map and filter

Learn collection transformation with map and filter.

map and filter

map applies a transformation to every element of a list and returns a new list. Example: numbers.map { it * 2 } doubles each number. The original list is unchanged.

filter keeps the elements that satisfy a condition. Example: numbers.filter { it > 5 } takes only numbers greater than five. Both functions take a lambda; it refers to the current element. They can be chained: transform with map first, then narrow with filter.

filter and conditions

filter takes a lambda and collects the elements for which the lambda returns true. The condition must always return a Boolean. Example: numbers.filter { it % 2 == 0 } selects even numbers.

On an empty list filter returns an empty list without error. filterNotNull takes the non-null elements. When map and filter are used together, transformation happens first then selection; order matters because each step produces a new list.

Exercise Transform with map and filter

Define val numbers = listOf(1, 2, 3, 4, 5). First double each number with map { it * 2 }, then keep those greater than five with filter { it > 5 } and print the result with println.

Show example solution

fun main() {
    val numbers = listOf(1, 2, 3, 4, 5)
    val result = numbers.map { it * 2 }.filter { it > 5 }
    println(result)
}

Test yourself

What does numbers.map { it * 2 } return?
- The first element
- Elements matching a condition
- A single total value
- A new list with each element doubled
Show answer

A new list with each element doubled — map applies a transformation to each element and returns a new list.

Complete the map call

numbers.____ { it * 2 }

Show answer

numbers.map { it * 2 }

Match the map and filter expressions

map

Transforms
filter

Selects
{ it * 2 }

Double
{ it > 5 }

Greater than five

Next lesson: fold and reduce

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