Re: Incorrect error?

[Adam Beneschan <adam@irvine.com>]

On Jul 18, 1:47 am, "ldries46" <bertus.dr...@planet.nl> wrote:
> I'm using Ada.Numerics.Generic_Real_Arrays. While Compiling I got the
> following error message:
>
> 65:21     expected type "Ada.Numerics.Generic_Real_Arrays.Real_Vector" from
> instance at line 6
> 65:21     found type "Ada.Numerics.Generic_Real_Arrays.Real_Matrix" from
> instance at line 6
>
> As can be seen Cross, a and b declared as Real_Vector .
> What do I do wrong?

As others have said, you're trying to do something that isn't
defined.  There is no "*" operation in Generic_Real_Arrays that takes
two Real_Vector arguments and returns a Real_Vector.

However, I think the error message GNAT is reporting is wrong, and
this should be reported as a GNAT bug.  It's no wonder you were
confused.  If there were just one function like this:

   function Product (Left : Real_Vector; Right : Real_Matrix) return
Real_Vector;

and you tried

   Cross := Product (A, B);

GNAT would say "expected type Real_Matrix ... found type Real_Vector",
which is pretty clear.  Changing "Product" to "*" and using it as an
operator causes GNAT to generate a different message "there is no
applicable operator".  But in the actual package, there are eight
definitions of "*", and the error message GNAT produces doesn't make
sense for any of them, since it's implying that you're actually using
a Real_Matrix as a parameter, when of course you aren't.

                                        -- Adam





> L. Dries
>
> Code:
>
> package Float_Arrays is new Ada.Numerics.Generic_Real_Arrays (float);
> use Float_Arrays;
>
> function Volume (ObjID : integer; Obj : ptrS_Object) return float is
>    ok     : boolean;
>    n      : integer := 1;
>    Volume : float;
>    a      : Real_Vector(0 .. 2);
>    b      : Real_Vector(0 .. 2);
>    X0     : Real_Vector(0 .. 2);
>    X1     : Real_Vector(0 .. 2);
>    X2     : Real_Vector(0 .. 2);
>    cross  : Real_Vector(0 .. 2);
>    nA     : Real_Vector(0 .. 2);
>    p      : ptrNode_Object;
>    TempS  : ptrS_Object;
>    TempT  : ptrTriangle_Object;
> begin
>    TempS := Obj;
>    while n /= ObjID loop
>       n := n + 1;
>       TempS := TempS.next;
>       if TempS = null then
>          declare
>             NO_NODE_OBJECT : exception;
>          begin
>             raise NO_NODE_OBJECT;
>          end;
>       end if;
>    end loop;
>    n := TempS.PosFaces(0);
>    TempT := stT_Object;
>    ok := TempT = null;
>    while not ok loop
>       if TempT.nr = n then
>          ok := true;
>       else
>          TempT := TempT.next;
>          ok := TempT = null;
>       end if;
>    end loop;
>    while n < TempS.PosFaces(1) loop
>       p := GetPoint(TempT.P1);
>       X0(0) := p.pRectangular(x);
>       X0(1) := p.pRectangular(y);
>       X0(2) := p.pRectangular(z);
>       p := GetPoint(TempT.P2);
>       X1(0) := p.pRectangular(x);
>       X1(1) := p.pRectangular(y);
>       X1(2) := p.pRectangular(z);
>       p := GetPoint(TempT.P3);
>       X2(0) := p.pRectangular(x);
>       X2(1) := p.pRectangular(y);
>       X2(2) := p.pRectangular(z);
>       a := X1 - X0; -- VecSub( a, X1, X0 );
>       b := X2 - X0; -- VecSub( b, X2, X0 );
>      Cross := a * b; -- VecCross( a, b, Cross );
> <---------------- line 65 the cursor is standing just before *
>       nA := Cross * 0.5; -- VecMultScalar( nA, Cross, 0.5 );
>       -- Volume = Volume + fabs(VecDot( X0, nA ));
>       Volume := Volume + X0(0)*nA(0) + X0(1)*nA(1) + X0(2)*nA(2);
>       n := n + 1;
>    end loop;
>    Volume := Volume/3.0;
>    return Volume;
> end Volume;