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From: Mart van de Wege <mvdwege@mail.com>
Newsgroups: comp.lang.ada
Subject: Beginners question: Compound types, how-to?
Date: Sun, 31 Oct 2010 23:00:52 +0100
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Date: 2010-10-31T23:00:52+01:00
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Hi,

I've been playing around with Ada for a bit, and like it. To teach it to
myself, I am rewriting some Perl code I wrote for hobby purposes, yet I
can't seem to get my head around how to use compound types.

I have the following code (yes, I'm an RPG geek, sorry):

games-rpg-dnd.ads:
--with Games.RPG;
-- Ignore parent package for now, we don't need the
-- utility functions yet.
package Games.RPG.DnD is
   type Ability_Type is (Str, Dex, Con, Int, Wis, Cha);
   type Ability is
      record
	 Score : Natural;
	 Modifier : Integer;
      end record;
   type Ability_Array is array (Ability_Type) of Ability;
   type Character is 
      record
	 Abilities : Ability_Array;
      end record;
end Games.RPG.DnD;

test-game.adb:
with Games.RPG.DnD, Ada.Text_IO, Ada.Integer_Text_IO;
use Games.RPG.DnD, Ada.Text_IO, Ada.Integer_Text_IO;

procedure Test_Game is
   C : Character;
begin
   C.Abilities(Str) := (Score => 10, Modifier => -1);
   Put (C.Abilities(Str).Score);
   put (C.Abilities(Str).Modifier);
end Test_Game;

When I do it like this:

with Games.RPG.DnD, Ada.Text_IO, Ada.Integer_Text_IO;
use Games.RPG.DnD, Ada.Text_IO, Ada.Integer_Text_IO;

procedure Test_Game is
   A : Ability_Array;
begin
   A(Str) := (Score => 10, Modifier => -1);
   Put (A(Str).Score);
   put (A(Str).Modifier);
end Test_Game;

It works, when I use the former code, it bombs with 'Invalid prefix in
selected component "C"';

What am I doing wrong? I am misunderstanding something, but what? C
should be a record containing an Ability_Array as its first field, so I
should be able to index into it using an Ability_Type subscript. How do
I do this?

Mart
-- 
"We will need a longer wall when the revolution comes."
    --- AJS, quoting an uncertain source.