langref.org - fsharp - Lists

Remove an element from a list by index

Given the list [Apple, Banana, Carrot], remove the first element to produce the list [Banana, Carrot]

fsharp

let split_at list n =
let rec split_at' list' n' left right =
match list' with
| [] -> (List.rev left, List.rev right)
| x :: xs -> if n' <= n then split_at' xs (n' + 1) (x :: left) right else split_at' xs (n' + 1) left (x :: right)
split_at' list 0 [] []

// ------

let (_, right) = split_at fruit 0

let drop list n =
if n <= 0 then
list
else
let (_, right) = split_at list (n - 1)
right

// ------

let result = (drop fruit 1)

Remove the last element of a list

fsharp

let take list n =
if n <= 0 then
list
else
let (left, _) = split_at list (n - 1)
left

// ------

let result = (take fruit ((List.length fruit) - 1))

let but_last list =
let rec but_last' list' acc =
match list' with
| [x] -> List.rev acc
| x :: xs -> but_last' xs (x :: acc)
if List.is_empty list then [] else but_last' list []

// ------

let result = (but_last fruit)

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"]

fsharp

let rotate list n =
if n <= 0 then
list
else
let (left, right) = split_at list (n - 1)
right @ left

// ------

let result = (rotate fruit 1)