From: Jon Harrop <jon@ffconsultancy.com>
To: caml-list@yquem.inria.fr
Subject: Re: [Caml-list] adding lots of elements to a list
Date: Fri, 2 Jun 2006 23:06:09 +0100 [thread overview]
Message-ID: <200606022306.10022.jon@ffconsultancy.com> (raw)
In-Reply-To: <1149262126.4480592e24237@webmailetu.univ-orleans.fr>
On Friday 02 June 2006 16:28, julien.michel@etu.univ-orleans.fr wrote:
> The number of created objects can grow very fast, and may raise an amount
> greater than 100 000 elements.
That's fine as long as you don't use the non-tail-recursive modules from the
List module (e.g. map, fold_right).
> let count = ref 3 ;; (* number of iteration *)
> let list = [] in
>
> while (!count > 0) do
> decr count;
> let list = list@[!count] in
> Printf.printf "The 1st element is %i \n" (List.hd list) ;
> done;
>
> Printf.printf "list contains %i elements \n" (List.length list) ;;
To get the desired behaviour you must use a list ref and replace the list each
iteration:
# let count = ref 3 ;; (* number of iteration *)
let list = ref [] in
while (!count > 0) do
decr count;
list := !list @ [!count];
Printf.printf "The 1st element is %i \n" (List.hd !list) ;
done;
Printf.printf "list contains %i elements \n" (List.length !list);;
The 1st element is 2
The 1st element is 2
The 1st element is 2
list contains 3 elements
- : unit = ()
OCaml's lists are designed to be consed and decapitated from the front,
so "h :: t" is O(1) whereas "t @ [h]" is O(n^2). ***
Also, you might find functional style easier to use here:
# let rec make = function 0 -> [] | n -> n-1 :: make (n-1);;
val make : int -> int list = <fun>
# make 3;;
- : int list = [2; 1; 0]
That version isn't tail-recursive, so it'll raise Stack_overflow or even
segfault if you give it a big "n":
# make 1000000;;
Stack overflow during evaluation (looping recursion?).
But you can write a tail-recursive version by accumulating the list in reverse
order:
# let rec make a = function 0 -> List.rev a | n -> make (n-1::a) (n-1);;
val make : int list -> int -> int list = <fun>
# make [] 1000000;;
- : int list =
[999999; 999998; 999997; 999996; 999995; 999994; 999993; ...]
*** actually I think this is O(n^3) in native code.
--
Dr Jon D Harrop, Flying Frog Consultancy Ltd.
Objective CAML for Scientists
http://www.ffconsultancy.com/products/ocaml_for_scientists
prev parent reply other threads:[~2006-06-02 22:06 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-06-02 15:28 julien.michel
2006-06-02 15:40 ` [Caml-list] " Jonathan Roewen
2006-06-02 15:46 ` Shawn
2006-06-02 15:55 ` Nils Gesbert
2006-06-02 22:06 ` Jon Harrop [this message]
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