From: Diego Olivier Fernandez Pons <Diego-Olivier.FERNANDEZ-PONS@cicrp.jussieu.fr>
To: caml-list@inria.fr
Subject: Re: [Caml-list] @, List.append, and tail recursion
Date: Fri, 31 Jan 2003 18:32:41 +0100 (NFT) [thread overview]
Message-ID: <Pine.A41.4.44.0301311831000.4055078-100000@ibm1.cicrp.jussieu.fr> (raw)
Bonjour,
Brian Hurt wrote :
> Here's an example I have run across. I'm working with sparse vectors, and
> basically storing them as (int * float) lists. Now, let's write the
> vector add function. The naive implementation would be:
let rec add_rec result = fun a b ->
match (a, b) with
| ([], _) -> a @ result
| (_, []) -> b @ result
| ((n, x) :: a_tail, (m, y) :: b_tail) ->
match compare m n with
| k when k < 0 -> add_rec ((n, x) :: result) a_tail b
| k when k > 0 -> add_rec ((m, y) :: result) a b_tail
| _ -> add_rec ((n, x +. y) :: result) a_tail b_tail
This is a tail recursive version of your function. The main problem is
that it reverses the list every time it is called.
Your first idea is to define
let add = fun a b -> List.rev (add_rec a b)
and you are right when you say that you are allocating a lot of
memory. But you can do much better : add_rec only needs the two lists
to be sorted, not to be increasing.
Why don't you try
type vector =
| Increasing of (int * float) list
| Decreasing of (int * float) list
then you just have to write the 4 corresponding add functions
Here is a second idea : is there a way to implement add in such a way
that it does not need the two arguments to be sorted ? If you were
working with arrays this would not be difficult.
Then, just try functional random acces lists. They give you a O(1)
access to the head element (in case both list are sorted in the same
way) or O(log n) acces to any element.
Diego Olivier
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next reply other threads:[~2003-01-31 17:33 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-01-31 17:32 Diego Olivier Fernandez Pons [this message]
-- strict thread matches above, loose matches on Subject: below --
2003-01-31 22:27 Harrison, John R
2003-01-31 19:58 Harrison, John R
2003-01-31 21:04 ` Brian Hurt
2003-01-24 15:35 Andrew Kennedy
2003-01-30 1:44 ` brogoff
2003-01-30 9:57 ` Christophe Raffalli
2003-01-30 16:03 ` Brian Hurt
2003-01-31 10:33 ` Mattias Waldau
2003-01-24 0:48 Brian Hurt
2003-01-30 18:10 ` Olivier Andrieu
2003-01-30 19:46 ` Brian Hurt
2003-01-30 20:52 ` Olivier Andrieu
2003-01-30 21:57 ` Brian Hurt
2003-01-31 2:16 ` james woodyatt
2003-01-31 17:05 ` Diego Olivier Fernandez Pons
2003-01-31 19:52 ` Brian Hurt
2003-01-31 21:34 ` Issac Trotts
2003-01-31 17:13 ` Brian Hurt
2003-01-31 17:42 ` brogoff
2003-01-31 19:18 ` Russ Ross
2003-01-31 19:32 ` Alexander V. Voinov
2003-02-01 2:30 ` brogoff
2003-01-31 23:12 ` Issac Trotts
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