How to Group List in Kotlin: Simple Guide with Examples

In Kotlin, you can group elements of a list using the groupBy function, which organizes items into a map based on a key you define. This function takes a lambda that returns the key for each element, grouping all elements with the same key together.

Syntax

The groupBy function is called on a list and takes a lambda expression that defines the key for grouping. It returns a Map<K, List<T>> where K is the key type and T is the element type.

• list.groupBy { element -> key }: Groups elements by the key returned from the lambda.
• The lambda receives each element and returns the key to group by.
• The result is a map where each key maps to a list of elements sharing that key.

kotlin

val groupedMap = list.groupBy { element -> key }

Example

This example groups a list of words by their first letter. It shows how groupBy collects words starting with the same letter into lists under that letter's key.

kotlin

fun main() {
    val words = listOf("apple", "apricot", "banana", "blueberry", "cherry")
    val grouped = words.groupBy { it.first() }
    println(grouped)
}

Output

{a=[apple, apricot], b=[banana, blueberry], c=[cherry]}

Common Pitfalls

One common mistake is forgetting that groupBy returns a map, not a list. Trying to use list functions directly on the result will cause errors.

Another pitfall is using a lambda that returns non-unique keys unintentionally, which groups elements unexpectedly.

kotlin

fun main() {
    val numbers = listOf(1, 2, 3, 4, 5)
    // Wrong: expecting a list but groupBy returns a map
    // val result = numbers.groupBy { it % 2 == 0 }
    // println(result.size) // This is fine

    // Correct usage:
    val grouped = numbers.groupBy { it % 2 == 0 }
    println(grouped) // Prints map with keys true and false
}

Output

{false=[1, 3, 5], true=[2, 4]}

Quick Reference

• groupBy { keySelector }: Groups list elements by the key returned from keySelector.
• Returns a Map<K, List<T>> where each key maps to a list of elements.
• Use map.keys to get all group keys.
• Use map.values to get all grouped lists.

Key Takeaways

Use Kotlin's groupBy function to group list elements by a key you define.

groupBy returns a map where each key points to a list of grouped elements.

The lambda passed to groupBy decides how elements are grouped by returning the key.

Remember that the result is a map, so use map operations to access groups.

Avoid using non-unique or unintended keys to prevent unexpected grouping.