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NNTP-Posting-Date: Wed, 22 Feb 2006 22:51:57 -0600
From: tmoran@acm.org
Newsgroups: comp.lang.ada
Subject: Re: Records that could be arrays
References: <WBaLf.18619$HU.13588@trnddc04>
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Date: Wed, 22 Feb 2006 22:51:57 -0600
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Date: 2006-02-22T22:51:57-06:00
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> type Coordinate is
>    record
>       Row    : Local_Float;
>       Column : Local_Float;
>    end record;
  In mathematics, one usually gives a coordinate as (x,y), while in
graphics it's often (vertical, horizontal).  Inadvertently switching
the order can be an annoying bug.  But with
   type Rows is new Local_Float;
   type Cols is new Local_Float;
the compiler's type checking can point out such errors.  In that case,
of course, you have to use a record with
        Row    : Rows;
        Column : Cols;
and cannot use an array.
  If your code might one day have to accomodate 3-D coordinates, then
        for A in Axis loop
          Sum := Sum + V(A)**2;
needs no change, while
        Sum := V.Row**2 + V.Column**2;
will need a makeover, and thus is much less desirable.

  The general point is that sometimes an array fits the problem better
and sometimes a record fits it better.  A rule that says "hey, they're all
Local_Float so it should be an array" is much too limiting.