CODEWITHSUNDEEP
f#
tofokevin
tofokevin

Reputation: 37

F# creating a new list in function

let rec findMatches str list =
        let hd :: tl = list
        match list with 
        | [] -> []
        | (s, _) as hd :: tl when s = str -> hd :: findMatches str tl
        | _::tl -> findMatches str tl

This is my current function and i am stuck on how to create a new list and returning the list, I would want to test my function with this

matchs "A" [("A",5); ("BB",6); ("AA",9); ("A",0)];;

and i want it to reutrn

val it : int list = [0; 5]

so i know that i need a int list returned

Upvotes: 0

Views: 161

Answers (2)

tranquillity
tranquillity

Reputation: 1685

This is a perfect candidate to use List.fold from the F# library

let toMatch = "A"

let test =
    [ ("A", 5)
      ("BB", 6)
      ("AA", 9)
      ("A", 0) ]

let findMatches toMatch items =
    List.fold
        (fun output item ->
            if toMatch = (fst item) then
                (snd item) :: output //Append if we find a match otherwise just return the same list
            else
                output)
        [] //Set initial output to the empty list
        items

findMatches toMatch test

Upvotes: 0

dumetrulo
dumetrulo

Reputation: 1991

It is easy to achieve your goal using a recursive inner function with an accumulator argument to collect the results one by one:

let findMatches str list =
    let rec inner acc = function
        | [] -> acc
        | (s, n) :: tl ->
            inner (if s = str then n :: acc else acc) tl
    inner [] list

Upvotes: 1

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