ocaml putting a integer into a list

Question

I a noobie with ocaml, my problems is how get the digit of an integer in the ocaml and how to put them into a list by recursive calls

OCaml function digits : int -> int list that takes an integer n as an argument and if the integer is positive, returns the list of digits of n in the order in which they appear in n ie:

# digits 3124;;
- : int list = [3;1;2;4] 


# let rec digits n =
  if n >0 then digits(n/10)::[]
  else [];;

lbonn lbonn · Accepted Answer · 2013-01-10T07:33:07

If you write digits directly as a recursive function, you'll find it hard to print the digits in the correct order without reversing the whole list at the end or appending elements to the end of the current list (both unefficient approaches).

This is best done with an intermediate terminal recursive function:

let digits2 d =
    let rec dig acc d =
        if d < 10 then d::acc
        else dig ((d mod 10)::acc) (d/10) in
    dig [] d
;;

Here, dig takes an accumulator which is used to build the list and is passed on the next recursive call and returned as a whole at the end (except for the cons of d here). This is a pretty common pattern when writing terminal recursive functions.

It may take a little time to get used to this style of writing, but eventually it will come. Trying to rewrite simple functions like this one is in my view a good way to train yourself.

ocaml putting a integer into a list

2 Answers