CODEWITHSUNDEEP
arrayslistelm
Misha Moroshko
Misha Moroshko

Reputation: 171351

How to remove an item at a given index from Array/List in Elm?

What is the most elegant way to remove an item at index i in a given Array? In a given List?

Upvotes: 10

Views: 4311

Answers (6)

Nikita Volkov
Nikita Volkov

Reputation: 43310

The "elm-community/array-extra" package now provides this function:

https://package.elm-lang.org/packages/elm-community/array-extra/2.1.0/Array-Extra#removeAt

Upvotes: 0

Ethan B. Martin
Ethan B. Martin

Reputation: 174

Use List.Extra.removeAt from elm-community.

Upvotes: 3

William Casarin
William Casarin

Reputation: 2582

This should be pretty efficient:

remove : Int -> Array a -> Array a
remove i a =
  let
    a1 = Array.slice 0 i a
    a2 = Array.slice (i+1) (Array.length a) a
  in
    Array.append a1 a2

Upvotes: 7

Alex Shroyer
Alex Shroyer

Reputation: 3829

Here's a different kind of solution based on J's from.

Usage:

[-1,2,0] `from` ["fran","world","san","hello"]

which returns: [Just "hello",Just "san",Just "fran"]

Definition, after import List exposing (..):

get idx lst =  -- get an element from a list (negative indexes from end)
  if idx >= 0 then
    head (drop idx lst)
  else
    head (drop (negate (idx+1)) (reverse lst))

from idxs lst =  -- atoms of idxs are indexes into lst
  case idxs of
    hd::tl -> (get hd lst)::(from tl lst)
    _ -> []

Upvotes: 2

Simon H
Simon H

Reputation: 21005

I needed an indexedFilter function recently. This could provide you a fold based alternative

indexedFilter : (Int -> a -> Bool) -> List a -> List a
indexedFilter p xs =
    let
        tup = List.map2 (,) [ 0 .. List.length xs - 1 ] xs
    in List.foldr (\(i,x) acc -> if p i x then x :: acc else acc) [] tup

Upvotes: 2

pdamoc
pdamoc

Reputation: 2923

Best I could think of:

removeFromList i xs =
  (List.take i xs) ++ (List.drop (i+1) xs) 

removeFromArray i =
  Array.toList >> removeFromList i >> Array.fromList

Upvotes: 18

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