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Remove first ocurrence from list

Removes from list the first ocurrence only of an element that satisfies the predicate. Additional elements that also satisfy the predicate remain in the list.

Remove first ocurrence from list

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let rec remove_first pred lst =
    match lst with
    | h::t when pred h -> t
    | h::t -> h::remove_first pred t
    | _ -> []

Usage:

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let somelist = [('a',2);('f',7);('a',4);('h',10)]

let removed = somelist |> remove_first (fun (x,y) -> x='a')
// Result is:
// [('f',7);('a',4);('h',10)]

Remove nth ocurrence from list

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// This requires a small modification:
let rec remove_nth pred n lst =
    match lst, n with
    | (h::t, 1) when pred h -> t
    | (h::t, _) when pred h -> h::remove_nth pred (n-1) t
    | (h::t, _) -> h::remove_nth pred n t
    | _ -> []

val remove_first : pred:('a -> bool) -> lst:'a list -> 'a list

Full name: Script.remove_first

val pred : ('a -> bool)

val lst : 'a list

val h : 'a

val t : 'a list

val somelist : (char * int) list

Full name: Script.somelist

val removed : (char * int) list

Full name: Script.removed

val x : char

val y : int

val remove_nth : pred:('a -> bool) -> n:int -> lst:'a list -> 'a list

Full name: Script.remove_nth

val n : int

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More information

Link:	http://fssnip.net/1T
Posted:	13 years ago
Author:	Alexander Rautenberg
Tags:	list