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  1. Practice
  2. Functional Programming
  3. Introduction
  4. Filter Array
  5. Discussions

Filter Array

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votes

  • nevertiree + 2 comments

    Haskell

    f n arr = filter (< n) arr
    • har33sh + 0 comments

      f n arr = [ a | a <- arr , a < n ]

    • gabrielhuff + 0 comments

      f = filter . (>)

  • Darshit_Shah + 0 comments

    Another solution approach in Scala using for expression:

    for (i <- arr if i < delim) yield i

  • PitrDubovich + 0 comments

    In Scala, assuming we're not able to use filter, we could instead use the fact that flatMap will filter out values of None.

    arr.flatMap(a => if (a < delim) Some(a) else None)

  • m0smith + 0 comments

    Another Clojure solution. Not lazy unfortunately.

    (fn[delim lst] (reduce #(if (< %2 delim) (conj %1 %2) %1) [] lst))

  • JCrazafy + 1 comment

    I am not particularly convinced that the specifications are tested fully. I used list comprehension to circumvent the use of "filter" and it worked.

    • abhiranjan + 0 comments

      Hi @JCrazafy, thanks for pointing. I have reworded the constraint. It is suggested to write own implementation, not enforced.

  • Technical_Rambo + 0 comments

    Scala : Create Either Single-Element List Or Empty List for each element based on condition and flatMap to merge them. 

    def f(delim:Int,arr:List[Int]):List[Int] = arr.flatMap(x=>if (x<delim) List(x) else List())

  • meincke911 + 0 comments

    An F# solution

    open System

[<EntryPoint>]
let main args =
    let x = Console.ReadLine() |> int
    
    let numbers =
        Seq.initInfinite (fun _ -> Console.ReadLine())
        |> Seq.takeWhile (String.IsNullOrWhiteSpace >> not)
        |> Seq.filter (fun v -> v |> int < x)
        |> Seq.iter (printf "%s\n")
    
    0

  • metaculus + 0 comments

    Here is how to read the stdin for this question in Ocaml:

    let f n arr = 
    (* implementation *)
    
let () =
    (* first line is parsed as the first arg to f *)
    let num = int_of_string (read_line ()) in
    
    (* the rest until EOF are parsed into an array for second arg *)
    f num (
        let rec aux arr =
            try
                let input = read_line () in
                    match input with
                    | "" -> arr
                    | a  -> aux (arr @ [int_of_string a])
            with End_of_file -> arr
        in aux [] )

  • jb_gn + 0 comments

    I decided I didn't want to use the Enum library at all and implemented my Elixir solution using pure naive comprehensions. One of the interesting things about recursion is that the list is represented backwards in memory immediately after make_list is run, but since filter walks the list again and places the last element in the first position, everything comes back out in the correct order once it is time to print :D

    defmodule NaiveList do
  def print([]), do:
    nil
  def print([value | rest]) do
    IO.puts(value)
    print(rest)
  end

  def filter(limit, list), do:
    filter(limit, list, [])
  def filter(_limit, [], output), do:
    output
  def filter(limit, [value | rest], output) when value < limit, do:
    filter(limit, rest, [value | output])
  def filter(limit, [_value| rest], output), do:
    filter(limit, rest, output)

  def make_list(), do:
    make_list(IO.gets(""), [])
  defp make_list(""<>number, list), do:
    make_list(IO.gets(""), [(number |> String.trim |> String.to_integer) | list])
  defp make_list(:eof, list), do:
    list
end

n = IO.gets("") |> String.trim |> String.to_integer
list = NaiveList.make_list()
NaiveList.filter(n, list) |> NaiveList.print

  • ravi_andrew_baj1 + 1 comment

    Any Lisp or Racket solutions?

    • meriororen + 0 comments

      This? Input thanks to @abhiranjan

      (define (read-threshold)
  (let ([x (read)]) x))

(define (read-list)
  (let ([x (read)])
    (if (eof-object? x)
        (list)
        (cons x (read-list)))))
      
(let ([t (read-threshold)]
      [l (read-list)])
  (for ([x l])
       (if (< x t)
           (printf "~a\n" x)
           #f)))
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