CODEWITHSUNDEEP
listocaml
Slavko
Slavko

Reputation: 23

Convert from int list to int Ocaml

How can i get number(int) from int list?

Example: I have list of int l=[1;2;3;4;5] and i want to get number n=54321.

This is my code:

let rec number ls =
match ls with
    |[]->[]
    |(h::t)->h*10**(List.length t) +  number t
;;

When i compile it, i get this error:

Error: This expression has type int but an expression was expected of type float

Upvotes: 1

Views: 2297

Answers (2)

Lhooq
Lhooq

Reputation: 4441

The easiest way to do it is to use an iterator in a tail-recursive way

List.fold_left f a [b1; ...; bn] is f (... (f (f a b1) b2) ...) bn.

let number_tr ls = 
  let n, _ = List.fold_left (
    fun (n, pow) e -> (e * pow + n, pow * 10) (0, 1) ls in
  ls

The accumulator contains two integers, the number that we are computing and the actual power. For each new element, we increment the number and then multiply the number by 10.

Another version uses a non tail-recursive iterator

List.fold_right f [a1; ...; an] b is f a1 (f a2 (... (f an b) ...))

let number_ntr ls = 
  List.fold_right (fun e n -> e + n * 10) ls 0

Runs :

number_tr [1; 2; 3]
> acc = (0, 1), e = 1 -> new acc = (1 * 1 + 0, 1 * 10)
  > acc = (1, 10), e = 2 -> new acc = (2 * 10 + 1, 10 * 10)
    > acc = (21, 100), e = 3 -> new acc = (3 * 100 + 21, 100 * 10)
      > acc = (321, 1000) -> returns 321

number_ntr [1; 2; 3]
> acc = 0, e = 3 -> new acc = 3 + 0 * 10
  > acc = 3, e = 2 -> new acc = 2 + 3 * 10
    > acc = 32, e = 1 -> new acc = 1 + 32 * 10
      > acc = 321 -> returns 321

Upvotes: 1

ScriptSurfer
ScriptSurfer

Reputation: 332

You can also do it using tail recursion: 

let f l = 
    let rec f_helper l num =
        match l with 
        | [] -> num
        | (h::t) -> f_helper t ((num * 10 ) + h)
    in f_helper (List.rev l) 0
;;

Upvotes: 0

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