Re: Incorrect error?

[stefan-lucks@see-the.signature]

On Mon, 18 Jul 2011, ldries46 wrote:

[...]
>   a      : Real_Vector(0 .. 2);
>   b      : Real_Vector(0 .. 2);
>   cross  : Real_Vector(0 .. 2);
> begin
[...]
>     Cross := a * b; -- VecCross( a, b, Cross ); <---------------- line 65 the

Why do you expect the product of two vectors to be an vector? 

Mathematical convention knows the "inner product" of two vectors (a 
single real value) and the "outer product" (a matrix).  

Ada.Numerics.Generic_Real_Arrays actually provides both:

   function "*"   (Left, Right : Real_Vector) return Real'Base;
   function "*"   (Left, Right : Real_Vector) return Real_Matrix;

So if M is a Real_Matrix(0..2,0..2) and F a float, you can write either

   M := a * b;  -- outer product 

or 
 
   F := a * b;  -- inner product

but not

  Cross := a * b; -- there is no such "*" defined in Ada.Num.Gen_Re_Arr. 

Please don't ask me why the inner product is of type "Real'Base", instead 
of "Real". I'd like to know that myself. ;-)
  
BTW, the error messages are not very helpful (at least not until you have 
seen many of them and are able to translate them for yourself):

>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

This really means is that there is no function "*" with two Real_Vector 
parameters as the input and returning a Real_Vector in scope. 



-- 
------ Stefan Lucks   --  Bauhaus-University Weimar  --   Germany  ------
               Stefan dot Lucks at uni minus weimar dot de
------  I  love  the  taste  of  Cryptanalysis  in  the  morning!  ------