99 Ocaml Problems

• #1 Tail of a list
  • Solution
• #2 Last two elements of a list
  • Solution
• #3 N'th element of a list
  • Solution
• #4 Find the number of elements of a list.
  • Solution
• #5 Reverse a list.
  • Solution
• #6 Palindrome
  • Solution
• #7 Flatten a list
  • Solution

https://ocaml.org/problems

#1 Tail of a list

Write a function last : 'a list -> 'a option that returns the last element of a list

# last ["a" ; "b" ; "c" ; "d"];;
- : string option = Some "d"
                      # last [];;
- : 'a option = None

Solution

let rec last = function
  | []    -> None
  | [x]   -> Some x
  | _::xs -> last xs

#2 Last two elements of a list

Find the last but one (last and penultimate) elements of a list.

# last_two ["a"; "b"; "c"; "d"];;
- : (string * string) option = Some ("c", "d")
# last_two ["a"];;
- : (string * string) option = None

Solution

let rec last_two = function
  | [] | [_] -> None
  | [x;y] -> Some (x,y)
  | _::xs -> last_two xs

#3 N'th element of a list

Find the N'th element of a list.

REMARK: OCaml has List.nth which numbers elements from 0 and raises an exception if the index is out of bounds.

# List.nth ["a"; "b"; "c"; "d"; "e"] 2;;
- : string = "c"
# List.nth ["a"] 2;;
Exception: Failure "nth".

Solution

let rec nth index = function
  | [] -> failwith "nth"
  | x::xs -> if index = 0 then x else nth (index - 1) xs

#4 Find the number of elements of a list.

OCaml standard library has List.length but we ask that you reimplement it. Bonus for a tail recursive solution.

# length ["a"; "b"; "c"];;
- : int = 3
# length [];;
- : int = 0

Solution

let length list =
  let rec length_rec acc = function
    | [] -> acc
    | _::xs -> length_rec (acc + 1) xs
  in length_rec 0 list

#5 Reverse a list.

OCaml standard library has List.rev but we ask that you reimplement it.

# rev ["a"; "b"; "c"];;
- : string list = ["c"; "b"; "a"]

Solution

let rev (list: int list) =
  let rec rev acc = function
    | [] -> acc
    | x::xs -> rev (x::acc) xs
  in rev [] list

#6 Palindrome

Find out whether a list is a palindrome.

HINT: a palindrome is its own reverse.

# is_palindrome ["x"; "a"; "m"; "a"; "x"];;
- : bool = true
# not (is_palindrome ["a"; "b"]);;
- : bool = true

Solution

let is_palindrome list = list = List.rev list

#7 Flatten a list

Flatten a nested list structure.

type 'a node =
  | One of 'a 
  | Many of 'a node list

# flatten [One "a"; Many [One "b"; Many [One "c" ;One "d"]; One "e"]];;
- : string list = ["a"; "b"; "c"; "d"; "e"]

Solution

let flatten nodes =
  let rec f = function
    | One x -> [x]
    | Many xs -> List.concat_map f xs
  in nodes |> List.concat_map f

(* Or without List.concat_map *)

let flatten2 nodes =
  let rec f acc = function
    | [] -> acc
    | One n::ns -> f (n::acc) ns
    | Many ns::rest -> f (f acc ns) rest
  in f [] nodes |> List.rev