take-nth - Clojure Standard Library

For a quick intro to this series of blog posts check out Clojure Standard Library - Intro. It includes a lot of useful info, including notes about presentation of examples and more.

This posts function:

take-nth

Quick Overview

Description

clojure.core/take-nth is a function that accepts a collection and returns every nth item in that collection.

Example

In this example I am going to use take-nth to get every third number from a collection that contains numbers 0 up to 20 (exclusive). The collection of numbers will be generated using range

(take-nth 3 (range 20))
; => (0 3 6 9 12 15 18)  ; Every third number from range(20).

How To Use

Parameters and Return Values

take-nth is a multiple arity function and uses arity overloading in order to provide different behavior based on the number of arguments you provide. take-nth takes one to two arguments.

• (take-nth n) Providing only n, returns a stateful transducer.

• (take-nth n coll) Providing both n and a collection (coll), returns a lazy sequence that contains every nth item from coll.

Note: Non-positive integers and zero return an infinite lazy sequence of zeros. ¹

Example Use Cases

One possible use might be to use take-nth to do a quick, naive downsample of a stream of data. If in the stream, items close together are likely to contain similar information or, if it is already random.

Ex: ²

(def random-stream (repeatedly #(rand-int 20)))  ; Created an infinite stream of random integers
; => #'user/random-stream
(def sample (take-nth 20 random-stream))  ; Created a sample of random-stream; contains every 20th integer.
; => #'user/sample
(take 10 sample)
; => (19 14 5 3 12 10 13 15 2 14)  ; The first ten items from sample.