In Kotlin, you can group elements with groupBy, then count how many items are in each group.

Basic example

val words = listOf("apple", "banana", "apricot", "blueberry", "avocado")

val countsByFirstLetter = words
    .groupBy { it.first() }
    .mapValues { (_, words) -> words.count() }

println(countsByFirstLetter)

Output:

{a=3, b=2}

Here:

• groupBy { it.first() } groups words by their first letter.
• mapValues { it.value.count() } converts each grouped list into its size/count.

You can also write it as:

val countsByFirstLetter = words
    .groupBy { it.first() }
    .mapValues { it.value.size }

Counting objects by a property

data class Person(val name: String, val city: String)

val people = listOf(
    Person("Alice", "London"),
    Person("Bob", "Paris"),
    Person("Charlie", "London"),
    Person("Diana", "Paris"),
    Person("Eve", "Berlin")
)

val peopleByCity = people
    .groupBy { it.city }
    .mapValues { it.value.count() }

println(peopleByCity)

Output:

{London=2, Paris=2, Berlin=1}

More efficient option: groupingBy + eachCount

If you only need counts, prefer:

val countsByFirstLetter = words
    .groupingBy { it.first() }
    .eachCount()

This avoids creating intermediate lists for every group.

println(countsByFirstLetter)
// {a=3, b=2}

So the common choices are:

items.groupBy { key }.mapValues { it.value.count() }

or, more efficiently:

items.groupingBy { key }.eachCount()