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Posted on Jan 26, 2020

Daily Challenge #172 - Find All in an Array

#challenge

Setup

Implement a function that will accept an array of integers and an integer n. Find all occurrences of n in the given array and return another array containing all the index positions of n in the array.

If n is not in the given array, return an empty array [].

Assume thatn and all values in the array will always be integers.

Example

find_all([6, 9, 3, 4, 3, 82, 11], 3)
> [2, 4]

Tests

[6, 9, 3, 4, 3, 82, 11], 3
[10, 16, 20, 6, 14, 11, 20, 2, 17, 16, 14], 16
[20, 20, 10, 13, 15, 2, 7, 2, 20, 3, 18, 2, 3, 2, 16, 10, 9, 9, 7, 5, 15, 5], 20

This challenge comes from MementoMori on CodeWars. Thank you to CodeWars, who has licensed redistribution of this challenge under the 2-Clause BSD License!

Want to propose a challenge idea for a future post? Email yo+challenge@dev.to with your suggestions!

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Top comments (14)

SavagePixie • Jan 26 '20

JavaScript

const findAll = (arr, n) => arr
   .reduce((a, b, i) => b === n
      ? a.concat(i)
      : a
   , [])

Kyle Jones • Jan 26 '20

In Python:

def find_all(list, value):
    position = 0
    results = []
    for element in list:
        if element == value:
            results.append(position)
        position += 1
    return results

print(find_all([6, 9, 3, 4, 3, 82, 11], 2))
print(find_all([6, 9, 3, 4, 3, 82, 11], 3))
print(find_all([10, 16, 20, 6, 14, 11, 20, 2, 17, 16, 14], 16))
print(find_all([20, 20, 10, 13, 15, 2, 7, 2, 20, 3, 18, 2, 3, 2, 16, 10, 9, 9, 7, 5, 15, 5], 20))
print(find_all(["happy", "birthday", "to", "you", "happy", "birthday", "to", "me"], "happy"))

Avalander • Jan 26 '20 • Edited

Haskell

Written on the phone, it might not work, I'll check it when I get home.

find_all :: [Int] -> Int -> [Int]
find_all xs n =
  [ snd x | x <- indexed if (fst x) == n ]
  where
    indexed = zip xs [0..(length xs)]

Jesse Phillips • Jan 27 '20

// D
pure nothrow @safe
auto findIndex(const(int)[] arr, size_t needle) {
  return arr.enumerate
     .filter!(x => x[1] == needle) 
     .map!(x => x[0])
     //.array
     ;
} unittest {
  assert(findIndex([6, 9, 3, 4, 3, 82, 11], 3).equal([2, 4]));
}

The commented out array line would allocate a new array to meet the specification.

Craig McIlwrath • Jan 26 '20

Haskell:

import Data.List (findIndices) 

findAll :: Eq a => a -> [a] -> [Int]
findAll x = findIndices (==x)

or alternatively,

findAll :: Eq a => a -> [a] -> [Int]
findAll x = map fst . filter ((==x) . snd) . zip [0..]

Ryan Beckett • Jan 26 '20 • Edited

Swift:

func findOccurances(of numberToFind: Int, in listOfNumbers: [Int]) -> [Int] {
    return listOfNumbers.enumerated().compactMap { (index, number) in
        number == numberToFind ? index : nil
    }
}

Lamonte • Jan 26 '20 • Edited

In Dart:

void main() {
  print(findAll([6, 9, 3, 4, 3, 82, 11], 3));
  print(findAll([10, 16, 20, 6, 14, 11, 20, 2, 17, 16, 14], 16));
  print(findAll([20, 20, 10, 13, 15, 2, 7, 2, 20, 3, 18, 2, 3, 2, 16, 10, 9, 9, 7, 5, 15, 5], 20));
}

List<int> findAll(List<int> items, int find) {
  var found = List<int>();
  for(var idx = 0; idx < items.length; idx++) {
    if(items[idx] == find) {
      found.add(idx);
    }
  }
  return found;
}

// alternative approach, which cuts it down maybe a line? lol
List<int> findAll2(List<int> items, int find) {
  var found = List<int>();
  items.asMap().forEach((idx,value){
    if(value == find)
      found.add(idx);
  });
  return found;
}

dartpad.dev/

Ciaran Treanor • Jan 26 '20 • Edited

Rust:

fn find_all(numbers: Vec<usize>, value: usize) -> Vec<usize> {
    numbers.iter()
        .enumerate()
        .filter(|&(_, val)| *val == value)
        .map(|(index, _)| index)
        .collect()
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test1() {
        assert_eq!(vec![2, 4], find_all(vec![6, 9, 3, 4, 3, 82, 1], 3));
        assert_eq!(vec![1,9], find_all(vec![10, 16, 20, 6, 14, 11, 20, 2, 17, 16, 14], 16));
        assert_eq!(vec![0,1,8], find_all(vec![20, 20, 10, 13, 15, 2, 7, 2, 20, 3, 18, 2, 3, 2, 16, 10, 9, 9, 7, 5, 15, 5], 20));
    }
}

Amin • Jan 26 '20

Elm

import List exposing (indexedMap, filter, map)
import Tuple exposing (pair, second, first)


findAll : List Int -> Int -> List Int
findAll integers integer =
    indexedMap pair integers
        |> filter (second >> (==) integer)
        |> map first