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[Caml-list] Multi-keyed lookup table?
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 Date: -- (:) From: ijtrotts@u... Subject: Re: [Caml-list] Array.filter (was Multi-keyed lookup table?)
```On Sun, Aug 10, 2003 at 09:53:07PM +0200, Michal Moskal wrote:
> On Sat, Aug 09, 2003 at 11:48:51AM -0700, ijtrotts@ucdavis.edu wrote:
> > # module Array = struct
> >    include Array
> >    let filter f a = Array.init (Array.length a) (fun i -> f a.(i))
> >   end;;
> > # Array.filter (fun x -> x * x) [| 1;2;3 |];;
> > - : int array = [|1; 4; 9|]
>
> This is map, not filter. Writing filter is more difficult, since you
> need to first make list of results and then put them into array (or use
> Obj.magic tricks).

Yikes, my bad.  Here you go:

# let filter f a =
let n = ref 0 in
for i = 0 to Array.length a - 1 do if f a.(i) then incr n done;
let k = ref 0 in
Array.init !n
(fun i -> while not (f a.(!k)) do incr k done; incr k; a.(!k-1));;
val filter : ('a -> bool) -> 'a array -> 'a array = <fun>
# let odd = fun x -> x land 1 = 1;;
val odd : int -> bool = <fun>
# filter odd [| 1;2;3;4;5;6;7;8;9 |];;
- : int array = [|1; 3; 5; 7; 9|]

Luckily, you don't have to make a list or use Obj.magic.

Issac

--
Issac Trotts
Programmer
Center for Neuroscience
University of California, Davis

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