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410 - PurelyFunctional.tv Newsletter

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.

@steffan-westcott
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steffan-westcott commented Jan 11, 2021

(defn nest [xs n]
  (map #(repeat n %) xs))

@mchampine
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mchampine commented Jan 11, 2021

(defn nest [l n] (for [e l] (repeat n e)))

@souenzzo
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souenzzo commented Jan 11, 2021

(letfn [(nest [els n]
          (for [el els]
            (repeat n el)))]
  {:a-a+b-b+c-c (nest [:a :b :c] 2)
   :empty       (nest [] 10)
   :1+2+3+4     (nest [1 2 3 4] 1)
   :3.empty     (nest [1 2 3] 0)})     

BONUS

let nest = ([... els], n) => els.map(el => Array(n).fill(el))

@steffan-westcott
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steffan-westcott commented Jan 11, 2021

As a point of comparison, here is my answer in Java. I did not enjoy wrestling with the types:

import java.util.Arrays;
import java.util.stream.*;

class Main {
  public static Object[] nest(Object[] xs, int n) {
    return Arrays.stream(xs).map(x -> Stream.generate(() -> x).limit(n).toArray()).toArray();
  }
}

@sztamas
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sztamas commented Jan 11, 2021

(defn nest [xs n]
  (map (partial repeat n) xs))

@kschltz
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kschltz commented Jan 11, 2021

(defn nest [es n]
  (->> es (map #(repeat n %))))

@proush42
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proush42 commented Jan 11, 2021

(defn nest [coll cnt]
  (map #(repeat cnt %) coll))

@NPException
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NPException commented Jan 11, 2021

Looks like I ended up with the same thing like most people:

(defn nest [col n]
  (map (partial repeat n) col))

Here's my solution to the slightly different original problem: https://edabit.com/challenge/5mANMR3X6gEfWephD

(defn array-multiplier [col]
  (map (partial repeat (count col)) col))

@diavoletto76
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diavoletto76 commented Jan 11, 2021

(defn nest [xs n]
  (map (partial repeat n) xs))

@chopmo
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chopmo commented Jan 11, 2021

(defn nest [list num]
  (map #(repeat num %) list))

@zackteo
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zackteo commented Jan 12, 2021

Happy to be able to make my first submission for this :)

(defn nest [l n]
  (map #(repeat n %) l))

@frndmg
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frndmg commented Jan 12, 2021

well, trying to bring functional to python too (ignore the imports 😉).

disclaimer: not as beautiful as the clojure version

from itertools import repeat
from functools import reduce, partial

compose = lambda *F: reduce(lambda f, g: lambda x: f(g(x)), F)
nest = lambda xs, n: compose(list, partial(map, compose(list, partial(repeat, times=n))))(xs)

@sztamas
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sztamas commented Jan 12, 2021

@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]

@ndonolli
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ndonolli commented Jan 14, 2021

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));

@Toni-zgz
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Toni-zgz commented Jan 17, 2021

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)

@cristiano-belloni
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cristiano-belloni commented Jan 19, 2021

Javascript oneliner for comparison:

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

@steffan-westcott
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steffan-westcott commented Jan 20, 2021

@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.

@Toni-zgz
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Toni-zgz commented Jan 20, 2021

@steffan-westcott thanks for your suggestions

@prairie-guy
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prairie-guy commented Apr 7, 2021

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

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