OCaml Tutorial: List, Array, Tuple

By Xah Lee. Date: . Last updated: .

Lists

The syntax for list is

[ element_1 ; element_2 ; element_3 ; … ]

The last ; is optional.

All elements must be of the same type.

(* list syntax *)
(* elements must all be the same type *)
let x = [ 2; 8; 5; ];;

list length

let x = [ 2; 8; 5; ];;
List.length x;;                 (* ⇒ 3 *)

list first, rest

let x = [ 2; 8; 5; ];;

(* first element, called “head”. *)
List.hd x;;                             (* ⇒ 2 *)

(* rest elements, from 2nd to last, called “tail”. *)
List.tl x;;                             (* ⇒ [8; 5] *)

The List.hd is a form of calling the “hd” function from the module “List”.

A list allows you to efficiently grow the list, by adding to or removing the first element of the list. However, accessing nth element takes time proportional to n.

Prepend List

x::list → prepends x to the front of list list

let x = [ 2; 8; 5; ];;

(* prepending a element *)
4::x;;                          (* ⇒ [4; 2; 8; 5; ] *)

(* prepending several elements *)
1::9::4::x;;                    (* ⇒ [1; 9; 4; 2; 8; 5] *)

The syntax

[a ; b ; c]

is equivalent to

a::b::c

The operator :: is right-associative. So, a::b::c means a::(b::c)

Join Lists

2 lists can be joined by the operator @.

(* join 2 lists *)
[ 2; 8; 5; ] @ [ 3; 4; ]

The time to join 2 list is proportional to the number of items in first list.

Array

Array is like a list of fixed length, and every element must be the same type.

The syntax for array is

[| element_1 ; element_2 ; element_3 ; … |]

(* array syntax. Elements must be same type *)
let x = [| 2; 8; 3 |];;

array get

Here's array element access.

(* array *)
let x = [| 2; 8; 3 |];;

(* getting a element *)
print_int x.(1);;         (* prints 8. Index starts at 0 *)

array set

Here's how to change a slot's value.

(* array *)
let x = [| 2; 8; 3 |];;

(* changing array element *)
x.(1) <- 9;;             (* set 1st index element to 9 *)

create array

(* creating array programmatically *)
(* first arg is num of items, second is init value *)
let x = Array.make 9 4;;

The difference between list and array is that you cannot add or remove items into a array. However, getting and setting arbitrary elements in array is fast, while getting/setting arbitrary elements in list is slow.

n-tuples

n-tuple is like array but the elements need not be the same type.

(* arbitrary n-tuple can be created *)
let x = 3,4,8 ;;                 (* 3-tuple *)
let y = 7.8, "toh", 4, 8;;      (* 4-tuple *)

(* when ambiguous, tuple needs to have parenthesis around *)

get tuple element, pattern matching

use pattern matching to access tuple element.

(* pattern matching, to access tuple elements *)
let (a,b,c) = 3,4,7;;

print_int b;;                   (* 4 *)

Nesting List-Like Things

List, array, tuple can be nested arbitrarily, but remember, for list and array, their elements must all have the same type.

(* nesting. Remeber, all elements must be same type. *)
let x = [ [1;2]; [3;4;5]; [6;7] ];;       (* list of lists *)
let x = [ [|1;2|]; [|3;4;5|]; [|6;7|] ];; (* list of arrays *)
let x = [ (1,2); (3,4); (5,6) ];;         (* list of tuples *)
(* nested array examples *)
let x = [| [|3; 4|]; [| 5; 6|] |] ;;    (* array of arrays *)
let x = [| [3; 4]; [ 5; 6] |] ;;        (* array of lists *)
let x = [| (3, "4"); (5, "7") |] ;;     (* array of tuples *)

Note: when you have tuples as elements to list or array, each tuple must be the same type. That means, you cannot have one tuples (3,4) and (3.1, 4) because the first is of type “int*int” and the second is “float*int”.

Note also the length of tuple must match. You cannot have elements (3,4) and (3,4,5) in a array or list. This is because, technically, (3,4) is of type “int*int” but (3,4,5) is of type “int*int*int”.

[| (3 , "4"); (3. , "4") |] ;;    (* compiler error. *)
[| (3 , "4"); (3  , "4", 5) |] ;; (* compiler error. *)
(* a tuple, with elements being int, list, tuple, array *)
let x = (3, [4;5;6], (7,8), [| 9; 10 |]);;

Tuple is the most flexible about types.