How do I remove elements from a list in OCaml?

In OCaml, lists are immutable, so you cannot directly remove elements from an existing list. Instead, you create a new list that excludes the elements you want to remove. Here are several common ways to achieve this:

1. Using `List.filter`:

- The `List.filter` function is the most common way to remove elements based on a condition. It takes a predicate function (a function that returns a boolean) and a list, and returns a new list containing only the elements for which the predicate returns `true`.

- Example: Removing all even numbers from a list:

let numbers = [1; 2; 3; 4; 5; 6];;
let odd_numbers = List.filter (fun x -> x mod 2 <> 0) numbers;;
( odd_numbers will be [1; 3; 5] )

2. Removing a Specific Element by Value:

- You can use `List.filter` to remove a specific element by checking if each element is not equal to the element you want to remove.

- Example: Removing the number 3 from a list:

let numbers = [1; 2; 3; 4; 3; 5];;
let without_three = List.filter (fun x -> x <> 3) numbers;;
( without_three will be [1; 2; 4; 5] )

3. Removing Elements at Specific Indices:

- Removing elements at specific indices requires a bit more work, as `List.filter` doesn't directly provide index information. You can use `List.mapi` to get the index and then filter based on it.

- Example: Removing the element at index 1:

let numbers = [1; 2; 3; 4; 5];;
let remove_at_index index lst =
  List.mapi (fun i x -> if i <> index then Some x else None) lst
  | List.filter_map (fun x -> x);;
let without_index_one = remove_at_index 1 numbers;;
( without_index_one will be [1; 3; 4; 5] )

4. Removing the First Occurrence of an Element:

- To remove only the first occurrence of an element, you can use a recursive function that keeps track of whether the element has been removed.

- Example: Removing the first occurrence of 3:

let rec remove_first_occurrence element lst =
  match lst with
  | [] -> []
  | hd :: tl ->
    if hd = element then tl
    else hd :: remove_first_occurrence element tl;;
let numbers = [1; 2; 3; 4; 3; 5];;
let without_first_three = remove_first_occurrence 3 numbers;;
( without_first_three will be [1; 2; 4; 3; 5] )

In summary, `List.filter` is the most versatile and idiomatic way to remove elements from a list in OCaml based on a condition. For more specific removal scenarios, you might need to combine it with other functions or write custom recursive functions.

Learn more about this