Array vs List in Elm

Question

I was suprised to learn that Array and List were two different types in Elm:

• Array

• List

In my case, I have a List Int of length 2,000,000 and I need about 10,000 of them but I don't know in advance which ten thousand. That will be provided by another list. In pseudo-code:

x = [ 1,1,0,30,...,255,0,1 ]
y = [ 1,4,7,18,36,..., 1334823 , ... 1899876 ]
z = [ y[x[0]], y[x[1]], ... ]

I am using pseudocode because clearly this isn't Elm syntax (it might be legal JavaScript).

Can these array selections be done in List or Array or both?

Answer 1

List is a linked list which provides O(n) lookup time based on index. Getting an element by index requires traversing the list over n nodes. An index lookup function for List isn't available in the core library but you can use the elm-community/list-extra package which provides two functions for lookup (varying by parameter order): !! and getAt.

Array allows for O(log n) index lookup. Index lookups on Array can be done using Array.get. Arrays are represented as Relaxed Radix Balanced Trees.

Both are immutable (all values in Elm are immutable), so you have trade-offs depending on your situation. List is great when you make a lot of changes because you are merely updating linked list pointers, whereas Array is great for speedy lookup but has somewhat poorer performance for modifications, which you'll want to consider if you're making a lot of changes.

Answer 2

Something like this should work:

import Array
import Debug

fromJust : Maybe a -> a
fromJust x = case x of
    Just y -> y
    Nothing -> Debug.crash "error: fromJust Nothing"

selectFromList : List a -> List Int -> List a
selectFromList els idxs = 
  let arr = Array.fromList els
   in List.map (\i -> fromJust (Array.get i arr)) idxs

It converts the input list to an array for fast indexing, then maps the list of indices to their corresponding values in the array. I took the fromJust function from this StackOverflow question.