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From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: Nested declares, constant size arrays and clarity
References: <fe1748$it1$1@nemesis.news.tpi.pl>
Date: Thu, 04 Oct 2007 01:49:19 +0200
Message-ID: <87641n92tc.fsf@ludovic-brenta.org>
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Date: 2007-10-04T01:49:19+02:00
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Mateusz Papiernik <mati@maticomp.net> writes:
> Hey!
>
> I'm starting off with Ada95. I'm having some problems with clarity of
> my solution, which - I think - is not as good as I would like to.
>
> I've got my root procedure in which I read some number from the
> standard input. Then I create an array of MyType with size read before.
>
> procedure mxp is
>   ... -- some record type declarations
>   ... -- some *functions*
> begin
>   ... -- read size
>   declare
>     type MyTypeArray is array (1...sizegiven) of MyType;
>     MyArray: MyTypeArray;
>     ... -- some *functions* doing something with newly made array
>   begin
>     ... -- these function calls
>   end;
> end;
>
> The problem is, I don't really like the idea of declaring some
> functions in the root procedure declaration, and some others in nested
> declare section. Due to that I've got function implementations in two
> places, which somehow feels awkward to me.
>
> Is there any way to accomplish the same without that nesting _and_
> without building external module/using OOP/pointers?
>
> Maybe there is a way to define general array type of unknown size,
> then declare functions doing something with it in the topmost
> declaration section, and then only somehow initialise the array with
> given size?

Yes, it is called an "unconstrained array type".  Here is how you
would do it:

procedure mxp is
   type My_Record_Type is record ... end record;
   type My_Array_Type is array (Positive range <>) of My_Record_Type;
   -- the type is unconstrained

   Size : constant Natural := Get_From_Keyboard;
   My_Array : My_Array_Type (1 .. Size); -- the array is constrained
begin
   -- Call the functions
end mxp;

Note: the array type uses Positive as the index type; the Size is a
Natural.  Positive and Natural are both subtypes of Integer and so are
compatible with one another:

subtype Positive is Integer range 1 .. Integer'Last;
subtype Natural  is Integer range 0 .. Integer'Last;

If the user types zero for the size, then My_Array will have the
bounds 1 .. 0.  THIS IS OKAY!!!  In that case, looping over the array
will simply do nothing.  You normally loop over an array like this:

   for K in My_Array'Range loop
      -- ...
   end loop;

PS. What I described above is exactly how the predefined type String
works; it is an unconstrained array of Character's.  Look at the
standard subprograms that operate on Strings for more inspiration.

HTH

-- 
Ludovic Brenta.