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Date: Fri, 29 Feb 2008 18:16:17 +0100
From: Georg Bauhaus <see.reply.to@maps.futureapps.de>
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Subject: Re: Array Syntax
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Date: 2008-02-29T18:16:17+01:00
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ma740988@gmail.com wrote:
> Trying to garner a feel for Ada - while perusing texts - and I'm
> having slight difficulties understanding array syntax.
> 
> 
>    Max_Num : constant := 6;
> 
>    type Pixel_Data is
>       record
>          Num_Of_Values        : Int32;
>          Is_Valid_Data           : Boolean;
>       end record;
> 
>    type Pixel_Number is Int16 range 1 .. Max_Num ;
>    type Pixel_Data_Array is array
>      ( Pixel_Number range 1 .. Max_Num  )
>      of Pixel_Data;
> 
> Pixel_Data_Array is an array 6 deep of composite type Pixel Data.  So
> far so good?

Yes, though either Pixel_Number should be a "new Int16"
or else a subtype of Int16. Below, you'll find an aggregate
object of your anonymous array Buffer_Pixel_Data.
The numbers in the leftmost column are index values of
this type's index.


   D1: constant Pixel_Data :=
        (Num_Of_Values => 42, Is_Valid_Data => true);

   D2: constant Pixel_Data :=
        (Num_Of_Values => -666, Is_Valid_Data => false);
begin
   Buffer_Pixel_Data :=
     -- the first component of Buffer_Pixel_Data
     -- is an array of six, assigned one by one:
     (1 => Pixel_Data_Array'(1 => (0, True),
                             2 => (-1, false),
                             3 => (0, False),
                             4 => D2, 5 => D1,
			     6 => Pixel_Data'(D2)),
      -- the components of the array are records of type Pixel_Data
      -- the last one, at index 6, emphasizes

      -- using range 1 .. Max_Num for collectively setting values
      2 | 4  => Pixel_Data_Array'(1 .. Max_Num => D2),

      -- explicitly mention the respective index type (as per Ludovic's
      -- remarks). The type of 6 is inferred by the compiler
      Int32'(6) => Pixel_Data_Array'(Pixel_Number'(1) .. 6 => D2),

      -- at index positions 3, 5, and 7 put an array with all
      -- components set to record value (1, True)
      3 | 5 | 7 => Pixel_Data_Array'(others => (1, True)),

      -- you can indicate the range of index values by not using
      -- single index values, but the 'Range of the index type,
      8 .. 9 => Pixel_Data_Array'(Pixel_Number'Range => D2),

      -- but it is sufficient to just name the index type
      10 .. Frames_in_Buffer => Pixel_Data_Array'(Pixel_Number => D1));

   -- the latter two work because of a coincidence:
   -- type Pixel_Number is new Int16 range 1 .. Max_Num ;
   -- and
   -- type Pixel_Data_Array is array
   -- ( Pixel_Number range 1 .. Max_Num  ) ... have the same range
constraint
end News6;