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From: "Nasser M. Abbasi" <nma@12000.org>
Newsgroups: comp.lang.ada
Subject: Re: basic question on adding 2 arrays
Date: Sat, 12 Jul 2014 23:09:13 -0500
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Date: 2014-07-12T23:09:13-05:00
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On 7/12/2014 10:34 PM, Nasser M. Abbasi wrote:

> ------------------------------------------------
> with Ada.Text_IO;                       use Ada.Text_IO;
>
> procedure foo is
>      nPoints : constant Integer := 100;
>      type grid_type is array (1 .. nPoints) of Float;
>      now : grid_type := (others => 0.0);
>      next: grid_type := now;
>      begin
>        next(2..now'length-1) := 1/2*
>          (now(1..now'length-1)+now(3..now'length));
> end foo;
> --------------------------------------------------
>
> I am basically trying to update an array (this is a finite difference
> scheme).  The error is that "+" is not defined for the grid_type,
> which I understand ofcourse. But wanted to ask if there is a way
> to do this in-line without having to define a method for this.
>
> For example, in Matlab or Fortran something like this works:
>
>      next(2:end-1) = 1/2*(now(1:end-2)+now(3:end));
>

This is what I tried. But after making special
"+" to add the 2 slices of grid_type, by making
new grid_type2 which is 2 elements smaller than
grid_type, how does not tell Ada to move this
grid_type2 into grid_type since they are of
different type?  Do I need another loop for this?

----------------------------
with Ada.Text_IO; use Ada.Text_IO;

procedure foo is
    nPoints : constant Integer := 100;
    type grid_type is array (1 .. nPoints) of Float;
    type grid_type2 is array (1 .. nPoints - 2) of Float;
    now  : grid_type := (others => 0.0);
    tmp:   grid_type2; -- just to do the slices
    next : grid_type := (others => 0.0);

    function "+" (left, right : in grid_type2) return grid_type2 is
       res : grid_type2;
    begin
       for i in grid_type2'Range loop
          res (i) := left (i) + right (i);
       end loop;
       return res;
    end "+";

begin
    tmp :=
      grid_type2 (now (2 .. now'Length - 1)) +
        grid_type2 (now (3 .. now'Length));
    
    next(2..now'length-1) := tmp;  -- stuck on this one ;)
    
end foo;
-----------------------------

I am sure there must be a better way to do all this, I can't
be making new type for each different slice I want to
work with?

--Nasser