List methods

Learn list processing with map, filter and sortedWith.

List methods: map, filter, sortedWith

To transform a list, map { } turns each element into a new value. filter { } keeps only those that match a condition. sortedWith(cmp) sorts by a comparator; sortedBy { } is its shortcut.

val nums = listOf(3, 7, 12)
val doubles = nums.map { it * 2 }       // [6, 14, 24]
val big = nums.filter { it > 5 }       // [7, 12]
val sorted = nums.sortedBy { it }      // [3, 7, 12]

These methods do not change the original list; they return a new one. it is the automatic name of the element inside the lambda.

sortedWith and compareBy

sortedWith takes a Comparator. The easiest way is to give a key with compareBy { } and sort by it. To sort in descending order, use reverseOrder().

val people = listOf("Selin", "Ayşe", "Mert")
val byName = people.sortedBy { it }            // [Ayşe, Mert, Selin]
val byLen = people.sortedBy { it.length }      // length ascending
val desc = people.sortedWith(compareByDescending { it })

These methods return a new list; the original is unchanged. To sort by two keys, chain compareBy({ a }, { b }).

Exercise Print the doubles with map

Create a listOf(3, 7, 12). Use map { it * 2 } to compute the double of each number and print the new list with println.

Show example solution

fun main() {
    val nums = listOf(3, 7, 12)
    val doubles = nums.map { it * 2 }
    println(doubles)
}

Test yourself

What does listOf(1, 2, 3).map { it * 2 } return?
- [1, 2, 3]
- 6
- [2, 4, 6]
- [1, 4, 9]
Show answer

[2, 4, 6] — map multiplies each element by 2 and returns a new list.
Fill in the blank: keep matching items
```
nums.____ { it > 10 }
```
Show answer
```
nums.filter { it > 10 }
```
Match method to purpose

map { }

Transform each element
filter { }

Keep matching items
sortedBy { }

Sort by a key
sortedWith(cmp)

Sort with a custom comparator

Next lesson: Summary - Lists

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies