Kotlin - how to find number of repeated values in a list?

Question

I have a list, e.g like:

val list = listOf("orange", "apple", "apple", "banana", "water", "bread", "banana")

How can i check how many times apple is duplicated in this list?

Solution 1

list.count { it == "apple" }

Documentation: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-list/, https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/count.html

Solution 2

One way to find all the repeated values in a list is using groupingBy and then filter the values which are > 1. E.g.

val list = listOf("orange", "apple", "apple", "banana", "water", "bread", "banana")
println(list.groupingBy { it }.eachCount().filter { it.value > 1 })

Output

{apple=2, banana=2}

Comments

[Simon Cedergren Malmqvist]

If you don't care of knowing which index has an overlapping value (e.g. for validation), println(list.groupingBy { it }.eachCount().any { it.value > 1 }) would be more efficient since it stops on the first occurrence.

[Caio]

I added this extension function on my projects for find repeated items in Iterable by a Predicate: inline fun <T, K> Iterable<T>.findRepeatedItemsBy(crossinline keySelector: (T) -> K):Map<K, Int> = groupingBy(keySelector) .eachCount() .filter { it.value > 1 }