BirnadinErick / 99-ocaml-problems

my attempt to 99 lisp problems in ocaml (not exhaustive and extras included)

OCaml Exercises

#hub/ocaml #hub/cs #hub/dsa #hub/algebra

Following are coding practice while engaging in OCaml's Execersises

Find length of a list

let rec len lst = match lst with [] -> 0 | _ :: t -> 1 + len t

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Generate list of positive integer range $[a, b)$

let rec range a b = if a = b then [] else a :: range (a + 1) b

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Find GCD using Euclid's method [[euclid-algo]] Exercise 32 #intermediate

let rec gcd p q =
  if q = 0 then p
  else
    let r = p mod q in
    gcd q r

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Check whether 2 positive integers are Co-prime Exercise 33 #beginner

let is_coprime a b = (gcd a b) = 1

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Find Euler's totient $\Phi(n)$ Exercise 34 #intermediate

let phi n =
  match n with
  | 0 | 1 -> 1
  | _ -> len (List.filter (fun x -> x = 1) (
		  List.map (gcd n) (range 1 (n + 1))
	))

---

get last element of a list #beginner

let rec last lst = match lst with
	| [] -> None
	| t :: [] -> Some t
	| _ :: u -> last u
	;;

---

get last two elements of a list #beginner

let rec last2 xs = match xs with
	| [] | [_] -> None
	| [t; t'] -> Some (t, t')
	| _ :: xss -> last2 xss
	;;

---

Find Nth element #beginner

(** here indeces gives list of indices of given list *)
let nth j lst = match ( last (List.filter (function (_, i) -> i = j) (List.combine lst (indices lst)))) with
| None -> raise (Failure "nth")
| Some (x,_) -> x;;

• given solution in the web-page

let nth k = function
	| [] -> None
	| h :: t -> if k = 0 then (Some h) else at (k-1) t
	;;

---

reverse a list #beginner did not complete

let rev list = 
	let rec aux acc = function
		| [] -> acc
		| h :: t -> aux (h :: acc) t
	in
	aux [] list
;;

☠️ formatting messes up code, click to reveal correct version $| h::t -> aux\ (h::acc)\ t$ 👆

---

convert string to character list #intermediate

let string_to_list s = 
	let rec aux i acc = 
		if i < 0 then acc
		else aux (i-1) (s.[i] :: acc)
	in aux (String.length s - 1) []
;;

---

check if list is palindrome #beginner

let is_palindrome xs = xs = List.rev xs;;

• enabling direct string

let is_palindrome str = 
	let xs = string_to_list str in
	xs = List.rev
;;

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duplicate the elements in list #beginner

let duplicate list = 
	let aux acc i = i :: (i :: acc)
	in List.rev (List.fold_left aux [] list)

---