Discussion on: Daily Coding Puzzles - Nov 4th - Nov 9th

[Ali Spittel]

Wednesday

Array.diff (6 KYU):

Your goal in this kata is to implement a difference function, which subtracts one list from another and returns the result.

CodeWars

[Jermaine]

Here's one in Dart:

arrayDiff(List a, List b) => a..retainWhere((i) => !b.contains(i));

[Tyler Leonhardt]

PowerShell

$a | ? { $_ -notin $b }

Expanding the ? alias:

$a = @(1,2,3,4)
$b = @(2,3,4,5)

$a | Where-Object { $_ -notin $b }

[Ben Halpern]

Ruby

a - b

[Garrett]

Similar effort when done in APL! :)

[Ali Spittel]

That's amazing!

[Ben Halpern]

[Ben Halpern]

On the other hand, Ruby probably allocated about 10,000 object references under the hood and used 1gb of memory to make that happen. 😄

[jess unrein]

Honestly, I just came out here to have a good time and I'm feeling so attacked right now 😅😅😅

[Ben Halpern]

github.com/thepracticaldev/dev.to/... 🙃

[jess unrein]

Haha nice. Honestly, I'm just so used to copy/pasting emojis from slack and discord. Fun fact, when you do that, it copies the colon emoji syntax, not the unicode glyph 😅🙃

[Ricardo Tomasi]

@bendhalpern some numbers (using /usr/bin/time -l on OSX):

Go: 0.0s, 1.5Mb
Go run: 0.10s, 24Mb
Node: 0.06s, 19Mb
Ruby: 0.08s, 11Mb
Crystal: 0.0s, 1.5Mb
Crystal run: 0.4s, 10.4Mb

[rakenodiax]

Rust: 0.00s - 1.12MiB
Rust (nightly): 0.01s - 0.88MiB
Go: 0.00s - 1.64MiB
Go run: 0.25s - 24.11MiB
Python: 0.21s - 5.48MiB

Disclaimer: I'm inexperienced with Go; using the example from below 😅

[jess unrein]

Oh, I know almost nothing about Go. I'm specifically using these exercises to learn the syntax. No clue if and how what I'm doing can be optimized. Go is famously, weirdly restrictive and intentionally verbose.

[Anologicon]

kkkkk it's very simple dude :D

[Sung M. Kim]

Ah, cannot unsee this answer... 😆

[Roman Elizarov]

Same in Kotlin :)

[Ben Berbass]

PHP

array_diff(a, b);

[Sung M. Kim]

It wasn't as easy as doing a set difference as the a needed to keep the duplicate values.

Below is the answer in C#.

Where is the same as filter in JavaScript.

using System.Linq;
using System.Collections.Generic;

public class Kata
{
  public static int[] ArrayDiff(int[] a, int[] b)
  {
    var hash = new HashSet<int>(b);
    return a.Where(_ => !hash.Contains(_)).ToArray();
  }
}

[Ali Spittel]

Python!

def array_diff(a, b):
    return [l for l in a if l not in b]

[jess unrein]

List comprehensions are truly a gift.

[Ali Spittel]

YES -- they are so elegant.

[Daniel Schep]

Or with sets!

def array_diff(a, b):
    return list(set(a) - set(b))

[Kasey Speakman]

F#

let diff a b =
    a |> Array.filter (fun v -> not (b |> Array.contains v))

also sequence expression syntax

let diff a b =
    [|
        for value in a do
            if not (b |> Array.contains value) then
                yield value
    |]

[Jakob Christensen]

Or (although it will also remove duplicates from a):

(a |> Set.ofSeq) - (b |> Set.ofSeq)

[Kasey Speakman]

Yes, this was my first thought, but I was trying to retain dupes.

[Christopher McClellan]

Just define the subtraction operator and done.

[Mateus Vahl]

Hey all, Why I don't see any java code here?

[David Wickes]

Common Lisp

I'll use lists because... well, it's Lisp, right?

(set-difference '(1 2 2 2 3) '(2))
;; => (3 1)

Can't beat a good standard library ;)

[Garrett]

APL

 a ~ b 

Replied to the post instead of the comment! My bad!

[Sebastian Martin Dicke]

Haskell

array_diff :: Eq a => [a] -> [a] -> [a] 
array_diff a b = filter (\x -> notElem x b) a

[Sebastian Martin Dicke]

Another Haskell solution:

array_diff :: Eq a => [a] -> [a] -> [a]
array_diff a b = [x | x <- a, x `notElem` b]

[jess unrein]

Go

func Difference(a []int, b []int) (diff []int) {
    m := make(map[int]bool)

    for _, item := range b {
        m[item] = true
    }

    for _, item := range a {
        if _, presence := m[item]; !presence {
            diff = append(diff, item)
        }
    }
    return
}

[Courtney]

function array_diff(a, b) {
  let counter = {};
  for(let i of b) {
    counter[i] = true;
  }
  let result = a.filter(item => counter[item] !== true);
  return result;
}

[Jay]

Rust

• With Liftime. The returned Vec<&T> has elements borrowed and valid until the list1 is valid.

fn diff<'a, T>(list1: &'a [T], list2: &[T]) -> Vec<&'a T>
where
    T: PartialEq,
{
    list1.into_iter().filter(|e| !list2.contains(e)).collect()
}

• Without lifetime We return a new Vec free from any list provided, by cloning the data. The generic data type only allows the data type that implements Clone trait to be passed.

fn diff2<T>(list1: &[T], list2: &[T]) -> Vec<T>
where
    T: PartialEq + Clone,
{
    list1
        .into_iter()
        .map(|x| x.clone())
        .filter(|e| !list2.contains(&e))
        .collect()
}

[Pavel Susicky]

Tried in Javascript

function arrayDiff(arr1, arr2) {
    return arr1.filter(val => !arr2.includes(val));
}

[Pawl]

this is beautiful. thanks

[Ali Spittel]

Nice!!

[Neocortexxx]

Wow, great! So clean!