Re: Normalizing array indices

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Sat, 29 Oct 2011 10:05:42 +0100, Simon Wright wrote:

> Stefan.Lucks@uni-weimar.de writes:
> 
>> In many cases, array ranges starting with an arbitrary index are
>> better (higher level) to model an application's demands. But
>> sometimes, like when applying a sorting routine, this extra
>> information is actually some ballast.
>>
>> Ideally, the specification of a subprogram would carry the information 
>> that the subprogram only uses "normalized" array indices, to free the 
>> tester from having to consider test cases with different A'First:
>>
>>   procedure Sort(A: in out Sort_Array_Type(1 .. <>)); 
>>
>> The user can still call Sort with any array of range, say, 4711 .. 9421, 
>> but Sort coldn't tell that apart from an array of range 1 .. 4711. Thus, 
>> there is no reason for additional test cases with different values 
>> for A'First. 
> 
> But your problem was with A'Last, surely?
> 
> I've had more surprises with
> 
>    type Arr is array (Integer range <>) of Float;
>    A : Arr := (1.0, 2.0, 3.0);
> 
> where A'First is Integer'First (on GNAT), ie -2**31.

The problem is that for some algorithms for any valid array index Index,
there should exist Index - 1 and/or Index + 1. One possible solution might
be some sugar for

   subtype Safe_Integer is range Integer'First + 1..Integer'Last - 1;
   type Arr is array (Safe_Integer range <>) of Float;

Another solution is array interfaces with universal integer as the position
(offset) in addition to index. Some people are fond of [] brackets,
positional interfaces could use them. Algorithms written in terms of
indices sometimes become simpler when rewritten in terms of offsets.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de