Return the middle value from list in OCaml

Question

I have a list

let derp = [1; 2; 3; 4; 5];;

Im such a beginner in this language that this may seem stupid, but if I wanted just the middle value (3), how would I return that?

Answer 1

如果您不介意使用List模块，那么可以这样做：

List.nth derp (List.length derp / 2);;

Answer 2

There are many ways to do this. You said, in your comments that there will always be 5 elements in this list.

1. First, you can use Pavel solution : List.nth derp (List.length derp / 2); . If you want to put it in a function, let middle list = List.nth list(List.length derp / 2);

2. The 2nd solution is to use pattern matching :

   let middle list = match list with | [] -> 0 | p :: q :: r :: s :: t -> r

   Note that here, I assume that the list is a list of Int

3. The 3rd is a rewritten version of the first 1 solution, but here I suppose that you know the lenght of the list :

   let rec middle list n = if(n == 0) if(list.isEmpty) failwith "Empty List" else list.head else middle list (n - 1)

Answer 3

exception Error

let rec middle = function
    []    -> raise Error
  | [x]   -> x
  | x::tl -> middle (List.tl (List.rev tl))

Test :

# middle [1];;
- : int = 1
# middle [1;2];;
Exception: Error.
# middle [1;2;3];;
- : int = 2
# middle [1;2;3;4;5];;
- : int = 3