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From: "Jeffrey R. Carter" <spam.jrcarter.not@acm.nospam.org>
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Subject: Re: Newbie's question
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Date: 2008-02-11T17:42:47+00:00
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Christos Chryssochoidis wrote:

Apparently you found a compiler error. Some comments on your code:

>       Result : access Int_Array;
>    begin
>       for I in Elements'Range loop
>          if Predicate(Elements(I)) then
>             Tmp_List.Append(Elements(I));
>          end if;
>       end loop;
> 
>       Result := new Int_Array(1..Index(Tmp_List.Length));

This is OK for a little test program like this, but in real code, you have a 
memory leak from this use of access values and heap allocation that you should 
want to avoid. You can do that by moving the declaration of Result into 
Copy_List below:

Result : Int_Array (1 .. Index (Tmp_List.Length) );

You might also find an unbounded array (Ada.Containers.Vectors) more suited than 
a list here, since you can use the same index for both.

>   Copy_List:
>       declare
>          Tmp_List_Cursor : Int_Lists.Cursor := Tmp_List.First;
>          I : Integer := 1;
>       begin
>          while Int_Lists.Has_Element(Tmp_List_Cursor) loop
>             Result(I) := Int_Lists.Element(Tmp_List_Cursor);
>             Tmp_List_Cursor := Int_Lists.Next(Tmp_List_Cursor);
>             I := I + 1;
>          end loop;
>       end Copy_List;

Rather than using a cursor and a loop, I would probably use the Iterate 
procedure to convert the list into an array.

You could also make Filter recursive and eliminate Result and Tmp_List altogether:

begin -- Filter
    if Elements'Length <= 0 then
       return Elements;
    elsif Predicate (Elements (Elements'First) ) then
       return Elements'First &
              Filter (Elements (Elements'First + 1 .. Elements'Last), Predicate);
    else
       return Filter (Elements (Elements'First + 1 .. Elements'Last), Predicate);
    end if;
end Filter;

>    function Greater_Equal_3(Element :Integer) return Boolean is
>    begin
>       if Element >= 3 then
>          return True;
>       else
>          return False;
>       end if;
>    end Greater_Equal_3;

This should be

return Element >= 3;

Personally, I'd make filter generic rather than passing the Predicate function 
as a subprogram parameter.

-- 
Jeff Carter
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