<T> {
    return LinkedList<T>(elements.toList())
}

// Usage:
val list = linkedListOf(2, 3)
list.push(1)
println(list) // [1, 2, 3]

Answer 3

Simple, just wrap the element to prepend in a List and then use the + operator (or List.plus()) to concatenate the two Lists:

val list1 = listOf(2, 3)        // [2, 3]
val list2 = listOf(1) + list1   // [1, 2, 3]

For your second question, in Kotlin 1.2 there are:

List.first()
List.last()

Both are O(1)

Answer 4

This could be done easily with extension functions as below

Prepending element

fun <T> MutableList<T>.prepend(element: T) {
    add(0, element)
}

Prepending list

fun <T> MutableList<T>.prependAll(elements: List<T>) {
    addAll(0, elements)
}

Answer 5

Inserts an element into the list at the specified index.

abstract fun add(index: Int, element: E)

Thus answer is

list.add(0,element)

Answer 6

If you do that often in your code for some reason, consider adding an extension operator method such as:

operator fun <T> T.plus(tail: List<T>): List<T> {
    val list = ArrayList<T>(1 + tail.size)

    list.add(this)
    list.addAll(tail)

    return list
}

Then your code could work Scala-like: 1 + listOf(2, 3)

Another way to achieve the same behaviour, shorter but sacrifices some memory:

operator fun <T> T.plus(tail: List<T>): List<T> {
    return mutableListOf(this).apply {
        addAll(tail)
    }
}

Answer 7

To be as close to Lisp as possible consider using immutable linked list.

You can use pcollections

val list = ConsPStack.from(listOf(2, 3))
val newList = list + 1
println(list)  // [2, 3]
println(newList) // [1, 2, 3]

Head:

list.first() // 1
list[0] // 1

(unfortunately this thing needs one allocation)

Tail:

list - 0 // [2, 3]
list.subList(1) // [2, 3]  

Looks rather ugly.

Hopefully we'll get better API when kotlinx.collections.immutable will be ready. It's an effort to create standard Kotlin immutable collections (not just read-only ones that we currently have). As of now this project is still at very early stage (I was unable to find structure that supports efficient prepend/head/tail there)

Answer 8

I'm not entirely sure what you want to do, so please try one of the following.

Mutating list:

val list = mutableListOf(3, 2)
list.add(1)

Copping an immutable list:

var list = listOf(3, 2)
list = list + 1