Re: How to access an array using two different indexing schemes

[AdaMagica <christ-usch.grein@t-online.de>]

Am Freitag, 24. November 2017 13:33:25 UTC+1 schrieb Jeffrey R. Carter:
> On 11/24/2017 12:42 PM, Jerry wrote:
> > I want to access an array such as Real_Vector (built-in for Ada >= 2005) with two different indexing schemes. For example, I would access a Real_Vector indexed (0 .. 4) when thinking of it as a times series and the same data indexed (1 .. 5) when thinking of it as a vector. Another application would be a vector indexed (-128 .. 127) because it fits my problem domain, perhaps a spatial variable, but I need to index it as (0 .. 255) when thinking of doing a Fast Fourier Transform on it.
> 
> I can't think of any situation when I'd need to access the same component of an 
> array using different indices, but I note that this compiles:
> 
> procedure Renaming is
>     subtype T1 is String (1 .. 5);
>     subtype T2 is String (2 .. T1'Length + 1);
> 
>     S1 : T1 := "Hello";
>     S2 : T2 renames S1;
> begin -- Renaming
>     null;
> end Renaming;

  procedure Renaming is
    subtype T1 is String (1 .. 5);
    subtype T2 is String (1 .. 0);   -- !
    S1 : T1 := "Hello";
    S2 : T2 renames S1;
  begin -- Renaming
    Put (S1'First'Image); Put (S1 (S1'First));
    Put (S2'First'Image); Put (S2 (S2'First));
    Put (S1'Last 'Image); Put (S1 (S1'Last ));
    Put (S2'Last 'Image); Put (S2 (S2'Last )); New_Line;
  end Renaming;

This results in
 1H 1H 5o 5o

Rememner that renamings ignore constraints.