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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: how to print an array range?
Date: Wed, 27 Jun 2012 00:17:13 +0200
Organization: cbb software GmbH
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Date: 2012-06-27T00:17:13+02:00
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On Tue, 26 Jun 2012 17:31:31 -0400, Robert A Duff wrote:

> Why didn't they make the index subtype Positive (or
> maybe Natural) instead of Integer?

Why should they limit index to a subset which would have more problems with
not being closed upon arithmetic operations than Integer? The only real
constraint here is the array bounds. Ideally index should be
Universal_Integer.

> Is it important to allow negative index values?

Yes, there were several cases I used a negative lower bound.

One important case is when you have a n-elements thick border around the
original matrix filled with some special values, usually zeros. This
technique is used when matrix elements are convoluted. E.g. let you compute
something like diagonal sums like:

   A(I - 2, J) + A(I - 1, J) + A (I, J) + A (I + 1, J) + A (I + 2, J)

You don't want to check I for special border cases A'First (1), A'First (1)
+ 1, A'Last (1) - 1, A'Last (1). Instead of that you add a border of zeros,
trading a little space for index checks, and get more uniform and efficient
code.

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