410 - PurelyFunctional.tv Newsletter

00 Nested lists.md

Nested lists

Write a function that nests the elements of a list one level deeper by repeating that element inside a new list a given number of times.

Examples

(nest [:a :b :c] 2) ;=> ((:a :a) (:b :b) (:c :c))
(nest [] 10) ;=> ()
(nest [1 2 3 4] 1) ;=> ((1) (2) (3) (4))
(nest [1 2 3] 0) ;=> (()()())

Thanks to this site for the challenge idea where it is considered Hard in Java.

Please submit your solutions as comments on this gist.

@frndmg I see what you're trying to do :-), but for python I would prefer the more idiomatic:

def nest(xs, n):
    return [[x] * n for x in xs]

My clojure solution was essentially identical to many solutions posted here already. So for fun, I'll show an ES6 javascript solution:

const nest = (xs, n) => xs.map(x => new Array(n).fill(x));

My clojure solution. I know isn't the best but i think is clearer than other solutions for beginners as me.

(ns nest)
(require '[clojure.test :as test])

(defn nest [vect num-rep]
    (let [funcion (fn [elt] (repeat num-rep elt))]
        (map funcion vect)))

(test/deftest nest-test
    (test/is (= '((:a :a) (:b :b) (:c :c))  (nest [:a :b :c] 2)))
    (test/is (= '() (nest [] 10)))
    (test/is (= '((1) (2) (3) (4))  (nest [1 2 3 4] 1)))
    (test/is (= '(()()()) (nest [1 2 3] 0))))

(test/run-tests 'nest)

Javascript oneliner for comparison:

(a, n) => a.map(a => new Array(n).fill(a))

@Toni-zgz Welcome to Clojure! Since you asked for some suggestions:

• To write formatted Clojure code in Github comments, write ```clojure as the first line and ``` as the last line. See the Github docs on syntax hightlighting
• map on an empty collection will evaluate to an empty sequence. You don't need to explicitly check for an empty input collection in your answer.

@steffan-westcott thanks for your suggestions

(defn nest [ns k] (map (partial repeat k) ns))