How to apply map function to array in Kotlin and change its values?

Question

I am working with this code.

fun main(args : Array<String>){

    val someArray : Array<Int> =  arrayOf(3,53,2,521,51,23,512,34,124);

    println("Original array is ")

    someArray.forEach {print("$it , ")}

    someArray.map({num -> num*2})

    println("Changed array is ")

    println()

    someArray.forEach { print("$it , ") }
}

But the map function does not seem to work. Here is what it prints

Original array is 3 , 53 , 2 , 521 , 51 , 23 , 512 , 34 , 124 , Changed array is

3 , 53 , 2 , 521 , 51 , 23 , 512 , 34 , 124 ,

My question is why is the array not doubling the value of its elements? What i am i missing here?

I don''t want to do that , how do i modify the existing array? — Dishonered, May 22 '18 at 10:27
Change the `val someArray` to `var someArray`. Then `someArray = someArray.map({num -> num*2})` — nayem, May 22 '18 at 10:28
Ok , is there any in which i can modify it without assiging it? — Dishonered, May 22 '18 at 10:36
Yes! Change the `val` to `var` to make it mutable. Then assign the result of the map to the same variable. That's what I meant in the previous comment. — nayem, May 22 '18 at 10:38
I understand what you are saying. My Question is , can i do this with a single function , like `someArray.someFunction({num -> num*2})` which will modify the elements ? — Dishonered, May 22 '18 at 10:40
No! You can't. Because they are value types. Modification to any of them creates new item. — nayem, May 22 '18 at 10:42

score 14 · Accepted Answer · answered May 22 '18 at 11:27

You could always define your own extension that does what you need:

fun <T> Array<T>.mapInPlace(transform: (T) -> T) {
    for (i in this.indices) {
        this[i] = transform(this[i])
    }
}

Usage would look like this:

someArray.mapInPlace { num -> num * 2 }

Note that there are special non-generic classes for primitive arrays, such as IntArray (see also this question), so you should probably be using those instead of the generic one. Of course then you'd have to define this extension for each of them separately, like:

fun IntArray.mapInPlace(transform: (Int) -> Int) { ... }
fun DoubleArray.mapInPlace(transform: (Double) -> Double) { ... }

user2340612 · Answer 2 · 2018-05-22T10:51:44.373

From the documentation, map doesn't modify the items in place but rather returns a List containing the result of the mapping function applied to all the items:

Returns a list containing the results of applying the given transform function to each element in the original array [or collection or map, depending on the receiver].

So, to get the expected result you should do something like:

val doubled = someArray.map { n -> n * 2}
doubled.forEach { print("$it , ") }

EDIT

From what I know there's no built-in function allowing you to map items in-place. However, in your case you're dealing with an Array, which is a mutable data structure, so the alternative is to use an old-style loop to iteratively transform its items (you can apply the same logic with MutableList or other mutable data structures using an Iterator to update values). Something like this:

fun main(args: Array<String>) {
    val someArray: Array<Int> = arrayOf(3, 53, 2, 521, 51, 23, 512, 34, 124);

    someArray.forEach { print("$it , ") }

    println()

    for (i in 0 until someArray.size) {
        someArray[i] = someArray[i] * 2
    }

    someArray.forEach { print("$it , ") }
}

Ok , is there any in which i can modify it without creating a new variable? — Dishonered, May 22 '18 at 10:36

score 6 · Answer 3 · answered Mar 25 '20 at 14:19

6

An alternative is to use forEachIndexed instead of map. Replace

someArray.map({num -> num*2})

by

someArray.forEachIndexed{ index, num -> someArray[index] = num * 2 }

answered Mar 25 '20 at 14:19

David Varela

101
1
4

score 2 · Answer 4 · answered May 23 '18 at 04:43

As many have stated, the map returns a new list and doesn't modify the original.

The most efficient way to rewrite your intended code is to do

// assuming you want to act upon individual elements in the list
fun main(args : Array<String>) {
    val someArray : Array<Int> =  arrayOf(3,53,2,521,51,23,512,34,124);
    println("Original array is $someArray")
    someArray
        .map { it * 2 }
        .forEach { print("$it , ") }
}

or if you just want to see the comma separated list of values

// really just want to print the transformed values instead
fun main(args : Array<String>) {
    val someArray : Array<Int> =  arrayOf(3,53,2,521,51,23,512,34,124);
    println("Original array is $someArray")
    someArray
        .map { it * 2 }
        .also { print(it.joinToString()) }
}

How to apply map function to array in Kotlin and change its values?

4 Answers4

EDIT