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Editorial Team
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Asked: 2026-06-04T06:24:29+00:00

I am trying to input a list into the function and it send me

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I am trying to input a list into the function and it send me a list with the first half of the elements taken away using f# with the below recursion but I keep running into a base case problem that I just cant figure out. any thoughts? I am using the second shadow list to count how far I need to go until I am half way into the list (by removing two elements at a time)

let rec dropHalf listToDrop shadowList = 
    match shadowList with
    | [] -> listToDrop
    | shadowHead2::shadowHead1::shadowTail -> if shadowTail.Length<=1 then listToDrop else 
                                    match listToDrop with 
                                    |[] -> listToDrop
                                    |listToDropHead::listToDropTail -> dropHalf listToDropTail shadowTail
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1 Answer

  1. Editorial Team
    let rec dropHalf listToDrop shadowList = 
    match shadowList with
    | [] -> listToDrop
    | shadowHead2::[] -> listToDrop   (* odd number! *)
    | shadowHead1::shadowHead2::shadowTail -> 
        match listToDrop with 
        | [] -> listToDrop   (* should never happen? *)
        | listToDropHead::listToDropTail -> dropHalf listToDropTail shadowTail

    i’m afraid i don’t use F#, but it’s similar to ocaml, so hopefully the following is close to what you’re looking for (maybe the comment format has changed?!). the idea is that when you exhaust the shadow you’re done. your code was almost there, but the test for length on the shadow tail made no sense.

    i want to emphasize that this isn’t anything like anyone would write “in real life”, but it sounds like you’re battling with some weird requirements.

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