View Category
Lists
Declaration

Define an empty list

Assign the variable "list" to a list with no elements
ocaml
let list = [];;

Define a static list

Define the list [One, Two, Three, Four, Five]
ocaml
let list = [ "One"; "Two"; "Three"; "Four"; "Five" ];;
Output

Join the elements of a list, separated by commas

Given the list [Apple, Banana, Carrot] produce "Apple, Banana, Carrot"
ocaml
let list = [ "Apple"; "Banana"; "Carrot" ];;

let f str_so_far elem = str_so_far ^ ", " ^ elem in
let str = List.fold_left f list;;

print_string str;;

Join the elements of a list, in correct english

Create a function join that takes a List and produces a string containing an english language concatenation of the list. It should work with the following examples:
join([Apple, Banana, Carrot]) = "Apple, Banana, and Carrot"
join([One, Two]) = "One and Two"
join([Lonely]) = "Lonely"
join([]) = ""
ocaml
let join list =
let rec join' list acc =
match list with
| [] -> ""
| [single] -> single
| one::[two] ->
if acc = "" then one ^ " and " ^ two
else acc ^ one ^ ", and " ^ two
| first::others -> join' others (acc ^ first ^ ", ")
in
join' list ""

Produce the combinations from two lists

Given two lists, produce the list of tuples formed by taking the combinations from the individual lists. E.g. given the letters ["a", "b", "c"] and the numbers [4, 5], produce the list: [["a", 4], ["b", 4], ["c", 4], ["a", 5], ["b", 5], ["c", 5]]
Access

Fetch an element of a list by index

Given the list [One, Two, Three, Four, Five], fetch the third element ('Three')
ocaml
let third = List.nth [ "One"; "Two"; "Three"; "Four"; "Five" ] 3;;

Fetch the last element of a list

Given the list [Red, Green, Blue], access the last element ('Blue')
ocaml
let list = [ "Red"; "Green"; "Blue" ] in
let last = List.nth list ( (List.length list) - 1 );;
let list = [ "Red"; "Green"; "Blue" ] in
let last = List.hd (List.rev list);;

Find the common items in two lists

Given two lists, find the common items. E.g. given beans = ['broad', 'mung', 'black', 'red', 'white'] and colors = ['black', 'red', 'blue', 'green'], what are the bean varieties that are also color names?
ocaml
let beans = ["broad"; "mung"; "black"; "red"; "white"]

let colors = ["black"; "red"; "blue"; "green"]

let f common c = if List.mem c beans then c::common else common

let common = List.fold_left f [] colors;;

(* common will contain a list with the common elements *)

Display the unique items in a list

Display the unique items in a list, e.g. given ages = [18, 16, 17, 18, 16, 19, 14, 17, 19, 18], display the unique elements, i.e. with duplicates removed.
ocaml
let ages = [18; 16; 17; 18; 16; 19; 14; 17; 19; 18]

let f res e = if List.mem e res then res else e::res

let unique = List.fold_left f [] ages;;
Modification

Remove an element from a list by index

Given the list [Apple, Banana, Carrot], remove the first element to produce the list [Banana, Carrot]
ocaml
let remove i list =
let rec rem i list' acc =
match list' with
| h::t when i > 0 -> rem (i-1) t (h::acc)
| h::t when i = 0 -> (List.rev acc)@t
| [] -> List.rev acc (* not enough elements *)
in
rem i list []

Remove the last element of a list

ocaml
let remove_last list =
match (List.rev list) with
| h::t -> List.rev t
| [] -> []

Rotate a list

Given a list ["apple", "orange", "grapes", "bananas"], rotate it by removing the first item and placing it on the end to yield ["orange", "grapes", "bananas", "apple"]
ocaml
let rotate list =
match list with
| head::tail -> tail@[head]
| [] -> []
Manipulation

Gather together corresponding elements from multiple lists

Given several lists, gather together the first element from every list, the second element from every list, and so on for all corresponding index values in the lists. E.g. for these three lists, first = ['Bruce', 'Tommy Lee', 'Bruce'], last = ['Willis', 'Jones', 'Lee'], years = [1955, 1946, 1940] the result should produce 3 actors. The middle actor should be Tommy Lee Jones.

List Combinations

Given two source lists (or sets), generate a list (or set) of all the pairs derived by combining elements from the individual lists (sets). E.g. given suites = ['H', 'D', 'C', 'S'] and faces = ['2', '3', '4', '5', '6', '7', '8', '9', '10', 'J', 'Q', 'K', 'A'], generate the deck of 52 cards, confirm the deck size and check it contains an expected card, say 'Ace of Hearts'.

Perform an operation on every item of a list

Perform an operation on every item of a list, e.g.
for the list ["ox", "cat", "deer", "whale"] calculate
the list of sizes of the strings, e.g. [2, 3, 4, 5]
ocaml
List.map String.length ["ox"; "cat"; "deer"; "whale"];;

Split a list of things into numbers and non-numbers

Given a list that might contain e.g. a string, an integer, a float and a date,
split the list into numbers and non-numbers.