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Date: -- (:)
From: Martin Jambon <martin_jambon@e...>
Subject: Re: [Caml-list] recursion/iterator question
On Sun, 16 Apr 2006, Tato Thetza wrote:

> Hi caml-list
> Given a list, I would like to iterate over all triplets in the list. For
> example, in mathematcs, its not uncommon to have expressions such as
> "for all i,j,k in set X, do f(i,j,k)"
>
> The only way I can think of is to create a list with all triplets of the
> list, so:
>  triplets([1,2,3,4]) = [(1,2,3),(1,2,4),(1,3,4),(2,3,4)]
> and take this list and map a function f to it.
>
> questions:
> 1) what would be the best way to write triplets?

You can use list comprehensions.
See http://oandrieu.nerim.net/ocaml/#pa_compr

let triplets l = [+ (x, y, z)
 		 | x <- l
 		 | y <- l
 		 | z <- l
 		 | when x < y && y < z ];;

That's not optimal, but it's pretty clear.


> 2) is there a cleaner way to iterate over all triplets in a list?

You can do that, it should perform better:

let rec iter_full f = function
     [] -> ()
   | x :: l -> f x l; iter_full f l

let iter_tail iter f l = iter_full (fun x l -> iter (f x) l) l

let iter_full3 f l = iter_tail (iter_tail iter_full) f l

let iter3 f l = iter_full3 (fun x y z _ -> f x y z) l



Martin

--
Martin Jambon, PhD
http://martin.jambon.free.fr

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