Collection Operations: Grouping

An introduction to the most important functions for grouping: groupBy, partition, groupingBy, fold, reduce, aggregate and their variants.

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THE CURRENT VERSION OF THIS ARTICLE IS PUBLISHED HERE.

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Tags: #FYI++

This article is part of the Kotlin Primer, an opinionated guide to the Kotlin language, which is indented to help facilitate Kotlin adoption inside Java-centric organizations. It was originally written as an organizational learning resource for Etnetera a.s. and I would like to express my sincere gratitude for their support.

It is recommended to read the Introduction before moving on. Check out the Table of Contents for all articles.

groupBy

inline fun <T, K> Iterable<T>.groupBy(
    keySelector: (T) -> K
): Map<K, List<T>>

The groupBy function is the most common function used for grouping. It allows you to group elements of a collection in a map, under keys given by keySelector.

Example