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Date: Sun, 18 Jun 2017 05:06:40 -0700 (PDT)
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Subject: Re: Arrays in Ada 2020
From: Robert Eachus <rieachus@comcast.net>
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Xref: news.eternal-september.org comp.lang.ada:46982
Date: 2017-06-18T05:06:40-07:00
List-Id: <comp.lang.ada>

On Saturday, June 17, 2017 at 11:00:15 PM UTC-4, Nasser M. Abbasi wrote:
> On 6/17/2017 9:14 PM, Ivan Levashev wrote:
> >=20
> > FYI Ada 2020 is about to bring some improvements:
> >=20
> > http://www.ada-auth.org/standards/2xrm/html/RM-4-3-3.html
> >=20
> >> G : constant Matrix :=3D
> >>      (for I in 1 .. 4 =3D>
> >>         (for J in 1 .. 4 =3D>
> >>            (if I =3D J then 1.0 else 0.0))); -- Identity matrix
> >=20
> > Best Regards,
> > Ivan Levashev,
> > Barnaul
>=20
> That is nice. But still too much code for such common operation.

What about:

with Ada.Numerics.Generic_Real_Arrays;
procedure Main is
   package My_Arrays is new Ada.Numerics.Generic_Real_Arrays(Long_Float);
   use My_Arrays;
   Identity_4: Real_Matrix :=3D Unit_Matrix(4);=20
begin null; end Main;

On a different but related subject, the body of Unit_Matrix should look lik=
e:
    function Unit_Matrix (Order           : Positive;
                         First_1, First_2 : Integer :=3D 1)
                                            return Real_Matrix is
      Result: Real_Matrix(First_1..First_1+Order,
                     First_2..First_2+Order) :=3D (others =3D>=20
                                                (others =3D> 0.0));
   begin
     for I in First_1..First_1+Order loop
        Result(I, I-First_1+First_2) :=3D 1.0;
     end loop;
     return Result;
   end Unit_Matrix;

On modern virtual memory machines, new space (as opposed to reused stack sp=
ace) is cleared to zeros and the compiler knows it.  If you are allocating =
a large chunk of memory (in theory on the stack), the compiler can grab new=
 memory and put a pointer on the stack.  If you are working with large matr=
ices, you either expect this to happen or handle it yourself.  Setting the =
diagonal values to 1.0 is an operation repeated Order times not Order squar=
ed times, so let the compiler do the zeros as fast as possible, then worry =
about the ones.

Oh, and the compiler should create Result in place, rather than copying bac=
k.  The fact that the view of the object that Unit_Matrix is assigned to ma=
y be constant won't affect things, the view of Result inside Unix_Matrix is=
 not constant.