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From: "jimmaureenrogers@worldnet.att.net" <jimmaureenrogers@worldnet.att.net>
Newsgroups: comp.lang.ada,comp.lang.fortran
Subject: Re: Ada vs Fortran for scientific applications
Date: 9 Jul 2006 20:08:23 -0700
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Xref: g2news2.google.com comp.lang.ada:5589 comp.lang.fortran:11870
Date: 2006-07-09T20:08:23-07:00
List-Id: <comp.lang.ada>

Brooks Moses wrote:
> As a curiousity question, how would this work for cases such as, say, a
> finite-volume grid where I have one range for the cells, and another
> range for the faces between the cells, and want to do soemthing like this:
>
>      type ncells is range 1..100;
>      type nfaces is range 0..100;
>
>      cell : ncells;
>      leftface : nfaces;
>      rightface : nfaces;
>
>      leftface = cell - 1;
>      rightface = cell;
>
> According your explanation those last two commands, while making logical
> sense, would throw a type-mismatch error.  How easy is that to "fix"?
Ada makes a distinction between types and subtypes. The Ada definition
of a subtype is a new alias for a type, with a possible subset of the
range of valid values.

Your example could be performed with a little subtype manipulation:

    type nfaces is range 0..100;
    subtype ncells is nfaces range 1..100;

   cell : ncells;
   leftface : nfaces;
   rightface : nfaces;

   leftface := cell - 1;
   rightface := cell;

This works in Ada because all instance of a subtype are also
instances of their base type. Nonetheless, any instance of ncells
is limited to the range of value specified for the subtype.
Assignment of 0 to an instance of ncells results in an
invalid object. The invalidity of the object becomes an error when
the invalid object value is evaluated. Ada provides the
'Valid attribute, which returns a boolean value, to indicate
whether or not an object is valid. Evaluating the 'Valid
attribute does not constitute an evaluation of the object,
and therefore does not raise the exception
Constraint_Error.

Jim Rogers