[Caml-list] Collateral effect with pointers

[Caml-list] Collateral effect with pointers

[ANDRÿffffc9 MOURA]

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Hi to all!
 
I am trying to implement in OCaml version 3.07 the Delaunay triangulation contained in the program 'Triangle' of  J.R. Shewchuk for divide-and-conquer,  but the results differ from those produced by the 
original program. After exhausting tests, I believe that the problem is related to the structure of pointers. Or either, with some moment the values of  'farleft' and 'farright' intervene one in another one. Please, it would like to know as to hinder collateral effect (improper propagation) provoked by pointers for the following situation:
 
type 'a pointer = Null | Pointer of 'a ref;;
let ( !^ ) = function
    | Null -> invalid_arg "Attempt to dereference the null pointer"
    | Pointer r -> !r;;
(*val ( !^ ) : 'a pointer -> 'a = <fun>*)
let ( ^:= ) p v =
    match p with
    | Null -> invalid_arg "Attempt to assign the null pointer"
    | Pointer r -> r := v;;
(*val ( ^:= ) : 'a pointer -> 'a -> unit = <fun>*)
let new_pointer x = Pointer (ref x);;
(*val new_pointer : 'a -> 'a pointer = <fun>*)
...
let dummytri = new_pointer ({tr_adjtri=(Array.init 3 (fun x -> {te_ptri=Null; ta_orient=0})); tr_no=(Array.init 3 (fun x -> dummypoint)); tr_adjedg=(Array.init 3 (fun x -> dummysh)); tr_area=0.0});;

let dummypoint = new_pointer {po_x = 0.0; po_y = 0.0; po_marker = 0};;

let dummysh = new_pointer {sh_edg=(Array.init 2 (fun x -> Null)); sh_no=(Array.init 2 (fun x -> dummypoint)); sh_adjtri=(Array.init 2 (fun x -> Null)); sh_marker=0};;
let triangleloop = ref [||];;
...

...
let farleft  = ref (new_pointer {te_tri={tr_adjtri=(Array.init 3 (fun x -> {te_ptri=dummytri; ta_orient=0})); tr_no=(Array.init 3 (fun x -> dummypoint)); tr_adjedg=(Array.init 3 (fun x -> dummysh)); tr_area=0.0}; te_orient=0})
and farright = ref (new_pointer {te_tri={tr_adjtri=(Array.init 3 (fun x -> {te_ptri=dummytri; ta_orient=0})); tr_no=(Array.init 3 (fun x -> dummypoint)); tr_adjedg=(Array.init 3 (fun x -> dummysh)); tr_area=0.0}; te_orient=0}) in
...
maketriangle farleft;
maketriangle farright;
(* It has advanced, geometric primitives modify values of farleft and farright and connect they. *)
...
 

(* It adds an element to the 'triangleloop' array and returns a pointer for its last element *)
let maketriangle newtriedge =
   let l = Array.length !triangleloop in
   aumenteTriangulos triangleloop 1;
   newtriedge ^:= !triangleloop.(l);;
(*val maketriangle : triedge pointer -> unit = <fun>*)
 

(* It increases the size of  'triangleloop' *)
let aumenteTriangulos trianguloArray aumento =
   trianguloArray := Array.append !trianguloArray
                             (Array.init aumento (fun x -> {te_tri={tr_adjtri=(Array.init 3 (fun x ->
                             {te_ptri=dummytri; ta_orient=0})); tr_no=(Array.init 3 (fun x -> dummypoint));
                             tr_adjedg=(Array.init 3 (fun x -> dummysh)); tr_area=0.0}; te_orient=0}));;
(*val aumenteTriangulos : triedge array ref -> int -> unit = <fun>*)
 

Yours truly,
 
André Luiz Moura




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Re: [Caml-list] Collateral effect with pointers

[skaller]

On Thu, 2004-01-22 at 00:07, ANDRÿffffc9 MOURA wrote:

> After exhausting tests, I believe that the problem is related to the
> structure of pointers. 

In C, an assignment:

*p = *q

copies the value q points at to the location p points at.

In Ocaml, with your construction, the same thing happens
BUT there is a caveat.

In Ocaml most values are boxed, which means the
value is presented by a pointer. So suppose
in the C assignment above the value being copied
was a struct, then the whole struct would be
copied .. but not in Ocaml.

This is irrelevant for immutable data types..

> let dummytri = new_pointer ({tr_adjtri=(Array.init 3 (fun x ->
> {te_ptri=Null; ta_orient=0})); tr_no=(Array.init 3 (fun x ->
> dummypoint)); tr_adjedg=(Array.init 3 (fun x -> dummysh));
> tr_area=0.0});;

.. but Ocaml arrays are not immutable.

To copy an array, you will have to call

Array.copy

Note that of course if the elements IN the array are
also not immutable .. that still won't work
(since the array is just an array of pointers,
Array.copy just copies the pointers).

Perhaps this may be the source of your problem?


-- 
John Max Skaller, mailto:skaller@tpg.com.au
snail:25/85c Wigram Rd, Glebe, NSW 2037, Australia.
voice:61-2-9660-0850. Checkout Felix: http://felix.sf.net




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