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recursion/iterator question
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Date: 2006-04-17 (06:50)
From: Li-Thiao-Té_Sébastien <sayan@c...>
Subject: Re: [Caml-list] recursion/iterator question
Tato Thetza wrote:
> 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?
> 2) is there a cleaner way to iterate over all triplets in a list?

I suggest turning the list into an array, then using for-loops instead 
of trying to explicitly generate the list of all possible triplets. If 
you need to accumulate the results, you can use a list ref. This amounts 
to implementing a set using an array instead of a list; unfortunately it 
is not functional :(

let iterate f l =
   let t = Array.of_list l in
   let n = Array.length t in
   let results = ref [] in
   for i = 0 to n-1 do
     for j = 0 to n-1 do
       for k = 0 to n-1 do
         results := f (t.(i),t.(j),t.(k)) :: results
done done done

Li-Thiao-Té Sébastien