Collection Operations: Aggregators

An introduction to the most important functions for aggregation: fold, reduce, average, count, sum, min, max, and their variants. A quick note on converting collections to strings using joinToString.

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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.

fold

inline fun <T, R> Iterable<T>.fold(
    initial: R,
    operation: (R, T) -> R
): R

The most fundamental aggregation operation is fold. The way it works is simple, but doesn’t lend itself well to a word-based…