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From: Matthew Heaney <matthew_heaney@acm.org>
Subject: Re: Dynamic Array Sizing
Date: 1999/06/20
Message-ID: <m3hfo2d10l.fsf@mheaney.ni.net>#1/1
X-Deja-AN: 491862557
References: <376B1811.666F042@hotmail.com> <m3vhckd737.fsf@mheaney.ni.net>
 <376C7D96.47FF7F2@hotmail.com>
NNTP-Posting-Date: Sun, 20 Jun 1999 14:15:08 PDT
Newsgroups: comp.lang.ada
Date: 1999-06-20T00:00:00+00:00
List-Id: <comp.lang.ada>

On 20 Jun 1999 15:09, Nick Wilson <snow_moose@hotmail.com> wrote:

> The trouble comes about because it's a real time system and my
> understanding is that it would be a good idea to do as much of the
> initialisation before the system actually starts running. It would be
> possible to just open, read from the file however many times as
> necessary then close it until the next lookup. I'm just looking for a
> nicer way if I can.

No, you don't have to open and re-open the file during run-time.  File
init can be done once, at system start-up.  See the code below.

 
> package Lookup_Array is
>    function Get_Array_Size return Positive;
>    procedure Initialise_Array;
>    function Lookup_Value(Index : in Positive) is
> end Lookup_Array;
> 
> 
> -- body
> package body Lookup_Array is
> 
> type Array_Store is array (Positive range <>) of Boolean;
> Array_Size : Positive;
> 
> function Get_Array_Size return Positive is
>     Open file
>     Finds size of array
>     return Array_Size := the size of array data in the file
> end Get_Array_Size;
> 
> declare
>     The_Array : Array_Store (Array_Size);
> 
>         procedure Initialise_Array is
>             reads values from file into the array
>         end Initialise_Array;
> 
>         function Lookup_Value(Index : in Positive) is
>             return The_Array(Index);
>         end Looup_Value;
> 
> end Lookup_Array;
> 
> I know this code isn't right, but hopefully it points out the problem of
> where I can declare The_Array so that it is visible to outside calling
> functions such as Lookup_Value ?


Some comments:

o If you insist on declaring a local array, then you can do it during
  package elaboration:

  package body Lookup_Array is

    function Get_Array_Size return Positive is ...;

    The_Array : Array_Storage (1 .. Get_Array_Size);

    function Lookup_Value ... is;

  begin

    <read values from file and populate The_Array>

  end Lookup_Array;


o The client of the package doesn't care whether he's reading from a
  a local array or from the file directly.  Since he's already calling a
  function to return a value, why not just implement the function to
  return a value from the file directly:

  package body Lookup_Array is

    File : <direct io>.File_Type;

    function Lookup_Value (Index : Positive) is
  
      Value : <value type>;

    begin
      
      Read (File, Index, Value);

      return Value;

    end Lookup_Value;

  begin

    Open (File, In_File, "myfile.dat");

  end Lookup_Array;


No local array is necessary.  And all initialization is done once,
during elaboration, so there's no run-time penalty.