Need help please about some exercices

Learning
1

hi e erybody
i have a list [“first” ; “second” ; “three”]

i want to return this list without the last element like that :retourne [“first” ; “second” ]*)

I try this but something doesn’t function:

let rec remove_last l =
  let lgth = List_length l in
  let ret = [] in
  if lgth < 2 then ret
  else List.hd l :: remove_last (List.tl l);;

print_string (remove_last l ["first" ; "second" ; "last"]);;
2

Please tell us what doesn’t work and the error messages you see, it makes it easier for us to help you.

In this case I suspect you need to replace List_length by List.length and your code should be right.

However note that calling List.length l on each recursive call needs to go through the whole l which is inefficient (see the implementation of List.length).

This is a case where you should use pattern matching. Here’s the structure, I’ll let you fill in the blanks:

let rec remove_last l = match l with
| [] -> …
| [v] -> …
| v :: vs -> …
2 Likes
3

i have this error "Error: This function has type 'a list → ‘a list
It is applied to too many arguments; maybe you forgot a `;’.

by the way I prefer to avoid the function match with

4

Ah it’s here:

you need to remove that l :–)

5

you say I have to cancel all this line or only the ‘l’
sorry I m beginner and I don’t understand any about this language

6

The way to work with lists is to use pattern matching:

let rec remove_last = function
  | [] | [_] -> []
  | x :: xs -> x :: remove_last xs

If you don’t understand anything about the language, then it may be a good idea to start by reading about the basics, I recommend OCaml from the Very Beginning.

Cheers,
Nicolas

7


The ocaml type system is like this toy for children, you can not put a triangle in the square shape.

Here remove_last returns a string list, but print_string expects a string.

So here you can use List.iter to apply print_string on every elements of the list, or you can also use String.concat to concatenate every string of the list into a single string:

$ ocaml
# let rec remove_last = function
  | [] | [_] -> []
  | x :: xs -> x :: remove_last xs
  ;;
val remove_last : 'a list -> 'a list = <fun>

# print_string (remove_last ["first" ; "second" ; "last"]) ;;
Error: This expression has type string list
       but an expression was expected of type string

# List.iter print_string (remove_last ["first" ; "second" ; "last"]) ;;
firstsecond
- : unit = ()

# print_string (
    String.concat " " (remove_last ["first"; "second"; "last"])) ;;
first second
- : unit = ()

(copy-pasted from the toplevel)

$ ocaml
# List.iter ;;
- : ('a -> unit) -> 'a list -> unit = <fun>

# String.concat ;;
- : string -> string list -> string = <fun>
8

please i would like to returns [first,second] and. not first second

9

Please tell us why you’re not able to figure out how to change the example, to fit what you want.

The answer is quite simple, just replace the space " " by a comma "," and use the concatanation operator ^:

# let s = String.concat "," (remove_last ["first"; "second"; "last"]) ;;
val s : string = "first,second"

# print_endline ("[" ^ s ^ "]") ;;
[first,second]
- : unit = ()