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

[Jerry <list_email@icloud.com>]

On Monday, November 27, 2017 at 6:25:26 PM UTC-7, Randy Brukardt wrote:
> 
> An evil and not guaranteed to work on all implementation technique.
> 
> The OP just needs to use an appropriate type conversion to change the bounds 
> of their array (odd no one has mentioned that):
> 
> If one has:
> 
>    subtype RV1_5 is Real_Vector(1..5);
> 
> Then the type conversion:
>     RV1_5(X) has the new bounds, and (by itself) doesn't copy any memory.
> 
> Thus, using a subprogram to temporarily bind the object shouldn't result in 
> any copying of the data (no guarantees, of course, but it will always work - 
> rememberm avoid premature optimization!):
> 
>     procedure Do_Y (Y : RV1_5) is
>     begin
>          -- Operations on Y.
>     end Do_Y;
> 
>     ...
>     Do_Y (RV1_5(X));
> 
> This is the best way to change the bounds of an array (or part of an array) 
> in Ada.
> 
>                                                             Randy.

Your method shows how to pass the re-dimensioned array to a subprogram for processing with a typecast. But I also need to process the array in the same program that the subtype is declared. How would I do the typecast without allocating more memory?

Something like this:

procedure Doubly_Indexed_Array is
   declare x with dimensions (0 .. 4)
   declare y with dimensions (1 .. 5) using the same memory as x but
	                              accessed with different indices
begin
   x(0) := 10.0;
   y(3) := 20.0;
   print x(0) resulting in 10.0
   print y(1) resulting in 10.0
   print x(2) resulting in 20.0
   print y(3) resulting in 20.0
end Doubly_Indexed_Array;

Jerry