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

[Per Sandberg <per.s.sandberg@bahnhof.se>]

Hi a reflection.
Why the different indexing ?
/P



Den 2017-11-24 kl. 12:42, skrev Jerry:
> 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 could declare two arrays such as
> 
> x : Real_Vector(0 .. 4);
> y : Real_Vector(1 .. 5);
> 
> Ada then allows the assignments
> 
> y := x;
> and
> x := y;
> 
> but this has two problems. First, I have wasted memory and cycles by declaring y and copying x into it. Second, I would then have a kind of version control problem where every time I modified one, I have to copy it to the other.
> 
> I have the notion that I should be able to accomplish this using pointers but I can’t seem to crack how do to this. I’ve tried all sorts of things and am resigned to doing the above duplication if I have to. (My arrays can be _much_ larger so the overhead might be significant. They might also be two-dimensional, as in Real_Matrix or Complex_Matrix.)
> 
> I'm currently looking at System.Address_To_Access_Conversions but I thought I would query the list in the meantime.
> 
> Any ideas?
> 
> Thanks,
> Jerry
>