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

[Jerry <list_email@icloud.com>]

On Saturday, November 25, 2017 at 8:37:25 AM UTC-7, Robin wrote:
> "Jeffrey R. Carter"  wrote in message 
> ...
> > 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,
> 
> It can arise in boundary value problems, where operations are carried out
> on all elements of a matrix except the first and last rows, and first and
> last columns.
> 
> In PL/I, such situations are handled using the iSUB facility. 
> 
If I understand correctly, this is actually different from what I described, which is accessing the same data with different indexing schemes.

You describe a situation where the data are actually different (deleting rows and columns). Your problem could be handled by array slicing for nD arrays where n > 1.

As has been discussed here before, Ada strangely and annoyingly lacks slicing for multidimensional arrays, leading the programmer to devise ugly error-prone ad hoc methods, and that is very un-Ada-like.

Your problem would also be solved by a sub-matrix capability which is apparently what you describe for PL/1. Of course Ada also lacks this capability along with matrix compositions, both of which Matlab excels at.

These missing array features and the fairly trivial but readability-reducing method of using () for array indexing instead of the highly suggestive [] are probably the main things that keep Ada from being a first-class language for numerical work.

It seems that if some of these concerns were addressed it would help Ada expand its fields of use, a desire which has been expressed in the past.

Jerry