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From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: OO Style with Ada Containers
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Date: Thu, 15 Nov 2007 10:36:54 +0100
Message-ID: <87oddv3l55.fsf@ludovic-brenta.org>
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Date: 2007-11-15T10:36:54+01:00
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braver <deliverable@gmail.com> writes:
> my sample application [...] gets a Vector of tokens and inserts them
> into an Ordered_Map to count word occurrences:

I don't think you need to store all the tokens.  All you need is a
Hashed_Set (which stores objects without duplication).  I would do
something like

   type Word_Counter is
      Word : Ada.Strings.Unbounded.Unbounded_String;
      Count : Natural;
   end Word_Counter;

   function Equivalent (L, R: Word_Counter) is
      use type Ada.Strings.Unbounded.Unbounded_String;
   begin
      return L.Word = R.Word; -- ignore Count in the comparison
   end Equivalent;

   function Hash (Element : Word_Counter) is
   begin
      return Ada.Strings.Unbounded.Hash (Element.Word);
   end Hash;

   package Word_Counter_Hashed_Sets is
     new Ada.Containers.Hadhed_Sets
       (Element_Type => Word_Counter,
        Hash => Hash,
        Equivalent_Elements => Equivalent);

   Counters : Word_Counter_Hashed_Sets.Set;
   Current_Word : Ada.Strings.Unbounded.Unbounded_String;
   Position : Word_Counter_Hashed_Sets.Cursor;
   use type Word_Counter_Hashed_Sets.Cursor;
begin
   ...
   Current_Word := Get_Next_Word;
   Position := Counters.Find (Current_Word);
   if Position = Word_Counter_Hashed_Sets.No_Element then
      Counters.Insert (New_Item => (Word => Current_Word, Count => 1));
   else
      declare
         E : constant Word_Counter := Counters.Element (Position);
      begin
         Counters.Replace_Element
            (Position => Position,
             New_Item => (Word => E.Word, -- [1]
                          Counter => E.Counter + 1));
      end;
   end if;
   ...
end;

(does [1] duplicate E.Word each time we increment the counter?  If so,
consider using
Ada.Containers.Hashed_Sets.Generic_Keys.Update_Element_Preserving_Keys).

> -- would need to import "=" for private type WC.Cursor:
> -- if Ngram_Cursor = WC.No_Element

declare
   use type WC.Cursor;
begin
   if Ngram_Cursor = WC.No_Element then

> I'd appreciate any improvements to the above, which I deduced from ARM

HTH

-- 
Ludovic Brenta.