From: Christian Stork <cstork@ics.uci.edu>
To: caml-list@yquem.inria.fr
Subject: Re: [Caml-list] recursion/iterator question
Date: Mon, 17 Apr 2006 02:36:35 -0700 [thread overview]
Message-ID: <20060417093635.GB13523@anthony.ics.uci.edu> (raw)
In-Reply-To: <200604170106.23660.jon@ffconsultancy.com>
On Mon, Apr 17, 2006 at 01:06:22AM +0100, Jon Harrop wrote:
> On Sunday 16 April 2006 22:11, 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)"
Just in case you didn't know, you're looking for an enumeration of all
"3-sets" or "combinations" out of the set X. See for example
http://mathworld.wolfram.com/Combination.html .
> > 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?
> As 3-tuples, as you have done.
Or, if you choose to represent triplets as lists of three elements, you can
generalize Jon's solution to
let rec combs = function
| (0, _) -> [[]]
| (n, es) when n > List.length es -> []
| (n, e::es) -> List.map (fun l -> e::l) (combs (n-1, es)) @ combs (n, es)
let triplets es = combs (3, es)
Question to the rest of the list: The ocaml compiler complains with
...
Warning P: this pattern-matching is not exhaustive.
Here is an example of a value that is not matched:
(1, [])
(However, some guarded clause may match this value.)
...
Am I right to assume there's no way to get rid of this warning short of
disabling P-warnings on the command line? (I can't list all the lacking
patterns since they depend on n, right?)
--
Chris Stork <> Support eff.org! <> http://www.ics.uci.edu/~cstork/
OpenPGP fingerprint: B08B 602C C806 C492 D069 021E 41F3 8C8D 50F9 CA2F
next prev parent reply other threads:[~2006-04-17 9:35 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-04-16 21:11 Tato Thetza
2006-04-16 22:00 ` [Caml-list] " David Powers
2006-04-16 22:27 ` Martin Jambon
2006-04-17 0:06 ` Jon Harrop
2006-04-17 9:36 ` Christian Stork [this message]
2006-04-17 17:07 ` Jon Harrop
2006-04-18 3:25 ` Jonathan Roewen
2006-04-18 8:58 ` Christian Stork
2006-04-18 16:15 ` Christian Stork
2006-04-20 11:53 ` Damien Doligez
2006-04-17 6:50 ` Li-Thiao-Té Sébastien
2006-04-17 11:26 ` Nils Gesbert
2006-04-17 13:09 ` Xavier Leroy
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